Experiments in Organic Chemistry 9781487583033

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Experiments in

ORGANIC CHEMISTRY

BY F. R. LORRIMAN J. J. RAE

G. H. SCHMID Department of Chemistry University of Toronto

UNIVERSITY OF TORONTO PRESS

PREFACE The experiments in this book are designed for students beginning the study of organic chemistry. The purposes of the book are to teach the student some of the techniques of organic chemistry and to familiarize him with the methods of preparation and chemical properties of representative members of the important classes of organic compounds. Each section contains a brief introduction to that part of the work and should help the student to understand the subsequent experiments. The authors acknowledge constructive suggestions from Miss V. Vlasinich, Dr. I. W. J. Still and Mrs. A. G. Sleep.

©

UNIVERSITY OF Tmi.oNTO PRESS

1966

PRINTED IN CANADA

Reprinted in 2018 ISBN 978-1-4875-8175-6 (paper)

CONTENTS I. Introduction to the Laboratory

1

2. Laboratory Techniques A. Dispensing Chemicals, 9 B. Melting Point, 9 C. Solubility and Crystallization, 15 D. Boiling Point and Distillation, 19 E. Extraction and Chromatography, 27

7

3. Qualitative Analysis

33

4. Aliphatic Hydrocarbons

37

5. Alkyl Halides

41

6. Alcohols

51

7. Aromatic Hydrocarbons and Their Derivatives

57

8. Aldehydes and Ketones

67

9. Carboxylic Acids and their Derivatives

81

10. Amines 12. Molecular Rearrangements

91 99 109

13. Polymers and Polymerization

117

14. Carbohydrates

123

15. Proteins

133

11. Phenols and Quinones

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1. INTRODUCTION TO THE LABORATORY Chemistry is an experimental science. All major theories of chemistry which have evolved through the years are based on experiments. This laboratory course is designed to acquaint the student with the experimental nature of organic chemistry. It will accompany and illustrate the lectures and also teach laboratory technique. The experiments in this book comprise a series of exercises which the authors have found from experience illustrate well the principles of organic chemistry and develop laboratory technique. Students are expected to cover the work assigned. Since organic reactions are often slow and sometimes do not fit themselves to a laboratory period, it is essential that the student plan his work prior to entering the laboratory. Often experiments overlap one another. Therefore it is essential that all con­ tainers should be clearly labelled, whether for permanent or temporary use. THE LABORATORY NOTEBOOK For reporting experiments a bound notebook of approximately eight and one-half by eleven inches is used. Observations must be recorded in the notebook IMMEDIATELY IN INK. Notes written on scraps of paper will not be tolerated. Every experiment must have a title and the date on which the experiment was performed. The left-hand page of the notebook may be used as a record of operations as they are carried out and data as observed. The right-hand page should be used to write up a formal report of the experiment in which the observations are organized, summarized and tabulated. All questions and problems must be answered in the notebook. The formal report of an experiment is due one week after it is completed in the laboratory. It may be written outside the laboratory employing the data recorded on the left-hand page. The form employed for the write-up will depend on whether or not a product is prepared in the experiment. Non-Preparative Experiments

In this type of experiment, no entries are made in the notebook before the experiment is performed. However, the student MUST be familiar with the experimental procedure before entering the laboratory. As the experi­ ment is performed, a record of observations is kept on the left-hand page of the notebook. The results of the experiment are then summarized in the formal report on the right-hand side. In tests involving chemical reactions, balanced equations for these reactions must be included in the write-up. 1

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3

Preparative Experiments These experiments, indicated by ( P) after the title, are reported in the manner listed below. It is required that all entries up to and including theoretical yield be made on the RIGHT-HAND page BEFORE entering the laboratory. I. II. III. IV. V. VI. VII.

Date and Title of Experiment Balanced Equation for the Main Reaction Table of Reactants and Products Theoretical Yield Experimental Procedure Weight and Percent Yield Answers to Questions

The following is an illustration of part of a proper laboratory report for a preparative experiment. I. Preparation of Nitrobenzene

October 3

III.

II. Main Reaction

Fommla

Molecular Weight

Ml.

Density gm/ml.

Gms.

Moles

C6H6 H 2SO 4

78.0 98.0

5 15

0.88 1.84

4.4 27.6

0.056 0.282

63.0 123.1

20

1.42 1.19

28.4

0.435

HNO 3 C6H 5NO2

IV. Theoretical Yield 1 mole C6H 6 = 1 mole C6H 5 NO2

0.056 mole C6 H 6

123.1 gm. C6 H 5 NO2

= 0.056 mole C6H 5NO2 X - - - - - - - - = 6.8 gm. mole

V. Experimental Procedure ( This account should be written in the past tense, not as written in the text, and must be a true record of what was actually done in the laboratory. Compare the following account with the method given in Chapter 7.) In a 250 ml. Erlenmeyer flask was carefully mixed 15 ml. of concentrated sulphuric and 20 ml. of concentrated nitric acid. To this mixture was added drop by drop with constant shaking 5 ml. of benzene. The flask was cooled occasionally under the tap to keep the reaction temperature slightly above that of the room. After the benzene was added, the solution was poured with vigorous stirring into 200 ml. of water. The lower oil phase was separated to give 4. 7 ml. of product.

Yield

4.7 ml. X 1.19 g/ml. = 5.6 g. C 6H 5NO 2 5.6 g. %Yield - - x 100% 82% 6.8g.

=

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5

After the preparation of the product, the material is turned-in at the storeroom in a labelled container. The label contains the following information neatly printed: (I) ( 2) ( 3) ( 4) ( 5)

Name of the substance Melting or boiling point of the substance Weight and percentage yield Student's name Locker number

LABORATORY SAFETY A chemistry laboratory is potentially an extremely dangerous place. There is danger of cuts from breaking glassware, danger of fires from flammable chemicals and danger from contact with or swallowing of hazardous and poisonous chemicals. Laboratory safety depends primarily upon the student. The student should keep his desk top clean and dry and free from extraneous equipment. The student should plan very carefully exactly what he intends to do in the laboratory, prior to beginning work. It is of prime importance that students work so that they are not rushed, are well prepared, alert, workmanlike and considerate of others. Wearing a laboratory coat or apron and goggles will minimize the severity of any accident. The principal sources of accidents in the organic laboratory are ( 1 ) improper handling of glassware, ( 2) flammable solvents and ( 3) corrosive chemicals. GLASSWARE:

To avoid cuts, glassware should always be handled gently.

BE EXTREMELY CAREFUL WHEN INSERTING GLASS TUBING INTO A RUBBER OR

Do not use any part of the anatomy as a backstop for the tubing because it might break. ( When pushing a cork by a corkscrew motion hold the glass tubing very close to the stopper.) If possible, lubricate the glass tubing with water, glycerol or ethanol before inserting into the stopper. This makes the stopper slip on easier. no NOT attempt to force glass tubing into a stopper. CORK STOPPER.

CHEMICALS

Flammable solvents must never be heated over a flame unless the solvent is in a flask under reflux or attached to a condenser for distillation. Use either a steam bath ( if the boiling point is not above 50° C.) or a hot plate. All solvents except water should be considered flammable and toxic. Do not place volatile solvents in beakers, even temporarily. When mixing, pouring, or measuring chemicals keep them away from directly in front of your face. Be especially alert against violation of this point. It is instinctive to hold a vessel directly in front of the face when pouring into it. Taste nothing in the laboratory unless specifically directed to do so. Materials that give off noxious fumes must be handled in the fume hood.

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7

IN CASE

OF

ACCIDENT

Minor Cuts: These are usually treated by washing thoroughly, applying an antiseptic and bandaging. First Aid equipment is available in the storeroom. Ma;or cuts: Apply a pressure bandage or a tourniquet and go immediately to the University Health Service. Fire: The student should remove himself from the vicinity of the fire. After it is established that all persons are safe, attention should be given to extinguishing the fire. Since the laboratory has adequate fire-fighting equipment, the major concern is for the safety of persons in the vicinity of the fire. Small fires in beakers are extinguished by exclusion of air with a wet towel or notebook. If a person's clothes are on fire, it is important not to run but lie on the floor and smother the flames. In such a case, a person usually needs help from his neighbour, or instructor to instantly smother the flames with a coat, fire blanket ( available in the corridor), towel or anything available. Burns: Minor skin burns are usually treated with special ointment available in the storeroom. DO NOT wash with water. For serious burns go immediately to the Health Service. Corrosive Reagents ( 1 ) EYES: Flood the eyes immediately with the solution in the dispenser provided. Be sure that the inside of the eyes are washed thoroughly. Force the eyes open if necessary to insure irrigation with plenty of water. Continue this treatment for several minutes, then go to the Health Service. (2) SKIN: Corrosive chemicals are best removed by thorough scrubbing with soap and water followed by a gentle rubbing with glycerine. Do not use bum ointments or alcohol since this helps the reagents penetrate the skin.

2. LABORATORY TECHNIQUES The manner in which a person handles chemicals and equipment in the laboratory is termed laboratory technique. A student entering the organic laboratory for the first time is presented with a wide variety of equipment which is to be assembled and used during the year. In this and subsequent chapters, detailed instructions will be given in the proper use of reagents and equipment. These instructions must be carefully followed. A. DISPENSING CHEMICALS There are certain general rules that must be observed particularly in obtaining the required amounts of solid and liquid chemicals.

7

IN CASE

OF

ACCIDENT

Minor Cuts: These are usually treated by washing thoroughly, applying an antiseptic and bandaging. First Aid equipment is available in the storeroom. Ma;or cuts: Apply a pressure bandage or a tourniquet and go immediately to the University Health Service. Fire: The student should remove himself from the vicinity of the fire. After it is established that all persons are safe, attention should be given to extinguishing the fire. Since the laboratory has adequate fire-fighting equipment, the major concern is for the safety of persons in the vicinity of the fire. Small fires in beakers are extinguished by exclusion of air with a wet towel or notebook. If a person's clothes are on fire, it is important not to run but lie on the floor and smother the flames. In such a case, a person usually needs help from his neighbour, or instructor to instantly smother the flames with a coat, fire blanket ( available in the corridor), towel or anything available. Burns: Minor skin burns are usually treated with special ointment available in the storeroom. DO NOT wash with water. For serious burns go immediately to the Health Service. Corrosive Reagents ( 1 ) EYES: Flood the eyes immediately with the solution in the dispenser provided. Be sure that the inside of the eyes are washed thoroughly. Force the eyes open if necessary to insure irrigation with plenty of water. Continue this treatment for several minutes, then go to the Health Service. (2) SKIN: Corrosive chemicals are best removed by thorough scrubbing with soap and water followed by a gentle rubbing with glycerine. Do not use bum ointments or alcohol since this helps the reagents penetrate the skin.

2. LABORATORY TECHNIQUES The manner in which a person handles chemicals and equipment in the laboratory is termed laboratory technique. A student entering the organic laboratory for the first time is presented with a wide variety of equipment which is to be assembled and used during the year. In this and subsequent chapters, detailed instructions will be given in the proper use of reagents and equipment. These instructions must be carefully followed. A. DISPENSING CHEMICALS There are certain general rules that must be observed particularly in obtaining the required amounts of solid and liquid chemicals.

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9

1.

SOLIDS

The analytical balances are not used in this laboratory. Weigh out on the triple-beam balance provided the required amount of material. Use a spatula for transferring. Place a piece of ordinary paper ( not filter paper) over the balance pan to prevent corrosion and to make subsequent transfer more convenient. Return unused material to the proper bottle, clean up any spilled material and leave the balance clean and empty for the next student. Do not use more than the required amount of material. 2.

LIQUIDS

Liquids are not weighed but poured out into graduates. Pour sufficient material into a test tube ( one inch equals approximately 5 ml.) and then remove from it the quantities needed. Pour unused chemicals down the sink. Clean up immediately all spills especially of acids and corrosive chemicals. 3.

CHEMICALS

All the chemicals needed are arranged in alphabetical order under either ( 1) Organic Chemicals OR ( 2) Inorganic Chemicals. Corrosive Chemicals are found in the FUME HOOD. If you cannot find a chemical in any of these three places look on the balance shell. Never take bottles of chemicals to your desk If a bottle becomes empty, ask a demonstrator to have it filled. Replace bottles in their proper alphabetical order! B. MELTING POINT

The melting point of a substance is defined as the temperature at which liquid and solid can exist in equilibrium with each other. For compounds which are solids at room temperature, the melting point is a most important characteristic. It is not only a means of identification of the substance but also a criterion of its purity. A sharp melting point ( within 1 degree limits) is regarded as indicative of a pure substance while a melting range of several degrees is caused by the presence of impurities. The reason for the constancy and sharpness of the melting point of a pure crystalline substance and the effect of impurities can be shown by means of a generalized equilibrium temperature-composition diagram for a two-component system. Such a diagram is illustrated in Figure 1. The melting point of pure component A is 70° C. and that of pure component B is 94 ° C. The lines XY, yz and VW in Fig. 1 indicate phase boundaries. Thus line XY separates the liquid phase from the solid A plus liquid phase. The lines yz and XY intersect at point Y. This point is known as the eutectic point. The mixture of A and B defined by this point acts like a pure compound, and melts at a single temperature. However this temperature is lower than the melting point of either pure A or B. From Figure 1 it is clearly seen

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11 100

100

z

u

•

80

l&I

X

Ict

60

0:: ::::, 0::

80

d

l&I 0.

SOLID B

+

:=;;

LIQUID

l&I

I-

SOLID A

40

Thermometer

60

+

40

LIQUID

Water

.c,a

V -r------~---------+W

y

20 - + - - - - - - - - - + - - - - - - - - - - 1 - 2 0 100%A

50%

0%

0°1.

100%B

COMPOSITION

FIG.

1

FIG.

2

that the melting point of the pure compounds are lowered by the addition of the other component. The addition of an impurity not only lowers the melting point of a pure substance but also the melting point range is greater. Consider the changes that will be observed as a 50:50 mixture of compounds A and B is slowly heated. At point a ( 30° C.), the sample will begin to melt. At point b ( 36° C.) solid will still be visible in the melt. At point c ( 43° C.) the last of the solid will disappear. At any point above c, such as d, the sample will be completely liquid. Thus the melting range of a 50:50 mixture of compounds A and B will be 30° to 43° C. In order to measure any temperature such as a melting point or boiling point, it is necessary to have an indication of the accuracy of the thermometer. Laboratory thermometers vary widely in their accuracy. Therefore it is worthwhile to calibrate your own thermometer. The thermometer is calibrated by comparing its readings at a series of known temperatures. The known temperatures are conveniently provided by the melting or boiling points of pure substances. In experiment 1, an ice-water mixture and the boiling point of water will be used to calibrate a thermometer.

Experimental 1.

CALIBRATION OF A THERMOMETER

By means of a support stand and clamp suspend a thermometer fitted through a cork above a 100 ml. beaker of water. Support the beaker on a wire gauze so that the bulb of the thermometer is suspended approximately

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13

1-2 mm. above the surface of the water. Add a boiling chip ( one piece) to prevent bumping. When the thermometer bulb is bathed by the vapor tabulate the thermometer readings at half-minute intervals for three minutes. Consult the blackboard for the prevailing atmospheric pressure and the relationship between pressure and the boiling point of water. Repeat the experiment using a mixture of ice and water in the beaker instead of boiling water. Insert the thermometer into the mixture so that the zero point is just above the surface. What corrections must be applied to your thermometer to make its reading correspond to the correct temperature? Prepare a graph of observed temperature as the ordinate and correct temperature as the abscissa. Place this graph in your notebook. 2.

MELTING POINT

Prepare some melting point tubes by heating to redness the centre portion of a soft glass test tube and slowly rotating it until it sags. Then remove it from the flame and immediately draw it out until it has an inside diameter of 1-2 mm. Cut this tube into lengths of about 12 cm. and seal each end. Store these in a clean, corked test tube. When required for use, scratch with a file at the centre and sever to form two melting point tubes. Pulverize a small amount of the material whose melting point is to be determined. Introduce sufficient to occupy a length of about 0.5 cm. into the melting point tube. This is accomplished by dipping the open end of the tube into the pulverized substance, then inverting it and causing the adhering particles to fall to the bottom of the tube by drawing a file slowly and gently back and forth across it. Attach the melting point tube by a small rubber band to the lower end of the thermometer or by making use of the principle of capillary action, i.e. wetting the thermometer with water, and bringing the tube in contact with it. The tube will adhere to the thermometer. Place the thermometer as illustrated in Figure 2 in a 100 ml. beaker, half full of water, and gradually heat, at the same time stirring the water. Note the temperature at which the substance liquefies. After finding the melting point approximately on the first trial repeat the experiment, heating the water very slowly as you approach the melting point found by the first trial. Record the "sintering" ( softening) and the final melting points. For solids that have a melting point above 100° C., it is necessary to use an oil bath rather than a water bath. An oil bath can be conveniently prepared by filling a small beaker approximately half-full with solid Crisco or a similar solid shortening. This material melts at about 40° C. As soon as it has completely melted insert the thermometer with the melting point tube. Continue with the melting point determination as previously described. 3.

MELTING

PoINT

OF MIXTURES

Pulverize and mix well approximately 0.5 g. of naphthalene and 0.2 g. p-dichlorobenzene ( 0.5 g. is approximately the quantity which can be

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15

heaped upon a ten-cent coin). Transfer some of the mixture to a melting point tube, place the tube in the bath and heat the bath slowly. Record the temperature at which the mixture first begins to soften and at which it has completely liquefied.

C. SOLUBILITY AND CRYSTALLIZATION The solubility of organic compounds in various solvents is determined to a large extent by the functional groups in the compound. Thus members of the same class of compounds usually exhibit similar solubility properties. One generality concerning solubility that has developed through the years is that "like dissolves like." Thus polar solvents, like water, will dissolve polar molecules while non-polar solvents ( like carbon tetrachloride) will dissolve non-polar molecules. Non-polar molecules will be insoluble in polar solvents and vice versa. Between these extremes lies a vast variety of molecules whose solubility can roughly be related to their polarity. In most cases, the upper limit of water-solubility is found with compounds containing four or five carbon atoms. Often a compound will dissolve in a solvent because it reacts with the solvent. Thus acids are soluble in 5% sodium hydroxide solution and bases, particularly the amines, are usually soluble in 5% hydrochloric acid. Compounds with oxygen, nitrogen or with double bonds, are usually soluble in concentrated sulfuric acid. This group includes olefins, alcohols, aldehydes, ketones, esters, amides and ethers. Compounds insoluble in concentrated sulfuric acid are inert and include aliphatic and aromatic hydrocarbons and halides. Thus the solubility or lack of solubility of an organic compound is extremely useful in ascertaining the identity of that compound. The solubility behavior of a solid compound is also important in its purification by recrystallization. Recrystallization is based on the premise that one can find a solvent in which, at elevated temperatures, the material to be purified is quite soluble but in which most of the impurities are practically insoluble or else much more soluble. Hence filtration of a hot solution would leave most of the impurities behind on the filter paper. When the hot saturated solution of the material is cooled, it becomes supersaturated and then crystallizes yielding pure material, with the other impurities remaining behind in solution. In order to obtain a maximum recovery, it is of advantage to cool the mixture to a relatively low temperature, since the solubility of most substances decreases with decreasing temperature. For a successful crystallization, it is also obviously necessary that the material being recrystallized be of rather low solubility in the solvent at these lower temperatures. It is hoped that any soluble impurities present will be much more soluble at the lower temperature so that the solution will not become saturated with respect to them also. It has been found by experience that it is frequently possible to remove most coloured

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17

impurities by treating the hot solution with a little decolourizing charcoal ( e.g. Darco, Norit). Since recrystallization is a very common laboratory procedure that will be used many times throughout the year, the techniques will be described in detail in the Experimental section.

Experimental 4.

SOLUBILITY OF ORGANIC COMPOUNDS

Complete in your notebook the blanks in the following solubility table, for typical members of the various classes of organic compounds. To save time some solubilities are indicated but you should be certain that you understand why a compound is soluble or insoluble. To test, consider a compound is soluble if 0.1 g. of solid or 0.2 ml. of liquid is soluble in 3 ml. of reagent, after vigorous shaking and mild warming if necessary. Some cases may be borderline. In the case of cone. sulphuric acid, note also if heat is evolved or colour is produced. Write structural formulas for all organic compounds. Solubility in Compound

10%

!',%

Water Ether NaOH NaHCO 3

Dil. Cone. HCl H 2 SO 4

Density Colour Relative or to Heat water

Hexane Cyclohexene

i i

Toluene

s

Cyclohexanol

s

n-Butyl bromide

s

Benzaldehyde

s

Butyric acid

s

p. Cresol

s

Dimethylaniline

s

s

Anthranilic acid

s

s

i

s

Sodium acetate

5.

CRYSTALLIZATION OF BENZOIC

Aero ( P)

Place 5.0 g. of crude benzoic acid in a 500 ml. Erlenmeyer fl.ask. Add 200 ml. of distilled water and a boiling chip. Heat the fl.ask either with a Bunsen burner ( since the solvent is non-flammable a Bunsen burner is satisfactory) or on a hot plate until the water boils. If all the benzoic acid

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19 does not dissolve continue adding small portions ( 5 to 10 ml.) of distilled water until all of the material has dissolved. The nearly saturated solution should be gently boiling at this point. Cool the solution slightly by removing the heat source for a minute or two, then cautiously add a heaping spatula­ tip of decolourizing charcoal. Reheat the solution and allow to boil for 20 minutes. In order to avoid crystallization in the funnel, add an additional 20 ml. of water. While the solution is gently boiling, fold a filter paper and fit it into a short stem funnel. Place the funnel in a 500 ml. Erlenmeyer flask and carefully pour the hot solution into it. Take care that no material passes over the edge of the filter paper. When all the material has filtered through, wash the filter paper with two to three ml. of boiling water to remove traces of benzoic acid crystals that may have appeared. Allow the flask to cool to room temperature and then briefly cool in an ice bath. Collect the crystals by suction through a Buchner funnel ( fitted with a filter paper which lies flat on the pedorated bottom), suck dry and spread on a clean watch glass to air dry. Weigh and determine the melting point of the material. Hand in the product. D. BOILING POINT AND DISTILLATION It has been shown experimentally that at a given temperature the pressure of the vapour of a pure substance in equilibrium with its own liquid is a constant and is independent of the amount of liquid and vapour present in the system. As the temperature of the liquid is increased the vapour pressure also increases, until the vapour pressure equals the pressure of the surround­ ings. At this temperature boiling will occur. For water at atmospheric pressure ( 760 mm.) the vapour pressure will reach 760 mm. at 100 ° C. and water will boil. The normal boiling point is a characteristic constant that is used in the identification of liquids. Since boiling points have a rather erratic behaviour to impurities, they are less useful as a criterion of identification and purity than are the melting points of solids. A pure liquid at constant pressure will boil at a constant temperature until it has all evaporated. A solution of two miscible liquids however presents a different situation. The vapour above this solution contains molecules of both liquids. The composition of the vapour will depend upon the concentra­ tion of each component and the individual vapour pressures. This can be illustrated by means of a temperature-composition diagram for an ideal solution as shown in Figure 3. Line XWZ represents the boiling point of a solution of A and B as a function of composition. The boiling point of a 50:50 mixture of A and B is given by the intersection of the vertical line a with the lower curve ( 84 ° C.) The vapour composition in equilibrium with this solution is obtained from the intersection of the horizontal line b with line XYZ, the vapour composition curve. This point Y 1 indicates that the composition of the vapour ( 80%B, 20%A) is considerably richer in B than the original solution. Now if the liquid of composition Y1, is condensed, a distillation has been pedormed.

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21 140

140

u

0

w

a,:

::>

Ic:(

a,:

w

100

n.

::::E Ill

I-

80

60-+--------~----------t-60 0% 100%A 50% 100%B 0% COMPOSITION

FIG.3

Distillation, therefore, is the process of converting a liquid to vapour by boiling it, removing the vapour and condensing it again to a liquid, called the distillate. The separation of A and B can be achieved in principle by collecting and redistilling the distillate numerous times. This is usually a very time consuming and impractical method. Instead a fractionating column is used. A fractionating column is simply a vertical column containing a material called "packing," which possesses a large surface area. The vapour can condense on this surface and start to flow back into the distilling flask. Before returning to the flask, however, it comes in contact with rising hot vapour from the flask which vaporizes some of the condensate. This amounts to a second distillation. This process occurs over and over again in the column with the result that the more volatile component gradually makes its way to the top of the column. The actual separation obtained depends upon many variables including the efficiency of the column and the difference in boiling points of the components. Sometimes it is not possible to completely separate the components of a solution no matter how efficient a column is used. This is due to the fact that a solution of two liquids does not always act like an ideal solution. The temperature composition diagrams for two such non-ideal solutions are shown in Figures 4 and 5. Figure 4 represents a system that forms an azeotropic solution of minimum boiling point while Figure 5 represents a system that forms an azeotropic solution of maximum boiling point. In these systems it is impossible to separate a solution into its two pure components. This can be illustrated by means of the diagram in Figure 4.

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23 0

100

100

..,a:

80

80

cc a:

60

60

u

...

::,

.., :E .., Q.

...

M

40

40

20

20 l50o/o

IOOo/oY

0%

0% 100%X

COMPOSITION

FIG.

u

4

110

110

90

90

70

70

50

50

30

30

•

.., a:

... ::, 2° > 1° regardless of mechanism. It is clear that one compound in the presence of base can undergo two different reactions. Thus 1-bromobutane will react with an aqueous sodium hydroxide solution to produce both 1-butanol and !-butene. In general it is difficult to suppress completely one of the two reactions. The factors that enhance nucleophilic substitution reactions also enhance elimination reactions resulting in a mixture of products. Experiments number 19, 20 and 21 are designed to illustrate the concept of relative rates of nucleophilic substitution reactions. A series of alkyl halides will be subjected to various reaction conditions. While the results are only qualitative, they serve to illustrate how reaction conditions and the structure of the alkyl group affect the rate of nucleophilic substitution reactions.

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47 Experimental 19. REACTION OF HALIDES wrrn SILVER NITRATE To 2 ml. of 5% alcoholic silver nitrate solution in six separate test tubes add 5 drops of chloroform, chlorobenzene, normal butyl chloride, secondary butyl chloride, tertiary butyl chloride and benzyl chloride respectively. Shake the tubes well at intervals of 5 minutes for 15 minutes. To another 1 ml. of each of these halides add 2 ml. of reagent sodium hydroxide and boil for 5 minutes. Cool the solution and acidify to litmus with nitric acid. Add to each 2 ml. of alcoholic silver nitrate. Explain the results obtained. 20. RATE OF HYDROLYSIS OF HALIDES

To 5 ml. of water in each of five test tubes add 2 drops of bromocresolgreen indicator solution. To these tubes add 4 drops of normal butyl chloride, secondary butyl chloride, chlorobenzene, tertiary butyl chloride and benzyl chloride respectively. Shake all the tubes and immediately place them in a water bath at 60° C. Note the colour before adding the chloride and at half-minute intervals thereafter for 5 minutes. Bring to a boil and heat for a further 5 minutes. Bromocresol-green is yellow at pH 3.8 and blue at pH 5.4. Draw conclusions regarding the relative rates of hydrolysis. Give a reasonable explanation of the results obtained. Write equations. Describe a reaction of alkyl halides in which this order of reactivity found in this experiment is reversed. Give alternative names for the reagents and products resulting from the above reactions using the I.U.P.A.C. system. Name the alcohols as derivatives of carbinol (methanol). 21. REACTION OF ALKYL HALIDES WITH SODIUM loDIDE

IN

ACETONE

To 2 ml. of sodium iodide in acetone in five separate test tubes add 2 drops of normal butyl chloride, secondary butyl chloride, tertiary butyl chloride, chlorobenzene and benzyl chloride. Note the time required to form a precipitate and whether or not iodine is liberated. If no reaction occurs within 3 minutes place the test tubes in a water bath at 60° C. for 6 minutes. Cool to room temperate and again note whether reaction has occurred. Explain the results obtained. 22. PREPARATION OF N-BUTYL BROMIDE (P) Set up the apparatus for heating under reflux as illustrated in Figure 7 using a 250 ml. Erlenmeyer flask as the reaction vessel. Partly fill the beaker with water to trap escaping hydrogen bromide keeping the funnel leading to the flask a few mm. above the liquid surface. Place in the flask 15 ml. of water and 21.7 g. of sodium bromide dihydrate (NaBr.2H2O) or its equivalent of anhydrous sodium bromide. Shake the flask while adding the water

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49

so that a hard cake of salt does not form. Add 12 ml. of n-butyl alcohol and re-attach the condenser. Through the top of the condenser add 12 ml. of concentrated sulphuric acid in about five portions, shaking the flask after each addition. Add a boiling chip and heat under gentle reflux for two hours. It is desirable but not essential that this procedure be completed in one period.

jftCond enser Inverted funnel

FIG.

7

Cool the flask slightly and arrange for downward distillation by connecting the neck of the flask to the condenser with a piece of 8--10 mm. glass tubing bent to about 70°. Rapidly distill the material until no more water insoluble material comes over. The crude distillate is freed from impurities in the following manner. Transfer to a 125 ml. separatory funnel and wash once with an equal volume of water. If the solution is coloured from a slight trace of bromine this can be removed by adding a small amount of sodium sulphite to the wash water. Separate the product and wash once with an equal volume of cold concentrated sulphuric acid. This reagent removes all the organic impurities which are likely to be present. Separate the product and wash carefully with water to remove all traces of adhering acid. Separate the layers and wash the product with an equal volume of 10% aqueous sodium bicarbonate solution ( CAUTION-Carbon dioxide may be liberated). After a final washing with water separate the layers and dry the n-butyl bromide over 5.0 g. of anhydrous calcium chloride. When dry ( after standing with occasional shaking for one half hour) filter into a 50 ml. distilling flask equipped for downward distillation and collect the fraction boiling at 99-103°. Weigh and hand in all but 2.0 g. of the product.

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51

23.

PREPARATION OF I-BUTENE

Set up the apparatus for heating under reflux as illustrated in Figure 7 with the following modifications. Replace the beaker containing water with a 125 ml. Erlenmeyer flask containing 75 ml. of carbon tetrachloride. Replace the inverted funnel with a piece of glass tubing and fix the tubing so that it extends below the surface of the liquid. Make sure that all corks fit tightly. Use a 125 ml. Erlenmeyer flask as the reaction vessel. In the 125 ml. Erlenmeyer flask used as the reaction vessel place 15 g. of potassium hydroxide, 50 ml. of 95% ethyl alcohol and 2.0 g. of n-butyl bromide. Heat the reaction mixture to boiling. Continue heating under reflux until gas evolution cases. Remove the flask containing the carbon tetrachloride before allowing the reaction mixture to cool. Add dropwise to the carbon tetrachloride a solution of bromine in carbon tetrachloride. Explain the observations.

6. ALCOHOLS Saturated alcohols are organic compounds that have the general formula CnH2n+20. The structure of an alcohol can be considered as being derived from a water molecule by the substitution of one of the two hydrogens by an alkyl group. The OH group that remains accounts for the chemical and physical properties of the alcohols. Thus alcohols react with sodium metal in a manner analogous to water. The oxygen atom of the OH group has unshared electrons and as a result can accept a proton in a manner similar to water (eq. I). (1)

However in the case of alcohols further reaction can occur. The protonated alcohol can lose a molecule of water to form a carbonium ion ( eq. 2). This carbonium ion can then react with an anion, (2) +

(CH3)2CH

+

-

Cl

--

(CH3)2CHCI

(3)

such as chloride ion, to form an alkyl chloride ( eq. 3). This reaction serves as the basis for the Lucas Test which is used to distinguish between primary, secondary and tertiary water-soluble alcohols. Tertiary alcohols upon protonation form a carbonium ion more readily than either secondary or primary alcohols. Thus a tertiary alcohol reacts faster with the Lucas

51

23.

PREPARATION OF I-BUTENE

Set up the apparatus for heating under reflux as illustrated in Figure 7 with the following modifications. Replace the beaker containing water with a 125 ml. Erlenmeyer flask containing 75 ml. of carbon tetrachloride. Replace the inverted funnel with a piece of glass tubing and fix the tubing so that it extends below the surface of the liquid. Make sure that all corks fit tightly. Use a 125 ml. Erlenmeyer flask as the reaction vessel. In the 125 ml. Erlenmeyer flask used as the reaction vessel place 15 g. of potassium hydroxide, 50 ml. of 95% ethyl alcohol and 2.0 g. of n-butyl bromide. Heat the reaction mixture to boiling. Continue heating under reflux until gas evolution cases. Remove the flask containing the carbon tetrachloride before allowing the reaction mixture to cool. Add dropwise to the carbon tetrachloride a solution of bromine in carbon tetrachloride. Explain the observations.

6. ALCOHOLS Saturated alcohols are organic compounds that have the general formula CnH2n+20. The structure of an alcohol can be considered as being derived from a water molecule by the substitution of one of the two hydrogens by an alkyl group. The OH group that remains accounts for the chemical and physical properties of the alcohols. Thus alcohols react with sodium metal in a manner analogous to water. The oxygen atom of the OH group has unshared electrons and as a result can accept a proton in a manner similar to water (eq. I). (1)

However in the case of alcohols further reaction can occur. The protonated alcohol can lose a molecule of water to form a carbonium ion ( eq. 2). This carbonium ion can then react with an anion, (2) +

(CH3)2CH

+

-

Cl

--

(CH3)2CHCI

(3)

such as chloride ion, to form an alkyl chloride ( eq. 3). This reaction serves as the basis for the Lucas Test which is used to distinguish between primary, secondary and tertiary water-soluble alcohols. Tertiary alcohols upon protonation form a carbonium ion more readily than either secondary or primary alcohols. Thus a tertiary alcohol reacts faster with the Lucas

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53

Reagent to form an insoluble alkyl chloride than either a secondary or primary alcohol. Carbonium ions can undergo two additional reactions. One is the loss of a proton to form an olefin ( eq. 4) (4)

while the other is a rearrangement ( eq. 5). (5)

This latter reaction will be discussed more fully in Chapter 12. Alcohols, of course, undergo certain reactions that do not occur with water. One of these is oxidation. A primary alcohol can be oxidized to either an aldehyde or a carboxylic acid depending upon the reaction conditions (eq. 6). Secondary alcohols can be oxidized to ketones (eq. 7) while tertiary alcohols are resistant to normal oxidization reaction conditions ( eq. 8).

(CH 3 )3COH

--+

OXIDIZES WITH DIFFICULTY

(8)

The experiments in this section are designed to illustrate the preparation and reactions of alcohols. Experimental 24.

PREPARATION OF ETHYL ALCOHOL BY FERMENTATION OF GLUCOSE

Take 50 ml. of 20% glucose solution ( diluted corn syrup), and dilute to 100 ml. with water. Make a paste by triturating 0.5 g. of yeast with 10 ml. of water. Pour the glucose solution, together with 5 ml. of tomato juice and 10 ml. of Pasteur salts solution (potassium or calcium phosphates, magnesium sulphate and ammonium tartrate) plus the yeast suspension into a 250 ml. Erlenmeyer flask provided with an absorbent cotton plug. Allow the apparatus to stand several days at about 37° C. Transfer about 50 ml. of the brew to a small distilling flask and distill off about 10 ml. of this liquid and test for ethyl alcohol by the iodoform test. Write the empirical equation for the formation of ethyl alcohol from glucose. How could you determine if carbon dioxide were evolved in the process? Enumerate the principal steps involved in the Meyerhoff Scheme for alcoholic fermentation.

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55 25. ACTION OF ACTIVE METALS ON ALCOHOLS

( i) To 5 ml. of methanol add one freshly-cut small piece of sodium or lithium. What reaction takes place? Equation? ( DESIBOY ALL UNCHANGED ACTIVE METAL BY ADDING MORE METHANOL TO IT!) Dilute the liquid with an equal volume of water and test with litmus. Equation? (ii) Dissolve 10 drops cyclohexanol in 5 ml. benzene and then add one small piece of freshly cut sodium or potassium or lithium. Shake and observe from time to time over a period of about 5 minutes. Write the equation. Write the equation for the reaction between a divalent metal such as calcium and cyclohexanol. 26. HALOGENATION OF ALCOHOLS

Add 5 ml. of Lucas Reagent ( cone. HCl - ZnCl2) to 1 ml. of each of the following: normal butyl, secondary butyl and tertiary butyl alcohols. Shake well, set aside and observe carefully over a period of about fifteen minutes. The alkyl chlorides of these alcohols are insoluble in aqueous solutions and the progress of their formation can be noted by the opalescence imparted to the systems and the subsequent separation of the halide as a discrete phase. List the alcohols in order of their reactivity in this reaction and write the equations for the reactions. Discuss the mechanism of the reactions. Compare your results with those of Experiment 20 where the reverse process was considered.

27. RELATIVE OXIDATION RATES OF ALCOHOLS Place 10 drops of normal butyl, secondary butyl and tertiary butyl alcohols respectively in each of three test tubes. Add to each 5 ml. dilute sulphuric acid, and then 5 ml. of 0.1 N. potassium permanganate solution. Shake at frequent intervals. Observe the time required for the decolorization of the permanganate in each case and thence compare their relative oxidation rates. If unable under these conditions to differentiate between any particular pair, repeat the experiment with 0.01 N. potassium permanganate solution. 28. OXIDATION OF A PRIMARY ALCOHOL-PREPARATION OF ACETALDEHYDE

Set up a distillation apparatus consisting of a 250 ml. flask, bent delivery tube, cold water condenser and adapter. In the 250 ml. distilling flask, place 10 g. powdered potassium or sodium dichromate, and add a solution of 40 ml. water and 40 ml. dilute sulphuric acid. When the dichromate has all dissolved, CAUTIOUSLY add 25 ml. of 50% aqueous ethanol, about 5 ml. at a time, with careful shaking, and cooling in running water if necessary. [This reaction is exothermic, and care should

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57

be taken to prevent the dichromate from forming a cake at the bottom.] As soon as all the alcohol has been added, connect the flask by means of the bent delivery tube and tight fitting corks to the condenser. The adapter at the other end of the condenser should dip into the water in a 50 ml. Erlenmeyer flask half filled with ice-water, supported in a beaker of cold water. Distill. Acetaldehyde boils at 20° C., and on distilling over will dissolve in the water, giving an aqueous solution of acetaldehyde. After distillation has proceeded for about five minutes, disconnect and cork the receiver and allow the liquid in the distilling flask to cool. Observe the colour of the liquid in the distilling flask. What does this colour indicate? Write the balanced equation for the reaction. Keep the aqueous solution of acetaldehyde for future experiments. 29.

OXIDATION OF A SECONDARY ALCOHOL-PREPARATION OF ACETONE

Oxidize a 5 ml. portion of 2-propanol using the same procedure and quantities of reagents as outlined for the oxidation of a primary alcohol ( Exp. 28). ( Write the complete equation for the oxidation.) Keep the distillate and test for the presence of acetone by means of the iodoform test.

30.

PREPARATION OF ETHYL

3, 5-DINITROBENZOATE (P)

Reflux together for fifteen minutes 0.5 g. of 3, 5-dinitrobenzoyl chloride, 1 ml. of ethanol, and 3 ml. of pyridine. Pour the reaction mixture with vigorous stirring into 10 ml. of water. Allow the precipitate to settle and decant the supernatant liquid. Wash the residue thoroughly with 10 ml. of 5% NaHCOa solution. Collect the product by suction filtration on a Hirsch funnel. Recrystallize the product from 95% ethyl alcohol and determine its melting point and hand it in.

7. AROMATIC HYDROCARBONS AND THEIR DERIVATIVES Aromatic hydrocarbons are compounds containing one or more six-carbonatom rings called a benzene nucleus. Kekule, in order to account for the apparent unsaturation of benzene ( CsHa), proposed a ring of six carbon atoms joined by alternate single and double bonds. This structure is unsatisfactory in view of the fact that aromatic compounds undergo substitution reactions instead of the usual addition reactions which are characteristic of alkenes and alkynes. For example benzene does not decolorize a potassium permanganate solution or decolorize a solution of bromine in carbon tetrachloride. The reason for this unusual stability is that the bonding electrons are not localized between any particular carbon atoms but are delocalized throughout the whole ring system. This makes the ring less susceptible to electrophilic attack.

57

be taken to prevent the dichromate from forming a cake at the bottom.] As soon as all the alcohol has been added, connect the flask by means of the bent delivery tube and tight fitting corks to the condenser. The adapter at the other end of the condenser should dip into the water in a 50 ml. Erlenmeyer flask half filled with ice-water, supported in a beaker of cold water. Distill. Acetaldehyde boils at 20° C., and on distilling over will dissolve in the water, giving an aqueous solution of acetaldehyde. After distillation has proceeded for about five minutes, disconnect and cork the receiver and allow the liquid in the distilling flask to cool. Observe the colour of the liquid in the distilling flask. What does this colour indicate? Write the balanced equation for the reaction. Keep the aqueous solution of acetaldehyde for future experiments. 29.

OXIDATION OF A SECONDARY ALCOHOL-PREPARATION OF ACETONE

Oxidize a 5 ml. portion of 2-propanol using the same procedure and quantities of reagents as outlined for the oxidation of a primary alcohol ( Exp. 28). ( Write the complete equation for the oxidation.) Keep the distillate and test for the presence of acetone by means of the iodoform test.

30.

PREPARATION OF ETHYL

3, 5-DINITROBENZOATE (P)

Reflux together for fifteen minutes 0.5 g. of 3, 5-dinitrobenzoyl chloride, 1 ml. of ethanol, and 3 ml. of pyridine. Pour the reaction mixture with vigorous stirring into 10 ml. of water. Allow the precipitate to settle and decant the supernatant liquid. Wash the residue thoroughly with 10 ml. of 5% NaHCOa solution. Collect the product by suction filtration on a Hirsch funnel. Recrystallize the product from 95% ethyl alcohol and determine its melting point and hand it in.

7. AROMATIC HYDROCARBONS AND THEIR DERIVATIVES Aromatic hydrocarbons are compounds containing one or more six-carbonatom rings called a benzene nucleus. Kekule, in order to account for the apparent unsaturation of benzene ( CsHa), proposed a ring of six carbon atoms joined by alternate single and double bonds. This structure is unsatisfactory in view of the fact that aromatic compounds undergo substitution reactions instead of the usual addition reactions which are characteristic of alkenes and alkynes. For example benzene does not decolorize a potassium permanganate solution or decolorize a solution of bromine in carbon tetrachloride. The reason for this unusual stability is that the bonding electrons are not localized between any particular carbon atoms but are delocalized throughout the whole ring system. This makes the ring less susceptible to electrophilic attack.

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59

While the structure of benzene is not correctly represented by a sixcarbon-atom ring of alternating single and double bonds, this symbol is still used. It must be remembered that the actual structure of benzene is a hybrid of the two Kekule structures ( I and II). Structures I and II are

0-0

II

I

frequently called resonance structures and do not by themselves have physical reality or independent existence. The double-headed arrow between the resonance structures indicates that they represent different electron-pairing schemes and not different substances in equilibrium. The mechanism proposed for substitution ( e.g. nitration) in the benzene ring is as follows. The first stage is the production of an electrophilic ion (N02+) (eq. 1). This ion attacks the benzene ring forming a carbonium ion (III) ( eq. 2). The HS04- ion abstracts a proton from this carbonium ion forming sulphuric acid and the product nitrobenzene ( eq. 3). HON0 2

+

N0 2

a:02

+

+

+ 2H2 so 4 -

+

H3 0

+Q HS04

-

--

+

2HS04

+

+ N02

2 o.N0 H

ON02

+

(2) III

+

(1)

H2 so4

(2) (3)

Groups attached to a benzene ring such as methyl, nitro and hydroxy affect both the reactivity of the ring and determine the orientation of substitution ( the position the next entering group occupies). Groups which make the ring more reactive chemically are called activating groups ( OH, CHa), those that make the ring less active are called deactivating groups (N02, COOH). With regard to orientation there are two kinds of groupsortho-para directors ( OH, CHa) and meta directors ( N02, COOH). Reactivity and orientation are both matters of relative rates of reactions. Methyl groups in a benzene ring cause the substitution reaction to go faster and also cause the ortho and para position to substitute more readily than the meta position. In electrophilic aromatic substitution there are two steps; formation of the carbonium ion and deprotonation by loss of a proton to a base. The rate determining step is the formation of the carbonium ion. The methyl group in toluene releases electrons to the ring tending to neutralize the positive charge on the ring and becoming at the same time positive itself. This stabilizes the carbonium ion and so we have a faster reaction. The nitro group on the other hand withdraws electrons from the ring which intensifies the positive charge, destabilizes the carbonium ion. An electron-

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61

releasing group activates the ring, an electron-withdrawing group deactivates the ring. An activating group activates all the positions in the ring but it activates the ortho and para positions more than the meta position. Similarly a deactivating group deactivates all the positions but deactivates the ortho and the para positions more than the meta position. The most stable intermediate carbonium ions formed by the attack of an electrophile on toluene are shown in Equation ( 4).

(4)

These are more stable because only these forms can have a positive charge on C attached to CHs ( an electron donor). The least stable carbonium ions formed by the attack of an electrophile on nitrobenzene are shown in Equation ( 5).

(5)

These forms are the least stable since the positive charge is forced to reside on an already electron-deficient carbon. The forms with a substituents in the meta position are more stable and as a result meta substitution is favoured.

Experimental 31.

PREPARATION OF BENZENE

Mix intimately by grinding 10 g. of dry sodium benzoate and 10 g. of calcium hydroxide. Transfer the mixture to a large test tube ( 15 X 2.5 cm. ) clamped almost horizontally but sloping slightly up towards the mouth. Fit the tube with a tight cork and a bent all-glass delivery tube dipping into a test tube standing in a beaker of ice-water. ( See Figure 8.) Tap the tube to spread the powder along its side and heat the powder strongly running the flame up and down the tube as long as a distillate is forming. Write an equation for the reaction. Why is this reaction called decarboxylation? Test the distillate to show that it is benzene by determining its boiling point using the micro method ( Experiment 5b). Is it soluble in water? Does it bum? Does it have a distinctive odour?

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63

FIG. 8 32. PREPARATION oF Nrrno DERIVATIVES OF BENZENE (WARNING: Aromatic nitro derivatives are very poisonous and corrosive. Wash off immediately, any you get on your hands.) (a) Preparation of N itrobenzene (P)

Mix very carefully in a 250 ml. flask 15 ml. of concentrated sulphuric and 20 ml. of concentrated nitric acid (GOGGLES!). To this mixture add drop by drop, with constant shaking, 5 ml. of benzene. Do not let the contents of the flask get hot. Cool if necessary under the tap. When all the benzene has been added pour the solution with vigorous stirring into 200 ml. of water in a large beaker. The oily phase that sinks to the bottom is nitrobenzene. Decant the upper aqueous layer. To the remaining liquid add 2 g. of calcium chloride and allow to stand for a few minutes. Transfer the liquid to a separatory funnel and complete the separation. From the volume and density of nitrobenzene calculate the yield. Use the product in section ( b). ( b) Preparation of M etadinitrobenzene (P) ( TO BE CARRIED OUT IN THE FUME HOOD. )

Into a clean, dry 50 ml. Erlenmeyer place 5 ml. of concentrated sulphuric acid and then cautiously (GOGGLES!) add 1 ml. at a time 9 ml. of fuming nitric acid. Now add slowly to these mixed acids 3 ml. of nitrobenzene. Heat the flask for 10 minutes with frequent shaking in a beaker of boiling water. Carefully pour the mixture slowly with vigorous stirring into 200 ml. of ice water. Allow to cool and filter with suction through a Buchner funnel. ( N.B. The filter paper must be small enough to lie flat on the bottom of the funnel and just cover the holes.) Wash with water twice and then air dry the product. When dry, weigh it and calculate the yield. Determine the melting point of the product and hand it in in a well-stoppered properly labelled, small test tube. Discuss briefly the directive influence of substituents on the position taken by the next entering group.

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65

33. REDUCTION OF .AROMATIC NITRO COMPOUNDS Place in a test tube provided with a tight-fitting cork, 2 drops of nitrobenzene, 10 ml. of 5% alcoholic potassium hydroxide and 10 ml. of 5% ferrous ammonium sulphate. In another test tube, put only the potassium hydroxide and the ferrous ammonium sulphate and use this as a control. Shake both tubes vigorously for one minute. Explain any changes in colour observed. Equation. Write names and formulas for three reduction products of nitro benzene containing only one benzene ring each.

34. PREPARATION OF PARA-TERTIARY BUTYL PHENOL (P) ( A FRIEDEL AND CRAFTS REACTION)

In a 125 ml. flask place 6.5 ml. of tertiary butyl chloride and 4. 7 g. of phenol ( CAUTION: phenol is very caustic, do not get it on your skin, but if you do, wash it off immediately.) Add to the flask ( FUME Hoool) 0.5 g. of anhydrous aluminum chloride, a few granules at a time, so as to maintain evolution of hydrogen chloride without the reaction mixture becoming too hot. If necessary cool the flask by placing it in a beaker of ice water. Shake the mixture occasionally for 60 minutes. At the end of this time, the contents of the flask should be solid. Add 25 ml. of water containing 2 ml. of concentrated hydrochloric acid and break up the solid with a spatula. Filter the product on a Buchner funnel. Recrystallize the compound from petroleum ether ( KEEP FLAMES AwAY! ) . Determine its melting point. Calculate the yield. Hand in the product in a properly labelled test tube. Equation. Discuss briefly the mechanism for this reaction. Why does the hydrocarbon radical take the para position? What is meant by an activating group? 35. FORMATION OF AN .AROMATIC FREE RADICAL ( SCHMIDLIN' S EXPERIMENT) ( p)

In a small Erlenmeyer with tight-fitting cork, dissolve 1 g. triphenyl chloromethane ( triphenylmethyl chloride) in 25 ml. benzene. Note the colour. Then add 3 g. zinc dust. Cork tightly and shake vigorously for several minutes. Allow the solid phase to settle, and decant about 5 ml. of the supernatant liquid into a test tube. Dilute with 5 ml. benzene and divide into two equal portions. To one portion add 10 drops of 1% iodine solution in benzene. Explain all colour changes. Shake the second portion so that air mixes with the liquid for several minutes. Allow to stand for about five minutes, observing closely. Decant 20 ml. of the remaining supernatant liquid into a small Erlenmeyer, shake it with air until the colour of the free radical has been discharged to a light yellow. Filter off the crystals of peroxide. Dry these crystals between filter papers and hand in. Write equations.

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67

8. ALDEHYDES AND KETONES Aldehydes and ketones are two large classes of organic compounds, both containing the carbonyl group ( ) C=O) and so exhibit a great many similar chemical reactions. The difference between them is that aldehydes have an alkyl group and a hydrogen atom bonded to the carbonyl group whereas the ketones have two alkyl groups bonded to the carbonyl group. Aldehydes are prepared by the oxidation (dehydrogenation) of primary alcohols while ketones are prepared by the oxidation of secondary alcohols. ( Chapter 6. ) Both types of compounds produce carboxylic acids on further oxidation. The ready oxidation of aldehydes is the basis for the Schiff's and Fehling's Tests. Ketones are harder to oxidize because the breaking of a carbon-carbon bond is involved. The reduction of aldehydes produces primary alcohols and of ketones, secondary alcohols. Aldehydes and ketones undergo two major types of reactions; addition reactions to the carbonyl group and condensation reactions. The addition reactions occur with alcohols, sodium bisulphite, hydrazines, hydroxylamines, and semicarbazides. The mechanism used to explain these reactions is based on the fact that the carbonyl group is polar. The oxygen atom is more electronegative than carbon and tends to pull electrons towards itself. This makes the carbon atom of the carbonyl group more susceptible to nucleophilic attack ( eq. 1). R

' c=o +

R/

R

'

z

R

/

/c,

OH Z

t

OH

(1)

Ketones are less reactive than aldehydes because alkyl groups release electrons to the carbonyl carbon making it less vulnerable to nucleophilic attack. Also a second alkyl group exerts a steric hindrance to the incoming group. A specific example of a nucleophilic addition to a carbonyl group is the reaction of cyanide ion with acetaldehyde. The cyanide ion ( the nucleophile) attacks the carbon of the carbonyl group to form the ion (I) ( eq. 2). CH 3 CHO

+

CN-

-

0 CH 3 ~H CN

I

+

H2 o

-----+

OH

CH

37•H

+

OH-

(2)

CN

II

This ion then removes a proton from the solvent to give the product (II). The addition of ammonia derivatives ( such as hydroxylamine, hydrazine, and semicarbazide) and the addition of a Grignard reagent all form addition products by a similar mechanism. By the elimination of water, the initial addition products of the ammonia derivatives form compounds with

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69

a double bond between the C and N. These addition compounds are solids with sharp melting points and are useful in identifying aldehydes and ketones. The second major reaction that aldehydes and ketones undergo is a condensation reaction. The self condensation of acetaldehyde is an example of this type of reaction ( eq. 3). This reaction occurs because the carbonyl 2CH3CHO

+ NaOH ~

OH CH 3 CHCH2 CHO

(3)

group in aldehydes and ketones makes the hydrogens on the carbon atom alpha to the carbonyl groups susceptible to removal by base. This acidbase reaction ( eq. 4) generates the anion that makes a nucleophilic attack (4) CH3CHO +

oCH 2 CHO

CHiHCH 2 CHO

(5)

on the carbonyl group of another molecule of acetaldehyde (eq. 5). This new anion then removes a proton from the solvent to give the final product ( eq. 6). This is the general mechanism for all base catalyzed condensation reactions involving aldehydes and ketones containing hydrogens alpha to the carbonyl group. The experiments in this chapter are designed to illustrate the preparation, properties and reactions of aldehydes and ketones. Experimental

36.

TESTS FOR ALDEHYDES

(a) Schiff' s test Shiff's reagent is prepared by decolorizing a 1% aqueous solution of fuchsine (magenta) or rosaniline with sulphur dioxide. When properly prepared it should be water-white. It should be kept in an amber bottle and must be used in the cold, and in the absence of alkalies. Keep the dye solution off your hands. Avoid spills. To each of seven test tubes, add 5 ml. Schiff's reagent. To one test tube add 10 drops of your prepared acetaldehyde solution ( Exp. 28); to the second 10 drops of formalin; to the third 5 drops of acetone; to the fourth 5 drops of methyl ethyl ketone; to the fifth 10 drops of a 5% alcoholic solution of phenylacetaldehyde; to the sixth 3 drops of acetophenone; and keep the seventh as a control. Observe during the first five minutes, and again at the end of one hour.

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( b) Fehling' s test Fehling's solution is made by dissolving copper sulphate in water, designating this solution as "Fehling's Solution A", and then dissolving sodium hydroxide and Rochelle salt ( Sodium potassium tartrate), in water and designating this as "Fehling's Solution B." Equal quantities of "A" and "B" are mixed together as required. Make ready a beaker half full of boiling water. Into each of seven test tubes place 5 ml. combined Fehling's solutions ( A plus B). To each of the tubes add, respectively, the materials listed in (a) and keep the seventh as a control. Immerse these in the boiling water, and heat for about five minutes. Write the equations for the reaction of the aldehydes with Fehling's assuming that the active part of Fehling's solution is cupric hydroxide. Tabulate the results of both these Experiments. Both these tests can be used to distinguish between aldehydes and ketones. Aldehydes are much more readily oxidized than ketones and so aldehydes reduce the fuchsine to its red colour and the copper ion to its univalent state, Cu20, (red) whereas ketones do not. Would you expect ethanol and methyl ketone to give positive iodoform tests? ( Exp. 37.) 37.

HALOFORM REACTION-loDOFORM TEST

Most aldehydes and ketones whose formulas contain the acetyl group, or substances that may be readily oxidized to aldehydes or ketones that meet with the above requirement will give chloroform or bromoform or iodoform if they are warmed with hypochlorites, hypobromites or hypoiodites, respectively. Since iodoform is easily detected, its formation is the basis of a test for such structures. In four separate test tubes place respectively 5 ml. of 5% solution of acetone in water, 5 ml. of 1% solution of ethyl alcohol in water, 5 ml. of a 2% solution of isopropyl alcohol, and 5 ml. of 2% methyl ethyl ketone in water. To each add 3 ml. 10% sodium hydroxide solution, and then 5 ml. of 10% solution of iodine in potassium iodide. Note the odour and the colour of the iodoform precipitated. Filter off all the iodoform formed and allow it to dry in the air on the filter paper until the next laboratory period. Then carefully heat some in a test tube for a few seconds, and hold a moistened strip of starch-paper at the mouth of the test tube. Conclusions? Perform the iodoform test on 5% aqueous solutions of methanol, ethyl acetate, diethyl ketone and acetic acid and explain the results. Even when the iodoform does not precipitate, its odour is nevertheless sufficiently distinctive to serve as a test. 38.

PREPARATION OF ALDEHYDES BY CATALYTIC OXIDATION OF ALCOHOLS (DEHYDROGENATION)

In two test tubes, place respectively, 10 drops of methanol and 10 drops of ethanol. Warm the alcohols slightly so that the test tubes will be filled

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73

with their vapours. Prepare a coil of copper wire by winding the wire around a pencil about half a dozen times. ( Remove the pencil. ) Heat the coil to redness by holding it for a few seconds in the upper oxidizing part of the Bunsen flame, withdraw it, and note its appearance. To what is the appearance due? Then heat it again as before, and while still at red heat, plunge it into the tube filled with the alcohol vapour. Note carefully the appearance of the wire, especially the part which was near the top of the test tube, smell the contents of each test tube, add five ml. of Scruff's reagent ( Exp. 36a) to each tube and allow to stand for about five minutes. Conclusions? Write equations for the reaction between the copper and the alcohols. 39.

PREPARATION OF ACROLEIN FROM GLYCEROL

Place about 3 drops of glycerol ( 1, 2, 3.-propanetriol) and 1 g. anhydrous potassium hydrogen sulphate in an evaporating dish. Stir while heating cautiously; smell the gaseous products and hold a strip of filter paper, moistened with Schiff's reagent, close to the top of the dish. Discuss the results and write an equation for the reaction. What is the I.U.C. name for acrolein? Why is the acrolein test useful for detection of vegetable oils and fats? Acrolein is a lachrymator or tear gas. 40.

OXIDATION REACTIONS OF ALDEHYDES AND KETONES

Into each of five test tubes place 5 ml. 3% potassium permanganate and 5 ml dilute sulphuric acid. To the first test tube add 10 drops formalin, to the second 10 drops of your prepared acetaldehyde solution ( Exp. 28), to the third 5 drops acetone, to the fourth 5 drops cyclohexanone, and to the fifth, a few crystals of benzophenone. Any tubes which do not decolorize after five minutes at room temperature should be warmed in a beaker half full of boiling water. Arrange these five substances in order of ease of oxidation under these conditions and account for any differences. Write the equation for the oxidation of acetone and of acetaldehyde. Note that the aldehyde group is oxidized to a carboxy acid without any loss of carbon but that with ketones although acids are also formed they always have fewer carbons, than the original ketone. What products would result on a similar oxidation of hexanone-2 and of cyclohexanone? 41.

OXIDATION OF BENZALDEHYDE

At the first of the laboratory period, make a smear of a couple of drops of benzaldehyde on a watch glass. Place a piece of starch-iodide paper which has been moistened with water, in contact with the smear. Observe at the end of an hour and then at the end of the laboratory period. Explain what is observed. Write an equation for the reaction. Perbenzoic acid

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(benzoyl hydrogen peroxide) which is formed here is an active oxidizer. Write the formula for peracetic acid. 42. REDUCTION OF A KETONE TO A SECONDARY ALCOHOL-PREPARATION OF DIPHENYL CARBINOL ( p) In a 125 ml. Erlenmeyer flask with wide mouth place 3 g. benzophenone and 75 ml. 10% alcoholic potassium hydroxide solution. Add 12 g. zinc dust connect the flask to a reflux condenser and boil gently for 2 hours. Filter by decantation while hot into another 125 ml. Erlenmeyer flask, add 25 ml. water, stir, evaporate to about 15 ml. total volume cork and set aside until the next laboratory period. Filter off the crystals, wash with 10 ml. 50% water-alcohol, then with 50 ml. water, 10 ml. at a time. Recrystallize from 50% ethanol. Dry the product, determine the melting point and hand it in. 43. ADDITION REACTION OF KETONES WITH SODIUM HYDROGEN SULPHITE To 3 ml. of acetone add 4 ml. of a cold freshly prepared concentrated solution of sodium bisulphite and shake vigorously. Is heat evolved? What else is observed? Repeat the test using 1 ml. cyclohexanone instead of acetone. Write the equation for the reaction. How can this reaction be used in identifying aldehydes and ketones? Why is the reaction called an addition reaction? Suggest a mechanism by which this addition reaction can occur. 44. INCREASING THE CARBON CHAIN LENGTH BY THE CYANOHYDRIN REACTION-PREPARATION OF MANDELIC Aero (P) N.B. From the time that the POTASSIUM CYANIDE is added this preparation must be carried out in a FUME HOOD. Care must be taken not to inhale the fumes and the hands must be thoroughly washed with soap and water after this experiment and any time during it if any of the liquid gets on them. In a 100 ml. beaker place 10.0 g. of sodium bisulphite and dissolve it in 25 ml. of cold water. To this add slowly with stirring 6.0 ml. of benzaldehyde (density= 1.09 g./ml.). Stir the system vigorously for 5 minutes at room temperature and then for 5 minutes in an ice bath. What is the formula for the product at this stage? Filter and allow to drain for 5 minutes but do not attempt to clean the solid from the beaker completely. Transfer the solid from the filter paper back to the original beaker and wash it down with not more than 25 ml. of water. Stir this into a uniform paste and add, with stirring, 15.0 ml. of freshly prepared 30% potassium cyanide solution. Stir for 5 minutes and separate the oily product ( Formula?) from the bulk of the aqueous phase by transferring the system to a large test tube and stirring well until the oil has risen to the top. Pour off the oil into a large evaporating dish. Add 25 ml. of cone. hydrochloric acid

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77

to hydrolyse the nitrile ( cyanohydrin) and bring to a boil. Allow to boil gently and stir occasionally until the oil has disappeared and crystals are forming around the edge of the container. Adjust the total volume to approx. 30 ml. Pour the system into a 100 ml. beaker, chill in an ice bath, cover with a watch glass and set aside until the next laboratory period. Filter off the crude product, air-dry, calculate the yield and hand it in. Write the equations for all the reactions involved and name the products at the various stages. 45.

PREPARATION OF PHENYL HYDRAZONES

(a) To separate 5 ml. portions of freshly prepared alcoholic 2, 4-dinitrophenylhydrazine ( DNPH) reagent add respectively about 5 drops of ( i) benzaldehyde and (ii) acetaldehyde drop by drop. Shake and warm. Observe the crystals that form when cooled. ( b) In a test tube add 5 drops of acetophenone to 10 ml. of a 1% solution of 2, 4-dinitrophenylhydrazine (DNPH) in 95% ethanol. Wann in a beaker of boiling water and allow to evaporate to about half volume. Then allow to cool. What is the substance formed? Equation? 46.

PREPARATION OF THE OxIME OF VANILLIN

(P)

In a small Erlenmeyer flask place 2.0 g. hydroxylamine hydrochloride in 20 ml. methanol. Add 1.0 g. vanillin and then 4 ml. 10% sodium hydroxide solution plus 10 ml. water and warm on a water bath at 60° C. for about ½ hour. Transfer to an evaporating dish and allow to stand until the next laboratory period, about two-thirds covered by a watch glass so that the mother-liquor will be in contact with the solid product. Filter off the oxime, wash twice with 10 ml. of water, dry in the air, weigh and hand in. Does it possess the fragrance of vanillin?

47.

PREPARATION OF THE SEMICARBAZONE OF CINNAMIC ALDEHYDE (P)

In a small Erlenmeyer flask, dissolve 2.0 g. semicarbazide hydrochloride and 2.0 g. sodium acetate in 40 ml. water plus 25 ml. ethanol. Then with vigorous shaking and stirring add slowly 1 ml. cinnamic aldehyde ( density 1.1 g./ml.). Wann on a water bath for about 5 minutes with occasional gentle shaking, then place the Erlenmeyer in an ice bath for about 10 minutes. Filter off the product, allow to air-dry and hand in. Does the product possess the fragrance of cinnamic aldehyde? Discuss by structural references, the type of isomerism possible in the product. Tabulate, with formulas and names, the ammonia derivatives which react with the carbonyl portion of aldehydes and ketones. Give the general type-formula of each product formed.

=

48.

PREPARATION OF HEXAMETHYLENETETRAMINE (P)

Mix 10.0 ml. formalin with 20.0 ml. of 28% ammonium hydroxide in an evaporating dish and evaporate to dryness in a fume chamber, by placing the dish over the mouth of a beaker half filled with water which is boiling,

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79

or on a steam bath. As the liquid thickens, it must be stirred occasionally. Dry well until the solid becomes powdery. Weigh the dry product to within 0.1 g., write the equation for the reaction, and calculate the yield, assuming that the formalin contains 37% W /W of formaldehyde, and has a density of 1.08 g./ml. and that the ammonium hydroxide has a density of 1.0 g./ml. Give another name for hexamethylenetetramine. State two uses for this compound. Hand in the product and when it is returned to you, put it away stoppered for a future experiment. 49.

ALDOL REACTION-PREPARATION OF 2,6-DIBENZALCYCLOHEXANONE (P)

In a 125 ml. Erlenmeyer flask place 2.0 ml. cyclohexanone ( density = 1.0 g./ml.). Add 4.0 ml. benzaldehyde ( density = 1.0 g./ml.). Then add 15 ml. 10% sodium hydroxide solution, 15 ml. water and 15 ml. ethanol. Warm under reflux over a beaker of boiling water for 2 hours, shaking occasionally. Then set aside until the next laboratory period to allow the product to crystallize. Filter and wash by decantation with two portions of 5 ml. cold 50% methanol-water then with cold water until almost neutral in reaction. Allow the product to dry, weigh it and calculate the yield. Hand it in properly labelled. Write the equation for the reaction in stages and account for the fact that the product is coloured. List the structural necessities for the aldol-type of reaction to occur and outline a mechanism for this reaction. Write the equations for the aldol-type reaction between (a) benzaldehyde and acetone and ( b) benzaldehyde and acetophenone and name the products. What will be the formulas and names of the substances formed on dehydration of these products? 50.

POLYMERIZATION REACTIONS OF ALDEHYDES

(a) Polymerization of Formaldehyde (P) Place 5 ml. formalin in an evaporating dish and leave until the next laboratory period. The solid product is paraformaldehyde. In a test tube, place sufficient of the polymer to cover the bottom of the tube. Carefully heat this and hold a strip of filter paper moistened with Schiff's reagent at the mouth of the tube. Record all observations and explain the results. Depolymerization by heat is a common property of addition polymers. Hand in the remainder of the product. ( b) Polymerization of Acetaldehyde to form a Resin Boil 10 ml. of acetaldehyde solution ( Exp. 28) with 10 drops of 40% sodium hydroxide solution. Note the appearance and odour of the product which is thought to be formed by the aldol reaction and subsequent dehydration and polymerization. Write the equation for the aldol reaction of acetaldehyde, also the dehydration of the aldol. Give the I.U.P.A.C. names for the products at each stage.

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81

9. CARBOXYLIC ACIDS AND THEIR DERIVATIVES Carboxylic acids are organic compounds that contain the carboxyl ( _cf'

0

)

functional group. They are generally prepared by the reaction

""'OH

of a Grignard reagent with carbon dioxide, by the oxidation of a primary alcohol or by the hydrolysis of a nitrile. Carboxylic acids undergo reactions that result in the breaking of either the C-O or the O-H bond of the functional group. The acid character of carboxylic acids is due to the breaking of the O-H bond to form an anion and a proton. RC-:-0

➔ Ka

'OH~

0 0-

RC_';'.

+

K

_ (H+)(RC02-)

a -

(RC02 H)

The tendency for any carboxylic acid to lose a proton is measured by the equilibrium constant Ka, The value of K.. depends upon the nature of the group R. If R is a substituent that withdraws electrons from the carboxyl group, Ka is increased, while if R donates electrons, Ka is decreased. As a result each carboxylic acid has a characteristic value for Ka, In general carboxylic acids are weak acids, i.e. acetic acid has a Ka of 1.8 X 10-5 • Carboxylic acids react with bases to form salts, i.e. benzoic acid reacts with sodium hydroxide to form sodium benzoate. The solubility of a salt often diHers significantly from the carboxylic acid, This will be illustrated in Experiment 61. Breaking of the C-O bond and subsequent replacement of the OH group by Cl, OR, and NH2 results in the formation of the following derivatives. ( 1) Carboxylic acid chlorides ( Acyl Halides) : The reaction of a carboxylic acid with thionyl chloride or phosphorus trichloride results in the formation of an acyl chloride ( eq. 1).

Acyl chlorides react vigorously with water to regenerate the carboxylic acid, with alcohols to form esters and with ammonia to form amides. ( 2) Esters: Carboxylic acids react with alcohols in the presence of a strong acid catalyst to form an ester ( eq. 2). CH3C::~H

+

C2 H50H

CH 3co2 c2 H 5

+

H2 0

(2)

This reaction is an equilibrium. Therefore, in order to obtain a quantitative yield of the ester it is necessary to shift the equilibrium in the direction of the formation of the ester. This can be accomplished by either removing the water as it is formed or by using a large excess of alcohol. Esters can

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83

also be prepared by the reaction of an acyl chloride ( eq. 3) or an anhydride with an alcohol ( eq. 4).

(3) Amides: Amides and N-substituted amides are most conveniently prepared by the reaction of an acid chloride with ammonia or an amine ( eq. 5). CH3COCI

+

CH3 ( CH2)2 NH2

-+

CH 3 CONH{CH 2 ) 2 cH 3

+

HCI

(

5)

One single mechanism has been postulated for the reaction involving C-O, C-N, or C-Cl cleavage. The reaction of acetyl chloride with water will serve to illustrate this mechanism. CH C.,,_O 3 'Cl

+

H2 o

----+

0

CH 3

¢-

Cl OH 2

(6)

+

(7)

0 + H+ cH 3 c~'OH (8) -----+ + 2 The first step of the mechanism is a nucleophilic attack by a water molecule on the carbonyl group ( eq. 6). The second step is the loss of chloride ion from the tetrahedral intermediate ( eq. 7), followed by the loss of a proton to form the product ( eq. 8). An analogous mechanism can be postulated for the hydrolysis of esters, amides, trans esterification and formation of an ester from a carboxylic acid and an alcohol. The experiments in this section are designed to show the preparations of carboxylic acids and correlate the reactions of carboxylic acids, their salts, esters, amides and acyl chlorides. ~0

CH3C,OH

Experimental

51.

PREPARATION OF BENZOIC

Acm

(a) Oxidation of Benzyl Alcohol In a 250 ml. Erlenmeyer flask fitted with a water condenser, add, in the order given, 4 g. of powdered KMnO4, 150 ml. of 2M H2SO4 and 2 ml.

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85

benzyl alcohol. Heat for about one hour under reflux and then cool the flask and contents in ice-water. Collect the solid by suction filtration and suck partly dry while in the Buchner funnel. Purify the solid by recrystallization from water. (b) Reaction of a Grignard Reagent with Carbon Dioxide. No -flames allowed in the laboratory Place 0.7 g. magnesium turnings and a solution of 1 ml. bromobenzene in 5 ml. dry ether in a large dry test tube. If no reaction is visible immediately ( i.e. no colour change) crush a piece of the magnesium against the wall of the test tube with a clean dry stirring rod and warm gently in a water bath, then remove and see if the boiling continues spontaneously. If it boils too vigorously cool in a beaker of cold water. Otherwise add an additional 2 ml. bromobenzene dissolved in 10 ml. dry ether in small portions to keep the reaction going. If too much of the ether boils away add a few ml. of dry ether from time to time. When the spontaneous reaction stops, warm the tube in a beaker of warm water for another 5-10 minutes. Take about 10 g. dry ice, wipe the moisture off with a towel, crush into small pieces, and place in a 250 ml. beaker. Add 5 ml. of anhydrous ether, and pour the Grignard solution over the dry ice with stirring. Just before the dry ice is gone add 30 ml. dilute hydrochloric acid in small quantities with vigorous stirring and filter the precipitate by suction filtration. Dissolve the benzoic acid in a solution of 5 g. sodium bicarbonate in 30 ml. water. Filter to remove any insoluble material, cool the solution in an ice bath and reprecipitate the benzoic acid by adding concentrated hydrochloric acid dropwise with stirring until the solution is acid to litmus ( w ARNING: effervescence). Filter the benzoic acid, wash with a little cold water, and crystallize from the minimum of hot water. Determine the melting point and percent yielcf of the pure dry product. Give equations for the above reaction, and a brief description of the mechanism. Mention at least three other uses to which Grignard reagents may be put, including specific examples and equations in your answer. 52. EFFECT OF ALPHA-HALOGEN SUBSTITUTION ON THE STRENGTH OF CARBOXYLIC Acms To separate 5 ml. portions of lN acetic and lN trichloroacetic acids, add 1-3 drops of Methyl Violet [pH ~2 (yellow to blue)]. Look up the degrees of ionization of these acids. What generality is illustrated here? What electronic explanation is given for the difference observed? 53. !'REPARATION OF THE ANILIDE OF ACETIC Aero (P) Mix 1 ml. of acetic acid with 2 ml. of thionyl chloride and let stand for 30 minutes. Cool the mixture and add 2 ml. aniline in 20 ml. of benzene. Warm the mixture for five minutes. Decant the benzene solution into a separatory funnel and wash successively with 2 ml. of water, 5 ml. of 5%

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87

hydrochloric acid, 5 ml. of 10% sodium hydroxide solution and 2 ml. of water. Evaporate the benzene by heating on a hot plate. Cool, recrystallize the product from water, determine the melting point and hand in. 54. SEMI-MICRO PREPARATION OF AcETANILIDE (ANTIFEBRIN)

In a fume cupboard, to 10 drops of aniline in a small test tube ( 1 X 10 cm.), add carefully 2 ml. of acetic anhydride drop by drop and shake or stir for 5 minutes. Allow to stand for 15 minutes. CAUTION: no NOT INHALE! Add 3 ml. water plus 2 ml. ethanol and heat until a clear solution results adding more hot water if necessary. Allow to cool slowly first in the room and then in an ice bath for 5 minutes. Equation? Filter through a Hirsch funnel fitted into a side arm test tube. Air-dry, determine the melting point. 55. AMMONOLYSIS OF AN ESTER-PREPARATION OF ACETAMIDE (P) Place 8.0 ml. of ethyl acetate and 20 ml. of concentrated ammonia ( 28%) in a corked labelled flask. Let stand until the next laboratory period. Describe the changes in appearance. Evaporate to one-fourth volume in a fume hood. Cool on ice and if the product does not solidify, evaporate off approximately 3 ml. more liquid. Collect the product, determine its melting point and hand it in. Show how (a) acetic acid and ( b) acetonitrile can be prepared from acetamide. 56. SCHOTTEN-BAUMANN REACTION-PREPARATION OF HIPPURIC Acm (P)

In a small Erlenmeyer flask dissolve 0.6 g. glycine in 8 ml. 30% aqueous sodium hydroxide solution. Cautiously in a fume cupboard, add 1.0 ml. benzoyl chloride, shaking vigorously until one phase is formed. At the end of one hour, pour the liquid with vigorous stirring into a mixture of 25 ml. ice water and 10 ml. concentrated hydrochloric acid. Filter the precipitate on a Hirsch funnel, wash twice with 5 ml. cold water, and allow to dry in the air. Determine the melting point and hand in the sample.

57. PREPARATION OF METHYL SALICYLATE Dissolve 1 g. of salicylic acid in 10 ml. of methanol in a 125 ml. Erlenmeyer flask. Add 1 ml. concentrated sulphuric acid and insert a water condenser in the top of the corked flask. Reflux in a beaker of boiling water for 30 minutes. Pour the contents of the flask into 100 ml. of water. Smell the product. Write the equation for the reaction. Why was sulphuric acid added? What evidence is there that synthetic oil of wintergreen was formed? What impurities would you expect to find in the ester? Formulate a reasonable mechanism for the reaction.

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89

58. PREPARATION OF ISOAMYL ACETATE (P) ( TO BE DONE IN FUME HOOD. ) CAUTION! Do not inhale the fumes of acyl halides such as acetyl chloride which are injurious to the mucous membranes. In a 125 ml. Erlenmeyer flask put 6.0 ml. of isoamyl alcohol ( density = 0.8 g./ml.) and 5.0 ml. acetyl chloride (density = 1.1 g./ml.). Allow to stand for two hours in the fume hood and then pour into 50 ml. of 1% sodium hydroxide solution. Observe the fragrance of the ester ( Synthetic Banana Oil). Measure the volume of the crude ester, and calculate the yield of ester. ( Density of isoamyl acetate = 0. 9 g. / ml. ) . Write the equation for the reaction. Label the crude ester with your name and yield and hand it in. 59. PREPARATION OF AcETYL SALICYLIC Acm (P) In a small flask put 2.0 g. salicylic acid, 6.0 ml. acetic anhydride ( density g./ml.) and 3 drops cone. sulphuric acid. Allow to stand for one hour with occasional shaking, then mix with stirring with 50 ml. ice water. Collect the product and recrystallize. Allow to dry in the air. Weigh, calculate the yield and hand it in.

= 1.1

60. ALKALINE HYDROLYSIS ( SAPONIFICATION) OF ETHYL ACETATE Add 5 ml. of ethyl acetate to 5 ml. of water in a test tube. Note the insolubility of the ester in water. Transfer the mixture to a 250 ml. flask and add 30 ml. of 10% sodium hydroxide solution. Fit the flask with a reflux condenser and heat gently for 20 minutes or until there is but one layer in the flask. Transfer to a small distilling flask and distil off 4 ml. Test the distillate for ethanol by the iodoform test. Acidify the solution remaining in the distilling flask with dilute sulphuric acid and again distil about 4 ml. Test this distillate for acidity by Bromothymol Blue indicator. What causes the observed acidity? Write equations for all reactions. 61. SOLUBILITY OF SALTS OF CARBOXYLIC Acms Place sufficient benzoic acid to cover the bottom of a test tube in each of four test tubes. To one of these add 5 ml. 10% NaOH solution, to the second 5 ml. of 5% NH4OH solution, to the third, 5 ml. of 5% Na2COa solution, and to the fourth, 5 ml. of water and record your observations. Write equations for the reactions. Prepare four more test tubes and in each of these add sufficient benzoic acid to cover the bottom and then add 5 ml. of 5% NH4OH to each. Boil until no more fumes of NHa can be detected by smell ( THIS IS IMPORTANT!). To the first test tube add 2 ml. of 5% CuSO4 solution. To the second add 2 ml. of 5% CaCl2 solution. To the third add 2 ml. of 5% Pb( NOa )2 solution.

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91 To the fourth add concentrated HCl until distinctly acid. Write equations for the reactions involved. If benzoic acid is characteristic of carboxylic acids having both hydrophylic and hydophobic portions, what generalizations can be drawn about solubilities of the salts of such acids in water? Compare the solubilities of the sodium, potassium and ammonium salts with the solubility of the free acid in water.

10. AMINES Amines are the only nitrogen-containing organic compounds that exhibit basic properties. As a result they are readily separated from other organic nitrogen-containing compounds by simple extraction with acid. Aromatic amines are generally prepared by the reduction of the readily available nitro compounds. Aliphatic amines can be prepared by a variety of methods including reduction of aliphatic nitro compounds, amides and nitriles. The structure of amines can be considered as being derived from an ammonia molecule by the progressive substitution of the hydrogens by alkyl or aryl groups. H-N-H

~

ammonia

methylamine ( a primary amine)

dimethylamine ( a secondary amine)

trimethylamine ( a tertiary amine)

This similarity of structure accounts for many of the chemical properties of the amines. Amines, like ammonia, are bases and react with acids to form salts ( eq. 1 ).

Ammonia, primary and secondary amines but not tertiary amines react with acyl halides ( eq. 2) and benzenesulfonyl halides ( eq. 3) to form amides. (2)

(3) The latter reaction is the basis of the Hinsberg test which is used to distinguish between primary, secondary and tertiary amines. Tertiary

91 To the fourth add concentrated HCl until distinctly acid. Write equations for the reactions involved. If benzoic acid is characteristic of carboxylic acids having both hydrophylic and hydophobic portions, what generalizations can be drawn about solubilities of the salts of such acids in water? Compare the solubilities of the sodium, potassium and ammonium salts with the solubility of the free acid in water.

10. AMINES Amines are the only nitrogen-containing organic compounds that exhibit basic properties. As a result they are readily separated from other organic nitrogen-containing compounds by simple extraction with acid. Aromatic amines are generally prepared by the reduction of the readily available nitro compounds. Aliphatic amines can be prepared by a variety of methods including reduction of aliphatic nitro compounds, amides and nitriles. The structure of amines can be considered as being derived from an ammonia molecule by the progressive substitution of the hydrogens by alkyl or aryl groups. H-N-H

~

ammonia

methylamine ( a primary amine)

dimethylamine ( a secondary amine)

trimethylamine ( a tertiary amine)

This similarity of structure accounts for many of the chemical properties of the amines. Amines, like ammonia, are bases and react with acids to form salts ( eq. 1 ).

Ammonia, primary and secondary amines but not tertiary amines react with acyl halides ( eq. 2) and benzenesulfonyl halides ( eq. 3) to form amides. (2)

(3) The latter reaction is the basis of the Hinsberg test which is used to distinguish between primary, secondary and tertiary amines. Tertiary

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93

amines do not form sulfonamides. The sulfonamide prepared from a primary amine is soluble in base while that formed from a secondary amine is insoluble. This serves to distinguish the three classes of amines. Amines, however, undergo certain reactions that do not occur with ammonia. One example is the reaction of amines with nitrous acid. The product of this reaction depends upon the class of amine. Primary aliphatic and aromatic amines react to form a diazonium ion. Aliphatic diazonium ions readily lose nitrogen to form a carbonium ion, which either reacts with solvent, rearranges or loses a proton ( see chapter 12). Aromatic diazonium ions, however, undergo two other reactions. These are coupling and replacement reactions. These are illustrated in Chart I.

N=N-0

roOH .,,;;

.

Cu 2 (CN) 2 ~

oc,+ N:N

.

roOH/4

COUPLING REACTION

oC=N

CuCI

~

OCI /4

HBF4 ~

REPLACEMENT REACTIONS

OF .,,;;

CHART I The coupling reaction gives highly coloured azo compounds as products which are commercially important as dyes and indicators. The replacement reaction is synthetically useful, since it permits the transformation of a nitro group into a wide variety of useful substituents. Secondary alkyl and aromatic amines react with nitrous acid to form N-nitroso amines ( eq. 4) while tertiary amines react to give complex products. ~NHCH 3

V

+

H0N0

~

(4)

The experiments in this section are designed to illustrate the preparation and reactions of aromatic amines using aniline as the model compound.

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95 Experimental 62. PREPARATION OF ANILINE Place 10 ml. of nitrobenzene in a 125 ml. Erlenmeyer flask with about 8 g. of granulated tin. Now add gradually, in small portions, about 30 ml of concentrated hydrochloric acid, shaking well after each addition. Finally heat in a water bath with frequent shaking until the oily layer of nitrobenzene has disappeared. Dilute to about twice its volume with distilled water and make distinctly alkaline with sodium hydroxide solution. Transfer the liquid to a 250 ml. distilling flask. Upon distillation, the distillate will be an emulsion of aniline and water. The oily drops of aniline can clearly be seen. Continue the distillation until the distillate loses its milky appearance, then collect an additional 25 ml. of distillate. This procedure is known as "steam distillation." Saturate the distillate with solid sodium chloride and then extract with two 25 ml. portions of methylene chloride. Combine the methylene chloride (b.p. 40° C.) washings and dry over 3 g. of potassium hydroxide pellets. Decant the organic layer into a small distilling flask and remove the methylene chloride by distillation using a water bath. Keep the residue for future experiments. Explain the principle of "co-distillation" and, in particular, "steam distillation." Assuming aniline and water to be completely immiscible, calculate the %W /W of aniline in the distillate. 63. REACTION OF ANILINE WITH BROMINE WATER To 5 ml. of a 1% aqueous solution of aniline, add bromine water drop by drop, until about 10 drops have been added. Equation?

64.

REACTION OF AMINES WITH BENZENESULFONYL CHLORIDE

THE

HINSBERG TEST

To 1 ml. of aniline in a test tube add 10 ml. of 10% sodium hydroxide, and about 1 ml. of benzenesulphonyl chloride. Stopper the tube and shake vigorously under the cold water tap. If all the material does not dissolve in 2 to 3 minutes, add additional alkali and shake again. Now acidify the solution with cone. hydrochloric acid ( litmus paper), cool, and scratch the inside of the tube with a glass rod. Filter, wash with water, dry thoroughly by pressing the solid between filter papers, and then test the solubility of the material in warm 10% sodium hydroxide, and in dilute hydrochloric acid. Repeat the above procedure using, instead of aniline, the secondary amine N-methylaniline. Note that a precipitate is obtained in the first stage of the reaction. Filter, wash with water, and then dry between filter papers roughly. Test the solubility of this precipitate in both dilute hydrochloric acid and in warm 10% sodium hydroxide. Similarly treat 1 ml. of N,N-dimethylaniline with alkali and benzenesulphonyl chloride. After shaking for 2 to 3 minutes, test the solubility of the oily layer in dilute acid and in 10% sodium hydroxide.

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97

65. REACTION OF ANILINE WITII Nimous Aero (a) At room temperature (Van Slyke) Dissolve 0.5 ml. aniline in 15 cc. of dilute hydrochloric acid. Add 5 ml. of a 10% solution of sodium nitrite. Shake and warm gently. Observe that a gas is given off. What is it? Note the odour of the solution. Equations? Repeat the experiment with N-methylaniline. ( b) At 0° (Griess) To 10 drops of aniline in a test tube add 6 ml. of dilute hydrochloric acid and cool in an ice bath. Add, with shaking, 4 ml. of this solution to 5 ml. of freshly prepared 10% sodium nitrite solution previously cooled in an ice bath. Divide this liquid into two equal parts. ( i) To one half add 2 ml. of the aniline solution in HCI. (ii) To the other half add a solution prepared by dissolving sufficient ,8-naphthol to cover a ten-cent piece in 5 ml. of 10% sodium hydroxide. Discuss the chemistry of the reactions in (a) and ( b). Write equations. 66. D ~ PREPARED BY THE COUPLING REACTION (P) (a) Preparation of m-Nitroaniline Slowly add a solution made by dissolving 8 g. of sodium sulfide and 4.0 g. of sodium bicarbonate in 25 ml. of water to a boiling solution containing 4.0 g. of m-dinitrobenzene dissolved in 40 ml. of methanol ( FUME Hoon). Distil off most of the methanol, and pour the residue into hot water ( 150 ml.). Allow to cool, filter and recrystallize the m-nitroaniline from water, with the addition of small amounts of ethanol if necessary. Determine its melting point. ( b) Preparation of m-Nitrobenzene azo {3-Naphthol (P) Dissolve 2 g. of m-nitroaniline as prepared in (a) in 8 ml. of concentrated hydrochloric acid, with warming if necessary. Add 20 ml. of water and cool in a freezing mixture to 0-5° C. Add a 20% aqueous solution of sodium nitrite dropwise, with stirring, until the mixture gives an immediate blue colour with starch-potassium iodide paper. It is advisable to wait after each addition of sodium nitrite before testing it with starch-iodide paper in order to give time for the reaction to take place. Keep the temperature below 10° C. Add the cold solution to a cold solution of 3 g. of ,8-naphthol in 30 ml. of 10% aqueous sodium hydroxide, with rapid stirring. Allow the mixture to stand for 30 minutes in the ice bath and then filter off the azodye. Wash the product with water and recrystallize it from glacial acetic acid. Determine the yield and the melting point of the pure product. Hand it in.

67.

DYES PREPARED BY THE OXIDATION OF ANILINE

(a) With Hypochlorites to give an Indamine To 5 ml. of an aqueous solution of aniline, add 5 ml. of a 2% aqueous solution of calcium hypochlorite. The coloured substance is Indamine. Equation?

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99

( b) With Dichromate to give Emeraldine

To 40 ml. of an aqueous solution of aniline, add 10 ml. dilute sulphuric acid and sufficient sodium dichromate to cover the end of a knife blade. Observe over the course of 20 minutes. The green dye is Emeraldine.

( c) Oxidation of Aniline to give Aniline Black

In an evaporating dish place 15 drops of concentrated sulphuric acid and 10 drops of sodium dichromate solution. Add 1 drop of aniline, stir and warm gently over a beaker of boiling water. Observe the Indamine and Emeraldine stages. Aniline Black was formerly a favourite dye for wool. 68. PREPARATION OF MAUVE-PERKIN'S HISTORIC EXPERIMENT

In a test tube, mix 2 drops of aniline, a few drops of o-toluidine and a pinch of p-toluidine. Add 5 drops concentrated hydrochloric acid and 15 ml. of 5% chromic acid solution. Warm in a beaker of boiling water for about five minutes. Dilute about 5 ml. of this liquid with water until the colour of the dye is discernible. Describe its colour. Write the formula assigned to magenta ( fuchsin) and indicate thereon the portion formed by each of the three amines. What was the function of the chromic acid in the reaction? How does Mauve compare structurally with Gentian Violet ( Methyl Violet)?

11. PHENOLS AND QUINONES In the discussion of alcohols ( chapter 6) it was pointed out that alcohols could be considered as derivatives of water. This, however, is not the case with aromatic hydroxy compounds which as a class are called phenols. The major difference between the chemical properties of alcohols and phenols is their acidity. Alcohols are neutral compounds while phenols are acids. Consider the difference between phenol C6H5OH and ethanol ( eq. 1).

-

0

-:::-0

/,

CH3CH20H

:;---

CH3 CH 2 0

+

(1) H+

( NOTE. Distinguish carefully between phenols, a class of compounds and phenol C6H5OH, as a particular compound.) The acid property of phenol is due to the phenyl ring adjacent to the hydroxyl group. The phenyl ring can stabilize the phenoxide anion relative to phenol by delocalization of the charge into the ring. Such resonance stabilization is not possible with the ethoxide ion. Thus phenol loses a proton more readily than ethanol.

99

( b) With Dichromate to give Emeraldine

To 40 ml. of an aqueous solution of aniline, add 10 ml. dilute sulphuric acid and sufficient sodium dichromate to cover the end of a knife blade. Observe over the course of 20 minutes. The green dye is Emeraldine.

( c) Oxidation of Aniline to give Aniline Black

In an evaporating dish place 15 drops of concentrated sulphuric acid and 10 drops of sodium dichromate solution. Add 1 drop of aniline, stir and warm gently over a beaker of boiling water. Observe the Indamine and Emeraldine stages. Aniline Black was formerly a favourite dye for wool. 68. PREPARATION OF MAUVE-PERKIN'S HISTORIC EXPERIMENT

In a test tube, mix 2 drops of aniline, a few drops of o-toluidine and a pinch of p-toluidine. Add 5 drops concentrated hydrochloric acid and 15 ml. of 5% chromic acid solution. Warm in a beaker of boiling water for about five minutes. Dilute about 5 ml. of this liquid with water until the colour of the dye is discernible. Describe its colour. Write the formula assigned to magenta ( fuchsin) and indicate thereon the portion formed by each of the three amines. What was the function of the chromic acid in the reaction? How does Mauve compare structurally with Gentian Violet ( Methyl Violet)?

11. PHENOLS AND QUINONES In the discussion of alcohols ( chapter 6) it was pointed out that alcohols could be considered as derivatives of water. This, however, is not the case with aromatic hydroxy compounds which as a class are called phenols. The major difference between the chemical properties of alcohols and phenols is their acidity. Alcohols are neutral compounds while phenols are acids. Consider the difference between phenol C6H5OH and ethanol ( eq. 1).

-

0

-:::-0

/,

CH3CH20H

:;---

CH3 CH 2 0

+

(1) H+

( NOTE. Distinguish carefully between phenols, a class of compounds and phenol C6H5OH, as a particular compound.) The acid property of phenol is due to the phenyl ring adjacent to the hydroxyl group. The phenyl ring can stabilize the phenoxide anion relative to phenol by delocalization of the charge into the ring. Such resonance stabilization is not possible with the ethoxide ion. Thus phenol loses a proton more readily than ethanol.

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101

Another difference in the reactions of phenols and alcohols is that the hydroxyl group of a phenol is not easily replaced by another group. For example phenol does not react with hydrochloric acid to form chlorobenzene. Phenols and alcohols can undergo some reactions that are similar. Thus phenols can form esters, ethers and can be oxidized. One product of oxidation of phenols is a class of compounds called quinones ( eq. 2). OH

0

0

OXIDATION REDUCTION

OH

0

(2)

0

Quinones can also be prepared by the oxidation of aminophenols and polyaromatic hydrocarbons ( eq. 3).

6

0

0

OXOOATION

NH 2

0

~ ~

0

OXIDATION

~ ~ 0 (3)

The presence of a hydroxyl group on an aromatic nucleus greatly activates the ring to electrophilic substitution reactions. Thus phenol reacts readily with bromine water, nitrous acid ( eq. 4), nitric acid and sulphuric acid. Phenol can be alkylated much more readily than benzene. Usually wanning the phenol with an olefin or an alcohol in the presence of sulphuric acid is sufficient for reaction to take place ( eq. 5).

0

0

OH

OH

+

HONO

~

0

~ ~

N~O

(4)

N'OH

OH

OH

0

0

+

R2 CHOH

+

H2

so 4

~

0

(5)

CHR2

It should be noted that phenol can undergo keto-enol tautomerism ( eq. 6). The equilibrium in this case is predominantly

0 0

0

(6)

HO

towards the enol. This is in contrast to acetone. Certain substituted phenols such as p-nitrosophenol can exist in the keto form. Since phenols are enols, they react with ferric chloride to give coloured water soluble complexes.

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Experimental 69. RELATIVE Aero STRENGTHS OF SoME PHENOLS To 3 ml. portions of 5% NaHCOs solution in separate test tubes add 3 ml. of a 5% aqueous-alcoholic solution of (a) phenol, ( b) picric acid, and ( c) 2,4,6-trichlorophenol. Immerse the tubes in a beaker half full of hot water ( about 70° C.) and observe at the end of fifteen minutes at which time any reaction will be apparent by the presence of micro bubbles along the walls of the test tube. Evaluate the phenols as to their relative acidities, making use of the rule that "salts of weak acids are decomposed by stronger acids to set free the weak acid." Consult the handbook for the K1on of two of these phenols and see whether your arrangement is in accord with the data cited in respect to these two examples. Write formulas for the phenols listed in this experiment. What explanation is given for the fact that picric acid is more acidic than phenol? 70. REACTION OF PHENOL WITH BROMINE WATER To 5 ml. of 1% aqueous phenol solution, add 10 drops of bromine water. Equation? Add 10 drops of bromine water to 3 ml. benzene, and observe how much more readily phenol is brominated. This is known as enhanced activity. Explain why the phenolic group is para and ortho directing and activating. Compare the enhanced activity shown here with that of aniline ( Exp. 63) and explain. 71. NITROSATION OF THYMOL (P) In a small Erlenmeyer flask dissolve 1.5 g. thymol in 30 ml. methanol, cool in an ice bath. Add 1 g. sodium nitrite dissolved in 2 ml. water and then 5 ml. of cone. HCl one ml. per minute with shaking. Allow to stand in the ice bath for 30 minutes with occasional shaking, then at room temperature for 15 minutes. Drown in 150 ml. of ice water, stir well, and allow to stand in the ice bath for a further 5 minutes. Filter off the nitroso compound ( or oxime), wash the solid thoroughly with 25 ml. cold water to remove the acid, dry in the air, and hand in the sample. Aromatic hydrocarbons will not form nitroso derivatives but phenols will do so easily. 72. SUSCEPTIBILITY OF PHENOLS TO OXIDATION CAUTION! Phenol produces painful wounds. Do not allow it to come in contact with the skin. Place a few crystals of (a) phenol and ( b) quinol in each of two 250 ml. Erlenmeyer flasks, add about 25 ml. water and 25 ml. dilute H2SO4 to each. Then carefully pour in about 10 ml. N / 10 KMnO4 solution and observe how quickly and completely the reaction proceeds. Benzoquinone-1,4 is

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105

one of the oxidation products of both of these phenols. Write the structure of the products formed. 73. REDUCING ACTION OF AMINOPHENOLS Dilute 5 ml. reagent ammonium hydroxide with 20 ml. water. Prepare some "ammoniacal silver solution" by adding this dilute ammonium hydroxide solution to 5 ml. of 5% silver nitrate solution until the precipitate that first forms just dissolves ( complex ion). To this liquid add a pinch of p-aminophenol, stir well and immerse in a beaker of boiling water for about 5 minutes. Explain the observations. 74. FORMATION OF QUINHYDRONE (P) Dissolve separately in two small Erlenmeyer flasks 1 g. quinol in 15 ml. of warm ( 60° ) ethanol and 1 g. benzoquinone in 20 ml. of warm ethanol. Mix the contents of the two flasks while still warm and stir. Allow to cool and then place on ice for 10 minutes. Filter off the product, air-dry and hand in.

75.

PREPARATION OF PHENOLPIITHALEIN

Grind 1 g. phenol and mix well with an equal quantity of powdered phthalic anhydride. Place in a Pyrex test tube, then add 10 drops of concentrated sulphuric acid. Mix well and heat in a Bunsen Harne for about 5 minutes or until the melt becomes dark red. Cool. Add carefully 30 ml. of 10% sodium hydroxide solution and stir for a few minutes. Write the equation for the reaction. What is the function of the sulphuric acid? What other product is formed simultaneously by an analogous reaction? To 5 ml. of the above alkaline solution in a beaker add 30 ml. of 30% sodium hydroxide solution. Write the equation. To a second 5 ml. portion add cone. HCl to acidity. Equation? What is the chromophoric group in the sodium salt of phenolphthalein? Show that phenolphthalein may be classed as a triphenylmethane derivative. Note also that it is a gamma lactone. 76. PREPARATION OF AN ETHER (METHYL ,B-NAPHTHYL ETHER)WILLIAMSON SYNTHESIS (P) Place 4 g. of ,8-naphthol in a 125 ml. Erlenmeyer flask. Add 10 ml. of methyl alcohol to dissolve the naphthol, then add 2.0 g. of finely crushed potassium hydroxide and 2 ml. of methyl iodide (CAUTION-POISON). Attach the flask to a reflux condenser and boil the contents gently for about 5 minutes. A granular white precipitate will appear. Distil away about one half of the alcohol from the mixture in the Erlenmeyer, being careful not to overheat.

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107

Allow the flask to cool, then add 20 ml. of water and wash the solid thoroughly, breaking any lumps. Decant the wash water into a Buchner funnel. Wash twice more, again pouring the water onto the funnel. Transfer the product to the funnel. Dry by suction, then in the air. Melt a little on a spatula and smell cautiously. Recrystallize, determine the melting point and hand it in.

77.

THE FERRIC Cm..ORIDE REACTION OF ENOLS AND PHENOLS

Add 5 drops 5% ferric chloride solution to 5 ml. portions of acetone, dilute aqueous solutions of phenol, quinol, resorcinol, ,8-naphthol, p-nitrophenol, o-nitrophenol, vanillin and salicylic acid and dilute aqueous-alcoholic solutions of acetyl acetone ( diacetone), acetoacetic ester, phenylacetaldehyde and 1% alcoholic benzoyl acetone. Tabulate the results, write formulas for the above substances and represent any expected tautomerism. Which of the above substances appear to be the least enolized under the existing conditions? Do you notice any notable difference in shade produced with phenols and with enols? This colour reaction is probably due to complex ions in which hexacovalent iron is bonded with the phenolic or enolic structure as the case may be to give co-ordination complexes.

78.

FORMATION OF THE POTASSIUM DIENOLATE OF BENZOIN

Boil a pinch of benzoin with 5 ml. Fehling's solution. Equation? This reaction is general for alpha-hydroxy ketones and is important in carbohydrate chemistry. In a 50 ml. Erlenmeyer flask, put 10 ml. ethanol and about 0.1 g. each of benzoin and benzil. Add 5 drops 10% alcoholic potassium hydroxide solution, allow to stand for a minute and then shake vigorously so that air will be mixed with the liquid. Repeat the operation of adding alcoholic KOH and shaking with air as long as the colour can be made to appear and to disappear again. The potassium dienolate of stilbendiol or isostilbendiol is formed, and can, if desired, be isolated as yellow crystals. The reddishpurple solution which is sensitive to the oxygen of the air and is decolorized thereby, is probably due to a free radical of para-quinonoid structure. On auto-oxidation, this free radical is converted partly into benzil and partly into potassium benzoate. Write the equations for the reaction and account structurally for the colour and its disappearance on shaking with air.

79.

ALKALINE REDUCTION (VATTING) OF ANTIIBAQUINONE

Into one test tube put 5 ml. 10% sodium hydroxide solution and sufficient zinc dust to cover the bottom of the test tube. Into a second test tube put 5 ml. 10% sodium hydroxide and sufficient sodium hydrosulphite to cover the bottom of the tube. Add a pinch of anthraquinone to each tube and heat carefully. The colour is due to the formation of the sodium salt of anthra-

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109

hydroquinone ( anthraquinol). Anthrahydroquione and oxanthrone are tautomers. Write the equilibrium expression and show the relationship between the respective scxlium salts. This salt is soluble in water and it is in this way that dyes of the indigoid or anthraquinonoid type are applied to a fibre. Show that the red colour can be caused to disappear by shaking the solution with air, which oxidizes the red salt to anthraquinone. Write equations for the reactions. Up until recently the Chinese vatted Indigo Blue with soda and glucose. What reaction would likely result here? What are the structural requirements for enolization? 80.

PREPARATION OF 1,4-NAPHTHOQUINONE

(P)

Dissolve 7.0 g. of chromic anhydride ( CrOa) in 35 ml. of ice cold 80% acetic acid in a 250 ml. beaker. CAUTION: avoid contact with chromic anhydride as it causes severe burns. To this slowly add with vigorous stirring, over a period of 5 minutes, a cold solution of 4.0 g. of naphthalene in 50 ml. of glacial acetic acid. Keeping the liquid in ice for the first hour, stir occasionally for the remainder of the period and allow to stand until the next laboratory period. Pour with vigorous stirring into 500 ml. of ice water. Allow to stand for one hour. Rinse the beaker with 25 ml. dilute acetic acid and add this to the drowned mass. Filter off the precipitate, wash acid-free with cold water, air-dry, determine its melting point and hand it in. Write the equation for the reaction. To what is the colour of the product due? Polynuclear aromatic hydrocarbons generally oxidize in the manner illustrated by this experiment.

12. MOLECULAR REARRANGEMENTS Many organic compounds undergo reactions that result in a reorganization of their molecular structure. In this section we will consider three such reactions. These are the Pinacol-Pinacolone and the Beckmann Rearrangements and the Hofmann Degradation. Pinacol-Pinacolone Rearrangement HO OH ( CH3)2

C-C (CH3 )2 +

H2S04

__,,.

(1)

Beckmann Rearrangement N,OH (CH3'3CCCH3

+

PCl5

---+

(2)

109

hydroquinone ( anthraquinol). Anthrahydroquione and oxanthrone are tautomers. Write the equilibrium expression and show the relationship between the respective scxlium salts. This salt is soluble in water and it is in this way that dyes of the indigoid or anthraquinonoid type are applied to a fibre. Show that the red colour can be caused to disappear by shaking the solution with air, which oxidizes the red salt to anthraquinone. Write equations for the reactions. Up until recently the Chinese vatted Indigo Blue with soda and glucose. What reaction would likely result here? What are the structural requirements for enolization? 80.

PREPARATION OF 1,4-NAPHTHOQUINONE

(P)

Dissolve 7.0 g. of chromic anhydride ( CrOa) in 35 ml. of ice cold 80% acetic acid in a 250 ml. beaker. CAUTION: avoid contact with chromic anhydride as it causes severe burns. To this slowly add with vigorous stirring, over a period of 5 minutes, a cold solution of 4.0 g. of naphthalene in 50 ml. of glacial acetic acid. Keeping the liquid in ice for the first hour, stir occasionally for the remainder of the period and allow to stand until the next laboratory period. Pour with vigorous stirring into 500 ml. of ice water. Allow to stand for one hour. Rinse the beaker with 25 ml. dilute acetic acid and add this to the drowned mass. Filter off the precipitate, wash acid-free with cold water, air-dry, determine its melting point and hand it in. Write the equation for the reaction. To what is the colour of the product due? Polynuclear aromatic hydrocarbons generally oxidize in the manner illustrated by this experiment.

12. MOLECULAR REARRANGEMENTS Many organic compounds undergo reactions that result in a reorganization of their molecular structure. In this section we will consider three such reactions. These are the Pinacol-Pinacolone and the Beckmann Rearrangements and the Hofmann Degradation. Pinacol-Pinacolone Rearrangement HO OH ( CH3)2

C-C (CH3 )2 +

H2S04

__,,.

(1)

Beckmann Rearrangement N,OH (CH3'3CCCH3

+

PCl5

---+

(2)

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111

Hofmann Degradation (3) While these rearrangements may appear different, the products can all be derived by means of a migration of an alkyl ( or sometimes an aryl) group from a carbon atom to an adjacent electron deficient atom as illustrated by equation 4.

The Pinacol-Pinacolone Rearrangement is an excellent example of this type of migration. The first step is protonation of the alcohol followed by the loss of water to form carbonium ion I. HO OH (CH3)2C-C(CH3)2

+

+

H2SO4

~

H2 o OH (CH3)2C-C(CH3)2

~

+ 9H

(CH3)2C-C(CH3)2

(I) +

OH ( CH3)2 ~- C-CH3 CH!

~

OH {CH3)2 ~-C-CH3 CH 3

(II)

Carbonium ion I is transformed into the more stable ion II by the migration of a methyl group. Loss of a proton by carbonium ion II results in formation of the final product. An analogous mechanism can be postulated for the Beckmann Rearrangement. The first step is the reaction of the oxime with PC15 to form a phosphorus ester which can ionize to the ion III N,.OPCl4

WOH (CH3bCCCH3

+

PCl5

~

(CH3)3ctcH3

+

( CH 3 l3 CN= C CH 3

+

~

HzO

~

0 ( CH3l3 CNHCCH3

(IV) Ion III is transformed into the more stable ion IV by the migration of the tertiary butyl group. Hydrolysis and workup gives as the final product an N-substituted amide. Again a similar mechanism can be postulated for the Hofmann Rearrangement. The first step is formation of the N-bromo compound from which the base abstracts a proton to form the ion V.

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113

-

OH

~

(V)

(VI)

(VII)

Ion V loses a bromide ion to form intermediate VI containing a neutral but electron deficient nitrogen. Migration of the alkyl group to the nitrogen results in the formation of the isocyanate VII which under the reaction conditions hydrolyses to the amine. While the reaction conditions of the three examples vary widely as do the preliminary steps, all three mechanisms have in common the migration of a group from a carbon atom to an electron deficient atom. Experimental 81.

PINACOL-PINACOLONE REARRANGEMENT (P)

(a) Preparation of Pinacol NOTE. The success of this experiment depends upon maintaining anhydrous conditions. In a 250 ml. Erlenmeyer flask fitted with a reflux condenser, place 4 g. of dry magnesium turnings and 40 ml. of dry benzene. Clamp the flask and condenser over a warm-water bath. Add through the condenser about 5 ml. at a time, 30 ml. of a 20% solution of mercuric chloride in dry acetone. If the reaction does not commence within a few minutes of the addition of the first portion as evidenced by a vigorous ebullition, warm the flask carefully on a water bath and be ready to cool it with running water should the reaction become too vigorous. Once the reaction has commenced, it is rarely necessary to supply further heat. Add the remainder of the mercuric chloride solution at such a rate that the reaction is as vigorous as possible and yet under control. After the addition of the mercuric chloride solution, add while still refluxing, a solution of 10 ml. of dry acetone and 10 ml. of dry benzene. When the reaction begins to subside reflux on the water bath for one hour. During this period the magnesium pinacolate which has been formed, swells up into a grayish-white spongy mass. This should be shaken vigorously from time to time over the period of another half-hour refluxing. Then add through the condenser, 12 ml. of water and 12 ml. of ordinary benzene and heat again on the water bath for half an hour with occasional shaking. This hydrolyses the magnesium pinacolate to pinacol which passes into the benzene solution.

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Pour off the supernatant liquid through a filter paper. Keep as much as possible of the solid in the flask. Extract this solid by refluxing it with 25 ml. of ordinary benzene for about five minutes, then filter while still hot and add it to the first filtrate. Place the combined filtrates in the distillation apparatus and distill to half the original volume to remove most of the unchanged acetone. Transfer the liquid in the flask ( not the distillate) to a wide mouth Erlenmeyer flask, and add 12 ml. of water with vigorous stirring. Cool in an ice bath, stopper and allow to sit until the next laboratory period. Filter the product by suction. Remove the crystals from the funnel and dry them as well as possible by pressing carefully between layers of filter paper. Store the product in a well-stoppered container as it sublimes easily. ( b) Preparation of Pinacolone Into a 250 ml. Erlenmeyer flask pour 40 ml. of reagent dilute H2SO4. Dissolve the pinacol ( or pinacol hydrate) previously prepared ( Experiment 81a) in this dilute acid. Attach a reflux condenser and boil for about five minutes observing any changes which take place. Cool until the boiling ceases, transfer to a 50 ml. distilling flask and distil off about one-third of the liquid. The distillate should consist of two layers. Separate the upper layer of distillate ( which contains the crude pinacolone) by means of a separatory funnel. Add 2 g. of anhydrous magnesium sulfate. Distil the crude pinacolone carefully. A few ml. of forerun will be collected at or near 70° C. The temperature will gradually rise and the pinacolone may be collected from 95 to 110° C. Hand in the product. 82. BECKMANN REARRANGEMENT (a) Preparation of Acetophenone Oxime In a 125 ml. Erlenmeyer flask place 3 ml. of acetophenone, 2 g. of hydroxylamine hydrochloride, 2.5 g. of sodium acetate, 20 ml. of water, and 5 ml. of 95% ethanol. Heat the solution under reflux for ten minutes and allow to cool to room temperature. If crystals do not form cool the flask in an ice bath and attempt to induce crystallization by scratching the sides of the flask with a stirring rod. After crystallization begins, keep the flask in an ice bath for 30 minutes. Collect the solid by suction filtration. Recrystallize the solid from water and determine its melting point. ( b) Beckmann Rearrangement (P) Place 2 ml. of concentrated sulphuric acid in a 25 ml. Erlenmeyer flask Heat on the hot plate until the temperature of the acid is at least 90° C. no NOT BOIL THE ACID. Add 2.0 g. of acetophenone oxime in SMALL PORTIONS while swirling the flask. CAUTION: A vigorous reaction occurs after each addition. Wait for the reaction to subside before adding another portion. After all the oxime has been added keep the acid solution at 90° C. for an additional fifteen minutes. Cool and then pour the contents of the flask onto approximately 50 g. of crushed ice. When the ice has melted, collect the product by suction filtration. Recrystallize the product from water, determine its melting point, and hand it in.

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83. THE HOFMANN DEGRADATION ( p) In a 50 ml. distilling flask place 8 ml. of 30% sodium hydroxide solution, and cool well in ice-water. In the hood, add through a funnel, I ml. bromine ( CAUTION! GOGGLES!). Shake, and cool again in ice-water. Add I g. acetamide, stopper with a cork, and allow the end of the side arm to dip into 10 ml. of distilled water in an open test tube. Heat carefully with a small, moving flame until the mixture becomes clear and colourless and a vigorous evolution of vapours ensues. Remove the flame from time to time until the main action has subsided. Test the reaction of the aqueous distillate to red litmus paper. To the distillate add 10 ml. of a saturated aqueous solution of picric acid, evaporate to half volume, cool and allow to stand covered with a watch glass until the next laboratory period. Filter off the methyl amine picrate, determine its melting point, and hand it in.

13. POLYMERS AND POLYMERIZATION Polymerization is a chemical reaction which results in the combination of a large number of molecules of a simple structure, called monomers to form a giant molecule called a polymer. The formation of polymers is of great industrial importance. The mechanism of polymerization processes is similar to that previously discussed for reactions leading to compounds of relatively low molecular weight. They can be divided into two classes; condensation and addition polymerization reactions. Condensation polymerization reactions are fundamentally nucleophilic addition reactions. The mechanism of most of these reactions is similar to that proposed for the formation of esters and the addition of amines to a carbonyl group. An example of this type of polymer is the glyptal or alkyd resins which are polyesters ( eq. I).

An example of an addition polymerization reaction is the polymerization of ethylene ( eq. 2). (2) Reactions of this type can proceed by three different mechanisms: these are free radical, cationic and anionic catalyzed reactions.

117

83. THE HOFMANN DEGRADATION ( p) In a 50 ml. distilling flask place 8 ml. of 30% sodium hydroxide solution, and cool well in ice-water. In the hood, add through a funnel, I ml. bromine ( CAUTION! GOGGLES!). Shake, and cool again in ice-water. Add I g. acetamide, stopper with a cork, and allow the end of the side arm to dip into 10 ml. of distilled water in an open test tube. Heat carefully with a small, moving flame until the mixture becomes clear and colourless and a vigorous evolution of vapours ensues. Remove the flame from time to time until the main action has subsided. Test the reaction of the aqueous distillate to red litmus paper. To the distillate add 10 ml. of a saturated aqueous solution of picric acid, evaporate to half volume, cool and allow to stand covered with a watch glass until the next laboratory period. Filter off the methyl amine picrate, determine its melting point, and hand it in.

13. POLYMERS AND POLYMERIZATION Polymerization is a chemical reaction which results in the combination of a large number of molecules of a simple structure, called monomers to form a giant molecule called a polymer. The formation of polymers is of great industrial importance. The mechanism of polymerization processes is similar to that previously discussed for reactions leading to compounds of relatively low molecular weight. They can be divided into two classes; condensation and addition polymerization reactions. Condensation polymerization reactions are fundamentally nucleophilic addition reactions. The mechanism of most of these reactions is similar to that proposed for the formation of esters and the addition of amines to a carbonyl group. An example of this type of polymer is the glyptal or alkyd resins which are polyesters ( eq. I).

An example of an addition polymerization reaction is the polymerization of ethylene ( eq. 2). (2) Reactions of this type can proceed by three different mechanisms: these are free radical, cationic and anionic catalyzed reactions.

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The mechanism of free radical polymerization is similar to the mechanism of the free radical chlorination of alkanes ( q.v.). Instead of light initiating the reaction, however, a compound which readily forms free radicals is added. Benzoyl peroxide is frequently used since upon gently heating it (3) (4)

forms two benzoyloxy radicals ( eq. 3). These radicals add to a molecule of ethylene to produce a new radical ( eq. 4). This process, called propagation, continues until a radical of high molecular weight is formed ( eq. 5). Termination of the chain process occurs when two large polymer radicals combine to form polyethylene ( eq. 6). Polymerization can also occur when Lewis acids are used as initiators ( cationic polymerization). In this case carbonium ions are formed ( eq. 7) and act as the chain carrier in the propagation step ( eq. 8). Chain termination occurs whenever a carbonium ion loses a proton, to form an olefin (eq. 9). (7) (8)

Polymerization by simple anions is also possible but usually leads to the formation of polymers of low molecular weight. As a result simple anionic initiators are little used in industry. However, certain organometallic compounds, which may be anionic in nature, have been found to be extremely useful. Thus the Ziegler catalyst ( a mixture of aluminum trialkyl and titanium tetrachloride) is extensively used in industry. Very little is known of the actual mechanism of polymerization initiated by the Ziegler catalyst. The experiments in this section will serve to illustrate condensation, cationic and free radical addition polymerization. Experimental 84.

CONDENSATION POLYMERIZATION OF BENZYL ALCOHOL (P)

In a small Erlenmeyer flask place 25 ml. of concentrated sulphuric acid.

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Then add carefully, with brisk stirring (GOGGLES!), 3.0 ml. of benzyl alcohol (density= 1.04 g./ml.). Cool the resulting system in running water, stirring to break up any lumps, and allow to stand at room temperature for about ten minutes. Then drown with vigorous stirring in about 300 ml. water. Break up any lumps as well as possible, filter by suction and wash acid-free with water. Dry the polymeric hydrocarbon. Write a portion of the structure assigned to it. Hand in the product. 85. FORMATION OF A GLYPTAL RESIN (P) In a crucible on an asbestos mat, over a low flame, heat a mixture of 3 g. powdered phthalic anhydride and I ml. glycerol plus a pinch of anhydrous sodium acetate. Stir occasionally and heat gently as long as water vapour is being evolved ( about ½ hour). Then, without charring, quickly raise the temperature of the system until large bubbles form and the liquid puffs up into a voluminous sticky mass. Allow to cool and powder the polymer. Write a portion of the structure attributed to it. Hand in the product. 86. PREPARATION OF A PHENOL RESIN Take I gram of hexamethylenetetramine made in a previous experiment

( Exp. 48), and mix thoroughly with an equal amount of phenol. Heat

evenly and gently in a crucible over a low flame, being careful not to char. Allow to cool. Result? Can the product be made plastic again on heating? This is the procedure for making the Bakelite type of resin. It consists in causing certain aldehydes and phenols to react in the presence of mild alkalis. Write a portion of the structure attributed to this resin. 87. PREPARATION OF CELLULOSE ACETATE (P) In a small flask, place 15 ml. of glacial ( 100%) acetic acid, 10 ml. of acetic anhydride, 10 drops of concentrated sulphuric acid and two 10 cm. filter papers, tom into small pieces. Stir, so that most of the air bubbles are removed. Stopper the flask with a cork and let stand until the next laboratory period. Then pour the liquid in a thin stream, and with stirring, into about 300 ml. of cold water. Filter the resulting slurry through a towel and squeeze out as much liquid as possible, wash free of acid, set aside in a warm place to dry. When dry, put about one-third of the product in a test tube, and add about 10 ml. chloroform. Cork and allow to stand with occasional shaking. When solution has been effected, pour some onto a clean watch glass and allow to evaporate slowly. When dry, lift the edges of the film, and remove it slowly from the glass. Will it ignite readily? A modification of this procedure is used in the manufacture of motion picture film. Hand in the remainder of the crude product.

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88. DEPOLYMERIZATION OF LUCITE Place 5 g. methyl methacrylate polymer (Lucite) in a small ( 50 ml.) distilling flask and attach to a water-condenser in which no water is circulating. Use a test tube surrounded by water as a receiver. The inside of the condenser tube must be dry. Heat with a small luminous flame ( no asbestos mat!), keeping the flame constantly moving around the bottom half of the flask. At about 300° the polymer will soften and undergo rapid depolymerization into the monomer which will distil over. Avoid local over-heating! Continue the distillation with a larger flame until the drops of distillate become yellow (charring). Depolymerization by heat is a common property of addition polymers. ( Compare paraformaldehyde-

Exp. 50a.)

89. POLYMERIZATION OF METHYL METHACRYLATE (P) CAUTION! Benzoyl peroxide is a very dangerous explosive and should not be brought near any warm object or touched with anything which is warm. Great care must be exercised when working with it! To the monomer obtained in the previous experiment, add a very small amount (just sufficient to cover the end of a knife blade) of benzoyl peroxide, shake, stopper the tube loosely and heat in a boiling water-bath for ten minutes. Allow to stand until the next laboratory period. Hand in the test tube containing this polymer. Write a suitable structure for a portion of the Lucite molecule. 90. PREPARATION OF A PoLYSULPIIlDE RUBBER-THIOKOL TYPE (P) Dissolve 2 g. of sodium hydroxide in 50 ml. of water contained in a 250 ml. beaker. Heat this solution to boiling, add 4 g. of powdered sulphur and stir until all the sulphur has dissolved. As the polysulphide is formed, the solution turns from light yellow to dark brown. Cool the solution to 70° and add 10 ml. of ethylene dichloride ( density = 1.26 g. / ml. ) . Stir the mixture vigorously so that the ethylene dichloride is suspended in the polysulphide mixture. The "synthetic" rubber slowly forms at the junction between the two liquids and collects a spongy lump at the bottom of the beaker as stirring is continued. After the reaction has been completed, remove the white-to-yellow rubber-like material from the solution, and wash thoroughly. Dry the polymer and hand in a portion of it.

14. CARBOHYDRATES Carbohydrates are polyhydroxy aldehydes or ketones. They can be subdivided into: monosaccharides ( which cannot be further hydrolysed) such

123

88. DEPOLYMERIZATION OF LUCITE Place 5 g. methyl methacrylate polymer (Lucite) in a small ( 50 ml.) distilling flask and attach to a water-condenser in which no water is circulating. Use a test tube surrounded by water as a receiver. The inside of the condenser tube must be dry. Heat with a small luminous flame ( no asbestos mat!), keeping the flame constantly moving around the bottom half of the flask. At about 300° the polymer will soften and undergo rapid depolymerization into the monomer which will distil over. Avoid local over-heating! Continue the distillation with a larger flame until the drops of distillate become yellow (charring). Depolymerization by heat is a common property of addition polymers. ( Compare paraformaldehyde-

Exp. 50a.)

89. POLYMERIZATION OF METHYL METHACRYLATE (P) CAUTION! Benzoyl peroxide is a very dangerous explosive and should not be brought near any warm object or touched with anything which is warm. Great care must be exercised when working with it! To the monomer obtained in the previous experiment, add a very small amount (just sufficient to cover the end of a knife blade) of benzoyl peroxide, shake, stopper the tube loosely and heat in a boiling water-bath for ten minutes. Allow to stand until the next laboratory period. Hand in the test tube containing this polymer. Write a suitable structure for a portion of the Lucite molecule. 90. PREPARATION OF A PoLYSULPIIlDE RUBBER-THIOKOL TYPE (P) Dissolve 2 g. of sodium hydroxide in 50 ml. of water contained in a 250 ml. beaker. Heat this solution to boiling, add 4 g. of powdered sulphur and stir until all the sulphur has dissolved. As the polysulphide is formed, the solution turns from light yellow to dark brown. Cool the solution to 70° and add 10 ml. of ethylene dichloride ( density = 1.26 g. / ml. ) . Stir the mixture vigorously so that the ethylene dichloride is suspended in the polysulphide mixture. The "synthetic" rubber slowly forms at the junction between the two liquids and collects a spongy lump at the bottom of the beaker as stirring is continued. After the reaction has been completed, remove the white-to-yellow rubber-like material from the solution, and wash thoroughly. Dry the polymer and hand in a portion of it.

14. CARBOHYDRATES Carbohydrates are polyhydroxy aldehydes or ketones. They can be subdivided into: monosaccharides ( which cannot be further hydrolysed) such

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as glucose and fructose; disaccharides (anhydrides of two monosaccharides) such as sucrose, maltose and lactose; polysaccharides ( condensation polymers of monosaccharides) such as starch and cellulose. The capital letters D and L are used to indicate the relation of sugars to D and L glyceraldehyde. Monosaccharides are either aldoses ( aldehyde group present) or ketoses ( ketone group present). The number of carbons is indicated by Greek numbers, e.g. an aldohexose contains six carbons. Most carbohydrates are now thought to be closed-ring structures having either a pyranose structure ( 5 carbons, 1 oxygen), or a furanose structure ( 4 carbons, 1 oxygen). Carbohydrates exhibit the reactions of aldehydes, ketones and polyhydroxy alcohols. They can be oxidized, reduced esterified and they form osazones (hydrazones) with phenylhydrazines. Experimental 91.

MoLISCH TEST FOR CARBOHYDRATES

In five separate test tubes place 1 ml. of 20% aqueous solutions of glucose, sucrose, maltose, lactose and fructose. Add to each 4 ml. of water and 10 drops of Molisch's reagent, ( alpha-naphthol in ethanol). Shake to mix. Incline the tube at 45° to the vertical and allow about 3 ml. of concentrated sulphuric acid to flow down the side of the tube thus forming a layer of acid beneath the sugar solution. Note the appearance of any colour at the interface. The colour is due to a combination of a furfural derivative ( formed by the action of the acid on the carbohydrate) and a-naphthol. 92.

TESTS FOR REDUCING SUGARS

(a) Benedict's Test Boil 5 ml. of Benedict's reagent; it should remain clear. Into each of five labelled test tubes, put 5 ml. of Benedict's solution, and add 10 drops of the five sugar solutions provided respectively to the test tubes. Place in a beaker of boiling water, and observe after five minutes. Write the equations for the reaction between Benedict's reagent and glucose assuming the active part of the former to be Cu++ ion. (b) Nylander's Test Nylander's solution is a solution of bismuth hydroxide in concentrated sodium hydroxide forming sodium bismuthate (NaBiOs). It is used in the same manner as Benedict's and reducing sugars reduce it to metallic bismuth which comes down as a fine black precipitate. Using the same procedure as in (a) test the five sugar solutions with it. Write the equation for the reaction between glucose and Nylander's solution.

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( c) T ollewl Test Clean the inside of a test tube by boiling some reagent sodium hydroxide to dissolve out all grease. Then rinse the tube out thoroughly, and place in it about 10 ml. of Tollens' ammoniacal silver solution and add 2 drops reagent sodium hydroxide solution. To this add 20 drops glucose solution, shake, then immerse in half a beaker of boiling water. Any reducing sugar will give a silver mirror in this manner. Write an equation for this reaction. 93.

EsTERIFICATION OF GLUCOSE BY BENZOYL CHLORIDE ( p)

In a 125 ml. stoppered Erlenmeyer, dissolve 2.0 g. dextrose in 25 ml. of reagent sodium hydroxide solution and then in a fume hood add 3 ml. of benzoyl chloride. (N.B. Benzoyl chloride is a power lachrymator!) Shake vigorously until the odour of benzoyl chloride has disappeared. Then drown in 250 ml. dilute hydrochloric acid, filter off the glucose pentabenzoate, wash twice with 15 ml. portions of cold water, dry in the air, and hand in. Write equation.

94.

FORMATION OF OSAZONES

Into three test tubes, place respectively 1 ml. of freshly prepared 20% solutions of glucose, maltose and lactose. Dilute each to 10 ml. with water, and to each add 3 ml. 10% phenylhydrazine hydrochloride solution and sufficient anhydrous sodium acetate to cover a twenty-five cent piece. Place in a beaker of boiling water and observe the comparative rates of appearance of the yellow crystals of the osazones, recording the order in which they appear. If they do not form in the heated solution within 5 minutes, cool it. Make a drawing of each type of crystalline osazone as viewed under the microscope provided. Write the equation for the formation of glucosazone. Write a tautomeric structure for the product which would account for the fact that it is coloured. Indicate why D-glucose and D-fructose should give the same osazone.

95.

HYDROLYSIS OF SUCROSE

To about 5 ml. of sucrose solution add about 3 drops of concentrated hydrochloric acid and boil for a few minutes. Cool, make alkaline with sodium hydroxide and test with 5 ml. boiling Benedict's solution. As a control, test the sucrose solution with Benedict's solution before boiling and with no addition of acid ( Experiment 92a). Write the equation for the hydrolysis of cane sugar using structural formulas. What monosaccharides are produced? Why is this process termed "inversion"? 96.

OXIDATION OF LACTOSE

(P)

Lactose on oxidation with nitric acid forms the two dibasic sugar-acids, mucic and D-saccharic, as well as decomposition products, e.g. oxalic acid.

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In a beaker on a water bath, heat 8 g. lactose with 75 ml. dilute reagent nitric acid, keeping the system at 70-80° for about one hour or until the evolution of oxides of nitrogen has ceased ( FUME HOOD). Evaporate to half volume on an asbestos mat. Allow to cool until next laboratory period. Filter off the mucic acid, wash once with 10 cc. methanol, dry, and hand in. Allow the filtrate to evaporate at room temperature in an evaporating dish and note whether oxalic acid and D-saccharic acid crystallize out. Write the formula for lactose and indicate from what portions of the structure the above mentioned three acids were probably derived. Why does mucic acid bear no D- or L-designation? Save the mucic acid for a subsequent experiment when it is returned to you. 97. DETERMINATION OF OPTICAL ROTATION Fill a polarimeter tube with a 5% sucrose solution and determine its observed optical rotation in a polarimeter. Make a sketch of the apparatus showing the essential parts. Take at least five readings. Calculate the specific rotation of sucrose as defined by this expression and compare it with the value recorded in the literature. alpha = observed rotation [a]2"o _ ..!!.. 1 = tube length in dm 0 cl c = grams solute per 100 ml. of solution 98. HYDROLYSIS OF STARCH (a) H ydrolysis by Acids Test 2 ml. starch suspension with 5 ml. boiling Benedict's solution. Heat 5 ml. of the starch paste with 1 ml. of dilute hydrochloric acid and at the end of every half minute withdraw a drop on a stirring rod, and touch it to a drop of very dilute iodine solution ( 0.01%) on a spot plate. Note the different shades of red imparted to the iodine as the starch passes through the dextrin stages. When the product becomes a clear solution, neutralize it, and test with an equal quantity of hot Benedict's solution. Indicate some of the recognizable stages in the hydrolysis of starch. What is the monosaccharide formed by its degradation? Why is this an important chemical reaction from the standpoint of nutrition?

( b) Hydrolysis by Saliva To 5 ml. of starch suspension, add about 2 ml. of fresh saliva (your own)

and immerse in a beaker of warm water for about ten minutes. Then test separate portions for the presence of ( i) starch ( see (a) ) , and (ii) reducing sugars ( Experiment 92). What is the enzyme in saliva which is responsible for this change? 99. PREPARATION OF CELLULOSE HEXANITRATE (GUNCO'ITON) GOGGLES! Into a beaker carefully pour 15 ml. of concentrated sulphuric

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acid into 15 ml. concentrated nitric acid. To this liquid, after cooling for several minutes, add about 1 g. absorbent cotton, and stir occasionally. After about seven minutes, withdraw the nitrated cotton, and remove most of the acid by pressing it with a glass rod against the side of the beaker. Transfer the nitrated cotton to a beaker containing 300 ml. cold water, wash until neutral in running water (litmus), squeezing out from time to time. Pull out and set aside to dry in the air until next day. When dry, CAREFULLY ignite it on your asbestos mat by means of your gas lighter. Why is the product called "hexanitrate"? Show that there is sufficient oxygen content in the molecule to completely bum the carbon and hydrogen content. What are some of the products of combustion of cellulose hexanitrate? What are some uses of the nitrates of cellulose? 100. PREPARATION OF CELLULOSE XANTHATE (VISCOSE) In a beaker add two filter papers, tom in shreds, to 15 ml. of cold 20% sodium hydroxide solution, and stir thoroughly. Cover the beaker and allow to stand for two hours. Then express as much of the sodium hydroxide as possible from the pulp, and place the pulp or "alkali-cellulose" loosely in a corked Erlenmeyer. Add 10 ml. of carbon disulphide (KEEP FLAMES AWAY) and stopper tightly. Put away to "age" until the next laboratory period. Note any change in colour. Pour off any unchanged carbon disulphide, then add 10 ml. of a 3% solution of sodium hydroxide and allow to stand for one hour, stirring occasionally. Add 5 ml. of water and stir until a smooth uniform product results. This is the viscose solution. Squirt some of the viscose solution into about 200 ml. of dilute sulphuric acid and note the regeneration of cellulose hydrate. This is the basis of the manufacture of viscose rayon, cellophane, cellulose sponges, cellulose sausage casings, etc. 101. HYDROLYSIS OF CELLULOSE Grind a 10 cm. filter paper carefully with about 1 ml. concentrated sulphuric acid in a mortar until a sticky mass is obtained. Cautiously add 5 ml. water, transfer to a test tube, and boil the resulting liquid for about two minutes. Neutralize the solution with reagent sodium hydroxide and test for a reducing sugar with 5 ml. boiling Benedict's solution. Indicate some of the recognizable stages in the hydrolysis of cellulose. Write a portion of the structure attributed to cellulose and indicate the recurring cellobiose unit therein. 102. REACTIONS OF AscoRBIC Aero (a) To 10 ml. of a 0.01% solution of 2,6-dichlorophenol indophenol add a few drops of glacial acetic acid until the red acid colour of the indicator persists. To this liquid add 1 ml. orange juice. Conclusion? Equations? ( b) Dissolve sufficient crystalline Vitamin C to cover the end of a knife

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blade in 5 ml. water and add 0.5% iodine solution, drop by drop, until 1 ml. has been added. Conclusion? Equation? ( c) Repeat ( b) but this time use tenth normal potassium permanganate acidified with H2S04 instead of iodine, adding it drop by drop as long as it is decolorized. This oxidation disrupts the Vitamin C molecule at the endiol double bond. Equation? 103.

HYDROLYSIS OF GUMS

To separate portions of about 0.1 g. gum acacia and 0.1 g. gum tragacanth add 5 ml. concentrated hydrochloric acid and heat for five minutes on the water bath. When cool, place about 10 drops of each separately in an evaporating dish, and to this with stirring, add freshly distilled aniline, one drop at a time, until about 5 drops have been added. Conclusion? What are the carbohydrates obtained on hydrolysis? They dehydrate to furfural or derivatives which give colour reactions with aromatic amines and phenols. Equations? 104.

TEST FOR LIGNIN AND GuMs IN

Wooo

With a stirring rod touch ( i) a 5% solution of resorcinol in concentrated hydrochloric acid and (ii) a 5% solution of aniline in hydrochloric acid separately to a piece of newsprint. Do likewise to a piece of filter paper. Explain the results.

15. PROTEINS Proteins are complex nitrogen containing organic compounds containing amino acids as the basic unit. These acids are joined together by primary bonds, called peptide bonds, to form a long chain. Proteins contain both acid and basic groups and so exhibit amphoteric behaviour. Some amino acids contain sulphur and disulfide linkages. The so-called tests for proteins are really tests for characteristic amino acids usually formed in proteins.

105.

Experimental BIURET REACTION

Heat about 0.5 g. urea in a small test tube until bubbles of gas are seen escaping, then hold a piece of moist red litmus in the test tube. Cool, add 5 ml. 20% sodium hydroxide solution, then Si drops of 1% copper sulphate solution. This colour reaction is known as the biuret reaction and is given by substances whose structures contain two or more peptide linkages. Write equations for the heating of urea as above, and indicate the peptide linkages in the products.

133

blade in 5 ml. water and add 0.5% iodine solution, drop by drop, until 1 ml. has been added. Conclusion? Equation? ( c) Repeat ( b) but this time use tenth normal potassium permanganate acidified with H2S04 instead of iodine, adding it drop by drop as long as it is decolorized. This oxidation disrupts the Vitamin C molecule at the endiol double bond. Equation? 103.

HYDROLYSIS OF GUMS

To separate portions of about 0.1 g. gum acacia and 0.1 g. gum tragacanth add 5 ml. concentrated hydrochloric acid and heat for five minutes on the water bath. When cool, place about 10 drops of each separately in an evaporating dish, and to this with stirring, add freshly distilled aniline, one drop at a time, until about 5 drops have been added. Conclusion? What are the carbohydrates obtained on hydrolysis? They dehydrate to furfural or derivatives which give colour reactions with aromatic amines and phenols. Equations? 104.

TEST FOR LIGNIN AND GuMs IN

Wooo

With a stirring rod touch ( i) a 5% solution of resorcinol in concentrated hydrochloric acid and (ii) a 5% solution of aniline in hydrochloric acid separately to a piece of newsprint. Do likewise to a piece of filter paper. Explain the results.

15. PROTEINS Proteins are complex nitrogen containing organic compounds containing amino acids as the basic unit. These acids are joined together by primary bonds, called peptide bonds, to form a long chain. Proteins contain both acid and basic groups and so exhibit amphoteric behaviour. Some amino acids contain sulphur and disulfide linkages. The so-called tests for proteins are really tests for characteristic amino acids usually formed in proteins.

105.

Experimental BIURET REACTION

Heat about 0.5 g. urea in a small test tube until bubbles of gas are seen escaping, then hold a piece of moist red litmus in the test tube. Cool, add 5 ml. 20% sodium hydroxide solution, then Si drops of 1% copper sulphate solution. This colour reaction is known as the biuret reaction and is given by substances whose structures contain two or more peptide linkages. Write equations for the heating of urea as above, and indicate the peptide linkages in the products.

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135 To 1 ml. of casein suspension, add 3 ml. of a 20% sodium hydroxide solution, shake, and add 5 drops of a 1% copper sulphate solution. Conclusion? 106.

XANTHOPROTEIC REACTION

To 2 ml. of the egg albumin or casein suspension, add 3 drops of concentrated nitric acid, and allow to stand for five minutes. Then heat to boiling. Cool and make alkaline with ammonium hydroxide or sodium hydroxide solution. Record all your observations. This test indicates the presence of the phenyl-particularly a hydroxy or amino phenyl group in a protein. What type of reaction is it? 107.

HOPKINS-COLE REACTION-TEST FOR TRYPTOPHANE

To 2 ml. of casein suspension, add 5 drops of Hopkins-Cole reagent ( glyoxylic acid). Mix well, then pour down the side of the tube, held on a slant, 3 ml. concentrated sulphuric acid so that two layers are formed. Observe at the end of several minutes. Write the structure for tryptophane. Its systematic name is ,B-3-indole-alpha-aminopropionic acid. 108.

TEST FOR SULPHUR IN PROTEINS

To sufficient powdered casein to cover the bottom of a test tube, add 10 ml. reagent sodium hydroxide and 10 drops lead acetate solution. Carefully bring to boil and interpret the result. Write the names and formulas for three amino acids containing sulphur which result from the hydrolysis of proteins.

109.

MILLoN's REACTION-TEST FOR HYDROXYPHENYL GROUP

Heat 3 ml. of Millon's reagent (solution of mercuric nitrate and nitrite in dilute nitric acid) with sufficient powdered casein to cover the bottom of a test tube. The development of a red colour or precipitate is an indication of the presence of the hydroxy phenyl group. Write the structure for tyrosine and show that it contains such a group. 110.

NINHYDRIN REACTION

This is a very delicate test for alpha amino acids or proteins in which at least one alpha amino group is free. Allow sufficient (a) glycine and ( b) alanine to cover the bottom of a test tube to stand with 3 ml. 5% aqueous Ninhydrin reagent ( triketohydrindene hydrate) for ten minutes and observe the development of the blue to violetred colour in each case. Ascertain whether casein contains a sufficient number of free alpha amino groups to give this test.

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INDEX

All numbers refer to experiments

Acetaldehyde, 28, 50 Acetamide, 55 Acetanilide,53,54, 82 Acetic acid, 52 Acetone, 29 Acetone Sodium Bisulphite, 43 Acetophenone oxime, 82 Acetylsalicylic acid, 59 Acids, 52, 61 Acrolein, 39 Alcohols, 24-27 Aldehydes, 36--50 Aldol reaction, 49 Alkyl halides, 19-23 Amines, 64 Aminophenols, 73 Aniline, 62, 63, 67 Aniline dyes, 67 Anthraquinone, 79 Ascorbic acid, 102 Azeotropic solution, 7 Baeyer's test, 17 Beckman rearrangement, 82 Beilstein's test, 15 Benedict's test, 92 Benzaldehyde, 41 Benzene, 31 Benzenediazonium chloride, 65 Benzenesulphonamide, 64 Benzhydrol, 42 Benzoic acid, 5, 51 1,4-Benzoquinone, 72 Benzyl alcohol, 51, 84 Biuret reaction, 105 Boiling point, 6 I-Butene, 23 n-Butyl bromide, 22, 23 Carbohydrates, 91-104 Carboxylic acids, 52, 61 Cellulose, 101 Cellulose acetate, 87 Cellulose hexanitrate, 99 Cellulose xanthate, 100 Chromatography, 8, 9 Cinnamic aldehyde, 47 Condensation polymers, 84, 85, 86 Cyanohydrin reaction, 44 Dehydrogenation, 38 Depolymerization, 88 2, 6-Dibenzalcyclohexanone, 49 m-Dinitrobenzene, 32 Diphenylcarbinol, 42 Distribution coefficient, 10 Dyes, 66, 67

Emeraldine, 67 Enols, 77 Ethanol, 24 Ethene, 16 Ethyl acetate, 60 Ethylene, 16 Ethyl, 3, 5-dinitrobenzoate, 30 Eutectic, 3 Extraction, 11 Fehling's test, 36 Fermentation, 24 Formaldehyde, 50 Fractional distillation, 7 Free radical, 35 Friedel-Craft reaction, 34 Glucosazone, 94 Glucose, 24, 94 Glucose pentabenzoate, 93 Glycerol, 39, 85 Glyptal resin, 85 Griess reaction, 65 Grignard reaction, 51 Gums (acacia, tragacanth), 103, 104 Haloform reaction, 37 Halogenation, 26 Hexamethylenetetramine, 48, 86 Hexaphenylethane peroxide, 35 Hinsberg test, 64 Hippuric acid, 56 Hofmann reaction, 83 Hopkins-Cole reaction, 107 Hydrazones, 45 Hydrocarbons (Aliphatic), 16-18 Hydrolysis, 20 Hydroxylamine hydrochloride, 46, 82 Indamine, 67 Inversion, 95 Iodofonn test, 37 Isoamyl acetate, 58 Ketones, 36--50 Lactose, 94, 96 Lassaigne test, 13, 14, 15 Lignin, 104 Lucas test, 26 Lucite, 88 Maltose, 94 Mandelic Acid, 44 Melting point, 2, 3 MethyTamine picrate, 83 Methylmethacrylate, 89 Methyl-,8-naphthyl ether, 76 Methyl salicylate, 57 Millon's reaction, 109 Molisch test, 91

Mucic acid, 96 1, 4-Naphthoquinone, 80 Ninhydrin reaction, 110 m-Nitroaniline, 66 Nitrobenzene, 32 m-Nitrobenzene azo-p-naphthol, 66 Nitro compounds, 33 Nitrosation, 71 Nylander's test, 92 Optical rotation, 97 Osazones, 94 Oximes, 46 Perbenzoic acid, 41 Perkin's experiment, 68 Pheno hthalein, 75 Phenof resin, 86 Phenols, 65, 69-77 Phthalic anhydride, 75, 85 Pinacol, 81 Pinacolone, 81 Polymerization, 50, 84-90 Polysulphide rubber, 90 Potassium dienolate of benzoin, 78 Proteins, 105-110 Qualitative analysis, 12-15 Quinhydrone, 74 Recrystallization, 5

Reducing sugar, 92 Reflux ratio, 7 D-saccharic acid, 96 Saponincation, 60 f Schif s test, 36 Schmidlin's experiment, 35 Schotten-Baumann reaction, 56 Semicarbazones, 47 Solubility, 4, 61 Starch, 98 Sucrose, 95 p-Tertiarybutylphenol, 34 Thermometer, 1 Thymol, 71 Tollen's test, 92 2, 4, 6-Tribromoaniline, 63 2, 4, 6-Tribromophenol, 70 Trichloroacetic acid, 52 Tryptophane, 107 Tyrosine, 109 Unsaturation test, 17 Van Slyke reaction, 65 Vatting, 79 Vanillin oxime, 46 Vitamin C, 102 Xanthoproteic reaction, 106