Cyclic and Noncyclic Organic Compounds [1 ed.] 1527553027, 9781527553026

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Cyclic and Noncyclic Organic Compounds

Cyclic and Noncyclic Organic Compounds: A Textbook of Organic Chemistry By

V. M. Abbasov, C. G. Rasulov, A. M. Askerova and S. A. Ismailov

Cyclic and Noncyclic Organic Compounds: A Textbook of Organic Chemistry By V. M. Abbasov, C. G. Rasulov, A. M. Askerova and S. A. Ismailov This book first published 2024 Cambridge Scholars Publishing Lady Stephenson Library, Newcastle upon Tyne, NE6 2PA, UK British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Copyright © 2024 by V. M. Abbasov, C. G. Rasulov, A. M. Askerova and S. A. Ismailov All rights for this book reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owner. ISBN (10): 1-5275-5302-7 ISBN (13): 978-1-5275-5302-6

The book discusses the main classes of cyclic and non-cyclic organic compounds, their structure, properties and methods of preparation. In close connection with the material under discussion, information is presented on theoretical concepts, spectral characteristics, issues of stereochemistry, kinetics and thermodynamics, and the most important modern methods of synthesis and analysis. The textbook is intended for students of chemical faculties of universities and chemical universities.

CONTENTS

Preface ......................................................................................................... x Introduction ............................................................................................... 1 History of organic chemistry ....................................................................... 3 Chapter 1. Theoretical foundations ......................................................... 5 1. The subject of organic chemistry ............................................................. 5 2. The complexity and originality of organic compounds ........................... 5 3. Variety of practical applications of organic compounds ......................... 6 4. Theory of the structure of organic substances ......................................... 6 5. Sources of organic compounds ................................................................ 7 6. Classification of organic compounds....................................................... 8 7. Nomenclature of organic compounds .................................................... 12 Chapter 2. Chemical bonds..................................................................... 21 1. Nature and types of chemical bonds. Employment methods ................. 21 2. Parameters of covalent bonds. Hydrogen bond ..................................... 27 3. Delocalized chemical bonds .................................................................. 33 Chapter 3. Chemical reactions ............................................................... 40 1. Basic concepts, classification of reagents .............................................. 40 2. Types of organic reactions and their mechanisms ................................. 41 3. Acids and bases. Proton and electron theories ....................................... 44 4. Methods for the study of organic compounds ....................................... 50 Chapter 4. Alkanes .................................................................................. 57 1. Acyclic saturated hydrocarbons ............................................................. 57 2. Cyclic saturated hydrocarbons............................................................... 69 3. Detonation properties of hydrocarbons.................................................. 76 4. Natural sources of saturated hydrocarbons ............................................ 78 Chapter 5. Unsaturated hydrocarbons .................................................. 81 1. Alkenes .................................................................................................. 81 2. Alkadienes ............................................................................................. 93 3. Alkynes ................................................................................................ 105

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Chapter 6. Aromatic hydrocarbons ..................................................... 115 1. General hydrocarbons of the benzene series characteristic ................. 115 2. Chemical properties ............................................................................. 124 3. Industrial methods for obtaining aromatic hydrocarbons .................... 134 Chapter 7. Hydroxyl derivatives of hydrocarbons ............................. 137 1. Monohydric alcohols ........................................................................... 137 2. Polyhydric alcohols ............................................................................. 145 3. Ethers ................................................................................................... 150 4. Phenols ................................................................................................ 152 Chapter 8. Oxocompounds ................................................................... 161 1. Aldehydes and ketones ........................................................................ 161 2. Separate representatives of aldehydes and ketones ............................. 174 Chapter 9. Carboxylic acids and their derivatives ............................. 177 1. Saturated monobasic acids................................................................... 177 2. Physical properties............................................................................... 179 3. Chemical properties ............................................................................. 180 4. Individual representatives of carboxylic acids..................................... 193 Chapter 10. Amines ............................................................................... 195 1. Physical properties............................................................................... 197 2. Chemical properties ............................................................................. 198 3. Individual representatives and application .......................................... 205 Chapter 11. Amino acids ....................................................................... 206 1. Nomenclature and isomerism .............................................................. 206 2. Physical and chemical properties......................................................... 207 3. Polycondensation of amino acids ........................................................ 209 4. Acquisition of amino acids .................................................................. 210 Chapter 12. Carbohydrates (hydrocarbons) ....................................... 211 1. Discovery and role of carbohydrates in nature .................................... 211 2. Monosaccharides ................................................................................. 212 3. Chemical properties of glucose ........................................................... 215 4. Oligosaccharides.................................................................................. 221 5. Polysaccharides ................................................................................... 222 Chapter 13. Rearrangement of chemical molecule ............................. 229 1. Molecular rearrangement ..................................................................... 229 2. Skeletal rearrangement ........................................................................ 232

ix

Cyclic and Noncyclic Organic Compounds: A Textbook of Organic Chemistry

References ............................................................................................... 238 Subject index ........................................................................................... 240

PREFACE

You my dear friend I love chemistry Chemistry takes water from fire, Where is such a science? Sokhrab Ismailov

The selection and systematization of the material for this textbook intended for undergraduate students of chemistry faculties of higher schools is designed to lay the foundations of active knowledge in accordance with the main goal of the course, to guide modern research in chemistry and to work effectively in the chosen field. In order not to deviate from the main goal and to ensure brevity, the textbook includes information acquired by students in high school, as well as advanced areas of organic chemistry that have become the subject of special research (dyes and heterocycles, steroids, terpenes, large alicycles, compounds of some organic elements, oils and waxes and etc.). Information on theoretical concepts, spectral characteristics, issues of stereochemistry, kinetics and thermodynamics, the most important modern methods of synthesis and analysis is presented in close connection with the material under discussion and therefore is easier to digest. For example, the essence of IR spectroscopy and mass spectrometry, the use of the energy profile of a reaction to judge its mechanism has already been described in the section “Chemical Reactions”; in the “alkenes” section, students will get acquainted with some important provisions of stereochemical theory; in the “chemical bonds” section, the theory of molecular orbitals, UV spectroscopy, kinetic and thermodynamic control of reactions and the basics of reactions; in the “alkynes” section about CHacidity, conjugated acids and bases; in the section of alkyl halides, with SN1- and SN2-substitution mechanisms, on the role of solvent and ion pairs, on the principle of soft and hard acids and bases in the section of alcohols, in the section of esters and Į-oxides - the use of crown -ethers and interfacial catalysis in organic synthesis, etc., occurs. The student begins the consideration of cyclic compounds, already knowing the properties of functions. In the case of alicycles, first discussed in Part II, this approach allows us to study how ring size and steric

Cyclic and Noncyclic Organic Compounds: A Textbook of Organic Chemistry

xi

structure properties affect their reactivity. In the section on the simplest benzoid systems described after alicyclic compounds, the material is systematized by function, as in non-cyclic compounds. Particular attention is paid to the specific properties of the latter (different from the properties of non-aromatic compounds), as well as the effect of substituents on the aromatic nucleus. In order not to increase the volume of the tutorial, the section “Heterocyclic Compounds” is not included. Relevant material is usually covered in special courses. In almost all textbooks, a review of each class of organic compounds begins with a listing of methods for their synthesis. This breaks the logic of the presentation because it includes material that the student is not yet familiar with. For example, speaking of alkanes, the methods for their preparation at the initial stage are usually given as the action of water on organomagnesium compounds, the electrolysis of carbonic acid salts, the action of metallic sodium on alkyl halides. However, the relevant material is discussed in detail later when considering the properties of organomagnesium compounds, carboxylic acid salts and halogen derivatives. Therefore, in this case, the reader is directed not to understanding, but to remembering. However, if the class of compounds is considered in the middle or at the end of the course, it would still be inappropriate to single out methods of synthesis in a separate section. This is because the student is already familiar with most of the material when describing the properties of the previously considered classes of compounds. So, for example, when he begins to study amines, he already knows such methods for their preparation as the alkylation of ammonia with alkyl halides, alcohols and epoxides, the reductive amination of ketones, the rearrangement and reduction of ketone oximes, the Mannix reaction, and the addition of ammonia to the reaction.

INTRODUCTION

Organic chemistry is the study of carbon compounds, which is why organic chemistry is also called the chemistry of carbon compounds. Organic chemistry can also be defined as the chemistry of hydrocarbons and their derivatives. Although such a definition more clearly reflects the content of organic chemistry, it also does not allow for a sharp distinction between organic and inorganic substances. Since all phenomena in nature are connected with each other, it is naturally impossible to roughly separate one field of science from another: there are natural dialectical transitions between related sciences. On the border between organic and inorganic compounds are soda, carbon disulfide, urea, carbon monoxide (II), etc., which can be considered equally as organic and inorganic compounds. The place of organic chemistry among other sciences is determined not only by its proximity to inorganic chemistry. By studying the most complex organic substances that play an important role in the life of animal and plant organisms, organic chemistry enters into close contact with biology. In the border area between these two sciences, a new young science has arisen and is successfully developing - biological chemistry. Finally, the connection between organic chemistry and physical chemistry and physics is becoming ever closer as a result of the ever wider application of physical methods to the study of organic substances. Among the reasons for the separation of organic chemistry into a separate science, the following can be distinguished: 1. The number of known organic compounds (about 16 million) far exceeds the number of compounds of all other elements of the periodic system of Mendeleev. Currently, about 700,000 inorganic compounds are known, while about 150,000 new organic compounds are obtained per year. This is explained not only by the fact that chemists are especially intensively engaged in the synthesis and study of organic compounds, but also by the special ability of the carbon element to give compounds with an almost unlimited number of carbon atoms linked in chains and cycles. 2. Organic substances are of exceptional importance in connection with their extremely diverse practical applications, and especially in connection with the fact that they play an important role in the life processes of organisms.

2

Introduction

3. There are significant differences in the properties and reactivity of organic compounds from inorganic compounds, as a result of which many special methods for studying organic compounds had to be developed. A special place among the elements of the periodic system of D. I. Mendeleev is occupied by carbon. This is due to the fact that its atoms, having the ability to form sufficiently strong bonds among themselves, form the so-called carbon skeletons - straight and branched chains, various rings, bulk structures of countless molecules. The valencies of free carbon in the carbon skeleton are saturated due to the formation of bonds with hydrogen (in this case, hydrocarbon molecules are obtained), as well as with other atoms and groups of atoms (the latter are called substituents). The ability to vary the number and arrangement of atoms in the carbon skeleton, as well as the number, types and arrangement of substituents means that an arbitrarily large number of carbon compounds can be “designed”. This was one of the reasons that prompted the chemistry of the latter to be singled out as a separate science. This science is characterized by an approach to the systematization of the material and the development of new ways of development. Since the first carbon compounds were isolated from living organisms or their metabolic products, it was called organic chemistry. Later it turned out that carbon compounds play a decisive role in almost all processes that determine the nature of the activity of a living cell, so the original name of the science of carbon compounds - organic chemistry - sounds relevant to the present time. Organic chemistry, on the one hand, comes into contact with inorganic chemistry, since its objects of study are, for example, derivatives of carbonic acid or compounds of silicon and boron, which have a relatively small proportion of carbon in their molecules, on the other hand. with biology, since a number of its sections are devoted to the study of substances actively involved in vital processes (nucleic acids, proteins, polysaccharides, lipids, steroid hormones, etc.). The great diversity and specificity of the properties of organic compounds led to the fact that the successes of organic chemistry began to significantly influence the development of already existing chemical disciplines (inorganic, analytical, physical chemistry, biochemistry) and contributed to the creation of new chemicals. The practical significance of organic compounds determined the emergence and development of many special areas of organic chemistry (the chemistry of paints, medicines, explosives and aromatics, plant protection products, combustible and structural materials).

Cyclic and Noncyclic Organic Compounds: A Textbook of Organic Chemistry

3

Organic chemistry penetrates deeper and more intensively into neighboring fields of knowledge. There is a natural process of the disappearance of clear boundaries separating the fields of science, the rapid growth of related disciplines, where discoveries are often made at the “crossroads of sciences”, which determines the direction of research for many years.

History of organic chemistry Substances in nature are divided into two types: inorganic and organic. It is no coincidence that the substances are named so. Water, salt, ash, etc. are inorganic substances. They are mainly found in inanimate nature. Organic matter is found in living organisms. Organic substances formed in organisms could not previously be obtained in laboratories. Proteins (meat, egg white), starch, fats, sugars, etc., are formed in plant and animal organisms. Currently, people can artificially synthesize organic substances in laboratories. All organic substances contain the element carbon. In addition to the carbon element, they contain oxygen, hydrogen, nitrogen, and other elements. If we burn organic matter, they turn into inorganic matter. For example, if we heat a seed in a test tube, it will turn black, char, and then burn to ashes. As a result of its combustion, carbon dioxide and water are released. At this time, inorganic substances are obtained from organic substances. On the other hand, starch is produced from carbon dioxide and water, which are inorganic substances, during photosynthesis in green plants. If the number of organic substances common in nature is approximately 700,000, then the number of organic substances formed by one element carbon is several million. While the science of inorganic chemistry has been known since ancient times, the concept of organic chemistry hardly existed until the early 19th century. However, people used natural dyes for dyeing fabrics, vegetable and animal oils as food, sugar obtained from beets, and in some areas of artisanal production, vinegar was obtained by fermenting alcohol. With the development of chemical analysis methods, it was found that the composition of plant and animal substances mainly consists of the element carbon. For the first time in 1807, the famous Swedish chemist J. Berzelius proposed to call substances obtained from living organisms organic substances, and the science that studies them, organic chemistry. Other chemists of the time believed that organic substances were fundamentally different from inorganic substances. In their opinion, organic substances cannot be obtained by the laboratory method, like

Introduction

4

inorganic substances. Organic substances can only be formed with the help of the “life force” of living organisms. At that time, this approach was called the vitalistic approach (from the Latin “vita” - life). In 1828, a student of J. Berzelius, the German chemist F. Wöhler for the first time synthesized organic matter from inorganic substances - urea (CO(NH2)2), and in 1824 - oxalic acid (C2H2O4), a big blow to the views of vitalists. AgNCO + NH4&Oĺ1+2)2CO + AgCl; (urea)

CNCN

H2O; OH-

NH2COCONH2

H+

C2H2O4 (Oxalic Acid)

Föller’s dicyanide CNCN, like oxalic acid nitrile, reacts in an alkaline medium to form oxalic acid amide (oxamide) NH2COCONH2, from which oxalic acid is readily formed. Later, the German scientist A. Kolbe, the French chemist M. Berthelot, and the Russian scientist A. Butlerov proved in principle the possibility of mutual transformation of inorganic and organic substances and the absence of a sharp boundary between them by synthesizing organic substances from inorganic ones. Unlike inorganic substances, most organic substances are flammable and easily decompose when heated.

CHAPTER 1 THEORETICAL FOUNDATIONS

1. The subject of organic chemistry In the body of plants and animals, a large number of carbon compounds are synthesized, many of which are necessary for humans. These are sugar, starch, vegetable oils and waxes, proteins, fats, dyes, fibers, etc. There are also simple carbon-containing substances (CO, CO2, CS2, cyanides, carbonates), which are classified and studied in the course of general and inorganic chemistry as inorganic compounds. In 1889, the German chemist K. Schorlemmer gave a definition of organic chemistry as a science, which is considered more successful: “Organic chemistry is the chemistry of hydrocarbons and their derivatives.” There are several reasons why organic chemistry was singled out as a separate science. Numerous and varied organic compounds now make up the majority of the 60 million compounds registered with the Chemical Abstracts Service (CAS) as of May 2012, and the number is steadily increasing.

2. The complexity and originality of organic compounds. They differ significantly from inorganic compounds in their properties and reactivity: organic compounds are usually less stable, and have a lower phase transition temperature (ter., tg.). Almost all of them burn or are easily destroyed when heated with oxidizing agents, and CO2 leaves. The molecules of most organic compounds do not decompose into ions. Reactions of organic compounds proceed very slowly and, in most cases, do not reach the end. The phenomenon of isomerism is widespread among organic compounds. Hence the need arises to develop special methods for the study of organic substances.

6

Chapter 1

3. Variety of practical applications of organic compounds. They play an important role in the life processes of animals and plants. Organic chemistry is of exceptional scientific and practical importance. The object of this research is a large number of compounds of synthetic and natural origin. It is for this reason that organic chemistry has become the largest and most important branch of modern chemistry. In the process of obtaining new knowledge from organic chemistry, the chemistry of macromolecular compounds, bioorganic chemistry and biochemistry emerged as independent scientific disciplines. Organic chemistry forms the chemical basis of biochemistry and molecular biology, a science that studies the processes occurring in chemicals and at the molecular level at the level of organism cells. Research in this area allows a deeper understanding of the nature of natural phenomena. Many synthetic organic compounds are produced on a large scale for use in various fields of human activity. These are petroleum products, fuel for various engines, solvents, explosives, polymeric materials (rubbers, plastics, fibers, films, varnishes, adhesives), surfactants, drugs, paints, plant protection products, perfumes and cosmetics, etc. Without knowledge of the basics of organic chemistry, it is impossible to organize the production and use of these products in an environmentally sound manner.

4. Theory of the structure of organic substances In 1861 A. M. Butlerov formulated a scientific theory of the structure of organic substances, which stimulated the development of organic chemistry as an independent science. The Scot A. Cooper and the German A. Kekule contributed to the formation of this theory. They formed the idea of tetravalent carbon and expressed the idea of the ability of carbon atoms to combine into long chains. The essence of Butlerov’s structural theory mainly consists of the following main provisions: 1. The chemical nature of each complex molecule is determined by the nature of the atoms that make up its composition, their number and chemical structure. 2. Chemical structure is a certain alternation of atoms in a molecule, their interaction with each other (both through neighboring and through other atoms).

Theoretical foundations

7

3. The chemical structure of substances determines their physical and chemical properties. 4. The structure of a molecule can be expressed by a structural formula that is unique to a particular substance. In the 1870s, the concept of the spatial arrangement of atoms in molecules was developed - this is called the stereochemical theory (J. Van’t Hoff). Since the beginning of the 20th century, the methods of quantum mechanics and computer technology began to penetrate into organic chemistry, with their help they tried to explain the reasons for the manifestation of the interaction of atoms in molecules and predict the ways of synthesizing new useful substances. Independent directions are formed: the chemistry of organic elements, macromolecular, heterocyclic, natural compounds. Physical and chemical research methods are actively used, which deepens the understanding of the structure and properties of substances. Many enzymes, hormones, vitamins, antibiotics, alkaloids, chlorophyll have been synthesized. The molecular structure of many proteins has been deciphered and their synthesis has been carried out in practice. The path has been opened for nucleic acids transcribed by the human genome.

5. Sources of organic compounds The main raw materials for the production of organic compounds are natural gases, oil, hard and brown coal, shale, peat, agricultural and forestry products. Natural gases usually accompany oil and have a different composition: as a rule, about 95% methane and 2% other hydrocarbons (ethane, propane, butane, etc.). They can be fractionated and separated into components by distillation at low temperature. According to scientists, oil is the geochemically altered remains of ancient plants and animals. There are also theories about the inorganic origin of oil from metal carbides and water in the thickness of the earth’s crust. Oil is a mixture of hydrocarbons with small amounts of oxygen, sulfur, nitrogen and other compounds. Oil, purified from gases, water and mechanical impurities by distillation at normal pressure, is divided into three fractions: gasoline (30žɋ NHURVHQH-300 žɋ DQGIXHORLOGLVWLOODWLRQUHVLGXH 2IWKHVH main fractions, those that boil at a lower temperature are distinguished: petroleum ether (30-žɋ QDSKWKD-žɋ DQGZKLWHVSLULW-

8

Chapter 1

 ž ɋ  $ ODUJH QXPEHU RI VXEVWDQFHV VXFK DV GLHVHO IXHl, lubricating oils, petrolatum and paraffin wax, are obtained from fuel oil by vacuum or steam distillation. Coal is the most important raw material for the chemical industry, as its reserves far exceed oil. In industry, several methods of coal processing are used: dry distillation (coking, semi-coking), hydrogenation, incomplete combustion, and the production of metal carbides. About 3% of coal tar is obtained in the coking process, but large quantities are obtained in large coke production. During the processing of coal tar during the hydrogenation of coal, hydrocarbons, phenols, naphthalene, anthracenes, heterocyclic nitrogen-containing compounds, mixtures of hydrocarbons are obtained. Incomplete combustion of coal leads to the formation of carbon monoxide (CO). The technology uses three types of gas mixtures containing CO - generator gas, water gas and synthesis gas (products of the conversion of methane and its homologues). Carbon monoxide is used to produce mixtures of hydrocarbons, alcohols, and also for the “oxosynthesis” process. Oil shale is converted into high-calorific natural gas. At present, technologies for obtaining motor fuel from shale gas have been developed. Oxalic acid is obtained by the oxidation of peat mass. The products of agriculture and forestry are used as raw materials for the purchase of certain chemicals (ethyl alcohol from starch and cellulose, turpentine and rosin resin, soap oils, etc.).

6. Classification of organic compounds The structure of organic compounds should be described by formulas. There are the following types of formulas: 1) Empirical - indicates the smallest absolute ratio of various atoms in a molecule, for example, CH3 (ethane C2H6); CH2O (acetic acid C2H4O2); 2) Molecular (gross) - shows the actual number of different atoms in the molecule: C2H6 (ethane); C2H4 (ethylene); C2H4O2 (acetic acid); 3) Structure - not only reflects the type and number of atoms in a molecule, but also shows the location of bonds between atoms (structure). Structural formulas can be represented in the following form: – Open structural formulas, for example, ethane;

Theoretical foundations

H

H

H

C

C

H

H

9

H

– Abbreviated structural formulas: propane CH3–CH2–CH3 or CH3CH2CH3; - Skeletal formulas of the structure, for example, hexane.

According to the structure of the carbon skeleton, organic compounds are classified according to the scheme (Fig. 1). Acyclic (aliphatic) - compounds with an open chain of carbon atoms. Saturated - compounds that cannot combine hydrogen or other substances. Unsaturated - compounds that can combine hydrogen or other substances. Cyclic - compounds with a chain of atoms connected in a ring. Carbocyclic - compounds in which a chain of only carbon atoms is connected to the ring. Heterocyclic - compounds containing, in addition to carbon atoms in the cyclic skeleton, one or more heteroatoms - usually nitrogen, oxygen, sulfur atoms.

Chapter 1

10

Organic compounds

Uncloused chain

Aliphatics

Satisfied

Cloused chain

Cycles

Unsaturated

But C atoms

Carbocyclic

Alicyclic

C and other atoms

Heterocyclic

Aromatic

Figure 1. Classification of compounds according to the structure of the carbon skeleton

Carbon skeleton (carbon chain) - a sequence of connections of carbon atoms in a molecule. A functional group is an atom or a group of atoms that determines whether a compound belongs to a particular class and is responsible for its chemical properties. Depending on the nature of the functional group, hydrocarbon derivatives are divided into classes (Table 1).

Theoretical foundations

11

Table 1 Main functional groups and their corresponding classes of organic compounds Group

The name of the group

1

The name of the class

2

General formula

3

4

–F, –Cl, – Br, –ø– Hal)

Halogen–

Halogenated

R–Hal

–OH

Hydroxy-

Alcohols, phenols

R–OH, Ar–OH

–SH

Mercapto-

Mercaptans

R–SH

–OR

Alkoxy-

Ethers

R–O–R1

–N=O

Nitroso–

Nitrosocompounds

R–N=O

Nitro–

Nitrocombination

R–NO2

Formyl-

Aldehydes

R–CH=O

O C

Oxo–(keto–)

Ketones

R–C(O)–R1

O C

Carboxy-

Carbonic acids

R–C(O)OH

Alkoxycarbonyl–

Esthers

R–C(O)OR1

O N

O C

O

H

O O C O

H

R

Chapter 1

12

O C

Carbomoyl–

Amides

R–C(O)NR2

–&ŁN

Cyano–

Nitriles

R–&ŁN

–ɋɇ ɋɇ2

Vinyl–

Alkenes

R1R2C=CR3R4

–&Ł&+

Ethynyl–

Alkynes

R1&Ł&52

Phenyl–

Aromatic hydrocarbons

NR2

R

All classes of organic compounds are interconnected. The transition is mainly carried out by the transformation of functional groups without changing the carbon skeleton.

7. Nomenclature of organic compounds Nomenclature is a set of rules by which the names of compounds are formed. With their help, you can build the name of any organic compound. In this case, it is necessary to observe an important principle: each name must correspond to a relationship. Several nomenclature systems are used in organic chemistry, the main ones being the trivial, rational, and IUPAC systems. 7.1. Trivial nomenclature. This is a historically established naming system that is still used today. They do not reflect the structure of molecules and often indicate what the organic compound consists of (formic acid, malic acid), its color (methyl orange, malachite green), method of preparation (acetone, phenolphthalein), etc. Scientific nomenclature. To become familiar with rational and IUPAC nomenclature, you need to learn some basic concepts and terms used in naming. Homologous series - a series of compounds similar in structure and chemical behavior, each subsequent (highest) member of the compound differs from the previous one by the CH2 group. Below is the homologous series of alkanes: Formula Name ɋɇ4 Methane ɋɇ3ɋɇ3ɋ2ɇ6) Ethane

Theoretical foundations

13

ɋɇ3ɋɇ2ɋɇ3ɋ3ɇ8) Propane ɋɇ3ɋɇ2ɋɇ2ɋɇ3 ɋ4ɇ10) Butane ɋɇ3ɋɇ2)3ɋɇ3 ɋ5ɇ12) Pentane ɋɇ3ɋɇ2)4ɋɇ3 ɋ6ɇ14) Hexsane ɋɇ3ɋɇ2)5ɋɇ3 ɋ7ɇ16) Heptane ɋɇ3ɋɇ2)6ɋɇ3 ɋ8ɇ18) Octane ɋɇ3ɋɇ2)7ɋɇ3ɋ9ɇ20) Nonane ɋɇ3(ɋɇ2)8ɋɇ3(ɋ10ɇ22) Decan Followed by: C11H24 - undecane, C12H26 - dodecane, C13H28 - tridecane, C16H34 - hexadecane (cetane), ..., C20 H42 - eicosane, etc. The general formula of the homologous series is a formula that reflects the molecular formula of each member of the homologous series at a certain value of the number of carbon atoms, expressed by the index n (n = 1, 2, 3, etc.). CnH2n+2 is the general formula for the homologous series of alkanes. From the logical series of alkanes we get CH4 - methane at n=1. The carbon skeleton (carbon chain) is a sequence of connections of carbon atoms in a molecule. The normal carbon skeleton is a linear chain of carbon atoms linked to each other in the sequence: C-C-C-C-C. A branched carbon skeleton is a chain of carbon atoms connected to each other in series by branches:

C

C

C C

C

C

C

C

Atoms connected to one carbon atom in the hydrocarbon chain are called single, to two - binary, to three - triple, and to four - quadruple. The branched hydrocarbon shown below consists of five single (terminal), three double, one triple and one quadruple carbon atoms:

H3C

C

C

C

CH3

CH3 C C CH3 CH3

Alkyl group (hydrocarbon radical) - a fragment remaining after the removal of a hydrogen atom from an alkane molecule.

Chapter 1

14

The Latin letter R is adopted as a general symbol for the designation of the alkyl group (Table 2). Table 2 Names of some alkyl groups The structural formula of the group CH3– CH3–CH2– CH3–CH2– CH2– CH3 H3C HC

Name and abbreviation Methyl (Me) Ethyl (Et) Propyl (Pr) Isopropyl (i-Pr)

CH3– CH2– CH2 –CH2– H3C H2C HC

Butyl (Bu) Secondary butyl (s-Bu)

H3C

CH3 HC H2C CH3 CH3

H3C

Isobutyl (i-Bu) Tertiary butyl (t-Bu)

C

CH3 CH3(CH2)3CH2– H3C HC H2C CH2

Pentyl (amil) Isopentyl (isoamyl)

CH3 CH2 =CH– CH2= CH– CH2– C6H5– CH3– C6H4– C6H5– CH2–

Vinyl, ethenyl Allyl Phenyl (Ph) Tolil (o, m, p) Benzyl (Bn)

A single hydrocarbon radical is a hydrocarbon radical (ethyl-, propyl-, butyl-, isobutyl-, etc.) with a free valence on one carbon atom. Binary hydrocarbon radical - hydrocarbon radical with free valency at a binary carbon atom (isopropyl, dibutyl, etc.). Tertiary hydrocarbon radical - hydrocarbon radical with free valency at the triplet carbon atom (tert-butyl-etc.).

Theoretical foundations

15

7.2. Rational nomenclature. The nomenclature is based on the name of the simplest (often the first, less often the second) member of the homologous series. All other compounds are considered derivatives formed by replacing hydrogen atoms with alkyl groups, atoms, or functional groups. To name a compound according to rational nomenclature, you must: - determine the class of the called connection; - choose the basis of the name (table 3); - list the substituents surrounding the base, according to their degree, and the radicals of the same name are grouped by the prefixes di-, tri- and tetra-; - form a name starting from simple pronouns to compound ones and ending with the name of the stem. Plural prefixes - di-, tri-, tetra-, etc., are used to denote the number of identical substituents (or multiple bonds). The normal carbon skeleton is a linear chain of carbon atoms connected in series: C–C–C–C–C.

Dimetylcarbinol

Substitutes

The main name of rationality is carbinol

Two metyl radicals CH3 Ethyl radical

H3C

C

OH

CH2

CH3

Chapter 1

16

Table 3 Fundamentals of rational names and endings according to the systematic nomenclature of the main classes of organic compounds. Classes of chemical bonds 1 Alkanes

Alkenes

Alkynes Arenas

General structural formulas 2 R–CH2– CH2–R1

R

The basis of rational names 3

Rational nomenclature name 4 Methane

Suffixes according to IUPAC 5 -an

Ethylene

-en

C

R2 C C

C C

R3 R1 5&Ł&– R1

–&Ł&–

Acetylene Benzene

-in Benzene

3

4

5

R

1

2

Alcohols

R–OH

C OH

O

Aldehydes

O

R

C

H

R1

O

O R

R

Keton

+on

Acetic acid

asid

Amine

Amine

OH

OH 1

+al

C

R2

Amines

Acetic aldehyde

O

O Acides

+ol

H

R

Ketones

Carbinol

3

N R

N

Theoretical foundations

17

7.3. The IUPAC systematic nomenclature is the most widely recognized and universal. Systematic names consist of words specially created or chosen to describe the structural features of a compound. To name a compound according to IUPAC nomenclature, do the following: 1) Choose the initial structure, which can be a carbon chain or a closed ring, including a large group; 2) Identify all functional groups present in the compound, and among them the old name is reflected by a suffix at the end of the compound name; all other groups are called prefixes in the form (see Table 1); 3) Unsaturation is indicated by the appropriate suffix (-en or -in), as well as the prefix (dehydro-, tetrahydro-, etc.); 4) Number the main chain with the smallest possible number, giving a large group; 5) List the prefixes in alphabetical order (di-, tri-, etc. multiplying prefixes are currently not taken into account); 6) Draw the full name of the connection according to the diagram (Fig. 2).

Compound Name Prefixes

Original name

Suffixes

Small functional groups with Main chain or basic cyclic

High-level functional

hydrocarbon radicals and locants

group only

structure

Figure 2. IUPAC name order of the compound.

Chapter 1

18

2-Hydroxy-3-methylbutanoic acid

Substitutes

Saturated backbon of butane-4C atoms

Methyl radical at the A small hydroxyl 3rd C atom of the functional group at the 3rd C atom of main chain the main chain

CH3 4 H3C

Large functional carboxsyl group

OH

3

2

HC

CH

First name

1 C

O

OH

Group size (desc): –COOH > –HSO3 > –CHO > >C=O > –OH > – NH2 > –C=C– > –&Ł&– > R, Br, I, Cl, F, N=O, NO2, –N=N–. The seniority among the last groups is determined alphabetically. The roots of the initial names depending on the length of the chain of carbon atoms are presented in Table 4. When writing the name of a substance, digits are separated from letters by a hyphen (–), and digits are separated from digits by a comma (,). Substituents in the prefix are listed in alphabetical order of the language from which the name is derived, regardless of their precedence.

OH 5 8

1 4

7

3 6

7-methyl-4-ethyloct-5-en-3-ol

2

Theoretical foundations

19

Table 4 Roots of primary names depending on the length of the chain of carbon atoms No. R-ane (-ene,in)

1 met

2 et

3 prop

4 but

5 pent

6 heks

7 hept

8 okt

9 non

10 dec

The composition of the main chain in acyclic molecules must necessarily include the highest functional group. The main chain should consist of the largest number of substituents, the maximum number of multiple bonds, the longest and most branched. If there are double and triple bonds in the chain, numbering at the same distance from the beginning of the chain is preferable to a double bond.

2 5

4 3

1

6

7

4-methylhept-1-en-6-in

It should be emphasized that unsaturation (the presence of double or triple carbon-carbon bonds) is indicated by replacing the suffix (-ane with -ene or -ine), and the highest functional group is indicated by adding the appropriate suffix indicating the highest group.

O O

1

3 2

4

5

OH 2, 3-dimethyl-4-oxopentanoic acid

The introduction of a structural formula by a systematic name, i.e., the solution of an inverse problem, usually begins with the designation of the original structure. Then the carbon atoms are numbered and the

20

Chapter 1

substituents are arranged. Finally, hydrogen atoms are added, provided that each carbon atom remains tetravalent.

CHAPTER 2 CHEMICAL BONDS

1. Nature and types of chemical bonds. Working methods A chemical bond is a set of forces connecting two or more atoms in polyatomic systems and is carried out with the help of electrons. According to quantum mechanical concepts, electrons in atoms are located in atomic orbitals (AO), which is understood as the most probable region where an electron is located in an atom. In the physical sense, each AO is described by its own set of quantum numbers that carry the wave function. The atom of each element has a certain type and number of orbitals: s-orbitals have a wave function with spherical symmetry. Three dumbbell-shaped p-orbitals are mutually perpendicular and each has a nodal plane in which the wave function changes sign (Fig. 3). z 1s x y z

z

z 2s

y

y

-

2pz

2py

2px x

z

+

x y

+ -

x

+ - -

x

y

Figure 3. Conditional graphic images of the orbitals of the atoms of the elements of the second period.

The presented AO images do not change depending on the electron filling (Table 5).

Chapter 2

22

Table 5 Filling the orbitals of some atoms of the 1st and 2nd periods with electrons Element

H C N O

Electronic configuration of an atom 1s1 2 2 1s 2s 2p2 1s22s22p3 1s22s22p4

Number of valence level orbitals 1 4 4 4

Number of valence electrons 1 4 5 6

Only valence electrons in the outer layers of atoms participate in the formation of chemical bonds. Accordingly, carbon in organic compounds has a valence of 4, since its electronic configuration changes upon excitation.

2s22p2 ĺV12px12py12pz1 A small amount of energy is required to move an electron from the 2s to the 2p orbital, which is then compensated by the energy released when two additional bonds are formed. The first explanation of the nature of the chemical bond was given by G. Lewis and W. Kossel in 1916. They proposed two main types of chemical bonds: 1) The main condition for the formation of an ionic bond is the transfer of one or more electrons from one atom to another; 2) Covalent bonds are formed due to generalized electron pairs. There are two ways to form a two-electron covalent bond. Collision - each atom participating in the formation of a bond gives up an electron to form a common electron pair.

2H CH3

+

CH3

HH CH4 CH3

Coordination - the formation of a donor-acceptor bond due to the electron pair of an atom. This can happen in two ways:

Chemical bonds

23

+

H N

+ H+

H

N

H

H Ammonium cation

+ R

O

+ R +H

R

O

R

H Alkyloxonium cation

a) The particle formed as a result of the transfer of an electron pair from a donor to a cation has a positive charge. The formation of “onium” salts occurs when the electron donor is N, O, S, or P. b) Transfer of an electron pair from a donor to an empty acceptor orbital.

H

H

H

N +

B

H

H

H H H

H

N B

H H ve ya

H

H

N

B

H

H

H

H

N

B

H

H

H

H H H

H

If the formation of a donor-acceptor bond is accompanied by the formation of opposite charges on neighboring binding atoms, then such bonds are called semipolar. To do this, charged dashes or unidirectional arrows (from donor to acceptor) are used in their designation. Like formation, the breaking of a chemical bond can be carried out by two mechanisms.

Chapter 2

24

1) A homolytic gap occurs in such a way that one electron remains in each of the previously bonded atoms. At this time, radicals are formed.

R

R +X

X

2) Heterocyclic splitting leads to the formation of two oppositely charged ions, the electron pair is preserved when the bond is broken, as a rule, at a more electronegative atom.

R

R+ + X-

X

2UJDQLFFRPSRXQGVDUHFKDUDFWHUL]HGE\ı- DQGʌ-covalent bonds. The ı-bond is formed by overlapping AO along the axis or overlapping along the line connecting the centers of the bonded atoms. Schemes of the IRUPDWLRQRIı-bonds when two s-AO, s- and p-AO overlap. s

s s +

+

+

p

p

+

-

-

p +

+

-

)LJXUH6FKHPHRIIRUPDWLRQRIı-bonds

ʌ-bonds are bonds formed by the maximum overlap of unhybridized p$2VDERYHDQGEHORZWKHSODQHRIı-bonds (Fig. 5). p

p

-++ )LJXUH6FKHPHRIIRUPDWLRQRIʌ-bonds

7KH ʌ ERQG LV D ZHDNHU ERQG WKDQ WKH ı ERQG DQG LWV HOHFWURQV DUH more easily separated from the atom. If two atoms are connected by a single bond, then this bond is always a ı-bond. When a double or triple ERQGLVIRUPHGEHWZHHQWZRDWRPVDıERQGLVIRUPHGILUVWIROORZHGE\D ʌERQG

Chemical bonds

25

,Q RUJDQLF FRPSRXQGV WKH FDUERQ DWRP IRUPV ı-bonds with hybrid AOs. Hybridization is understood as the mixing of valence orbitals and their arrangement in shape and energy. The four valence orbitals of the C atom can be replaced by a number of equivalent hybrids AO sets 2s12px12py12pz1. Depending on the combination of hybridized and unhybridized orbitals, the C atom can be in the state of sp3-, sp2-, or sphybridization. ,IRQHFDUERQDWRPIRUPVIRXUıERQGVWKH\DUHHTXLYDOHQWVLQFHWKH\ are formed by covering four atomic orbitals of the C atom (sp3 hybridization).

1s + 3p = 4sp3

+

+

+

109o28`

In the formation of the CH4 molecule, the use of sp3 hybrid orbitals when the C atom is bonded to four H atoms leads to the formation of stronger C–+ ı ERQGV 0HWKDQH LV D SHUIHFW WHWUDKHGURQ ZLWK DQ +–C–H angle of 109º 28´. The geometry of such a molecule ensures minimal repulsion between the four bonding electron pairs.

26

Chapter 2

In ethylene, each C atom is bonded to only three other atoms, not four. In this case, the electronic structure of the molecule is described as sp2 hybridization.

1s + 2p = 3sp2 Three sp2 hybrid orbitals, formed from one 2s and two 2p orbitals, lie in the same plane at an angle of 120º. In ethylene, the C–& ı ERQG LV formed when hybrid orbitals overlap along their axes. The remaining two sp2 orbitals of each of the C atoms overlap with the s orbitals of the H atoms and form four C–+ıERQGV8QK\EULGL]HGS-AO lie at right angles WRWKHı-ERQGSODQHRYHUODSSLQJDERYHDQGEHORZIRUPLQJʌ-bonds.

,QDON\QHVHDFKFDUERQDWRPRIWKH&Ł&WULSOHERQGFDQEHERQGHGWR two more atoms. In acetylene, both carbon atoms are sp-hybridized. Hybrid orbitals lie on the same straight line at an angle of 180°.

1s + 1p = 2sp

Chemical bonds

27

When a triple bond is formed, a hybrid orbital of carbon atoms SDUWLFLSDWHLQWKHIRUPDWLRQRIDıERQG7KHWZRunhybridized p-orbital of each of the two carbon atoms are parallel to each other and can overlap in SDLUVIRUPLQJWZRʌ-bonds located in perpendicular planes.

2. Parameters of covalent bonds. Hydrogen bond A covalent bond is characterized by: length, energy, polarity and direction. The bond length is the equilibrium distance between the centers of the nuclei of the atoms that form the bond. Half the length of a covalent bond between identical atoms in a molecule is called the covalent radius. In general, the bond length is equal to the sum of the covalent radii of the atoms that make up the bond.

Chapter 2

28

A

B

rAB The lengths of some of the bonds are presented in Table 6. The lengths of the bonds surrounding the C atom depend on its hybridization state. The C–C single bond tends to decrease in length as the ratio of scharacters of the hybrid orbital increases. As the number of bonds between atoms increases, their length always decreases. Table 6 Length, energy, polarity, refractive index of some covalent bonds Type of chemical bond C-H C-C C=C &Ł& C-O C=O O-H

Length, nm

Energy kC/mol

Dipole moment, D

Refractive index

0,109 0,154 0,133 0,120 0,143 0,121 0,096

415 348 620 810 340 710 465

0,4 0 0 0 0,7 2,4 1,5

1,7 1.3 4,2 6,2 1,5 3,3 1,7

Bond energy is the energy used to break a chemical bond (or released when it is formed). The energy required to break a single bond homolytically is called the dissociation energy (ED). The energy serves as a measure of the strength of the bond: at higher values, the bond is more stable. Double bonds are stronger than the corresponding single bonds, but not twice as VWURQJ 7KLV PHDQV WKDW WKH ı ERQG LV VWURQJHU WKDQ WKH ʌ ERQG The bonding energy can vary greatly depending on factors related to the structure of the molecule. The C–H bond energy for singlet, doublet, and triplet carbon atoms is not the same and decreases as one goes from singlet–H to triplet–H.

Chemical bonds

29

The direction of a chemical bond is determined by the hybrid state of the atoms involved in its formation. The angles between two bonds to a common atom are called bond angles. These angles in organic compounds must correspond to the hybridization state of the carbon atom. Bond polarity is the asymmetry in the distribution of electron density between atoms due to the difference in electronegativity. If the atoms forming a covalent bond are equivalent, the pair of electrons belongs equally to both atoms. Most covalent bonds are formed by dissimilar atoms, which causes a shift in the electron density towards one of them. The greater the electronegativity of an element, the stronger the attraction between the nucleus and outer electrons in the atom (Table 7). Table 7 Pauling electronegativity scale

F O N Cl Br I C 4,0 3,5 3,0 3,0 2,8 2,6 2,5 S H P Si Mg Li Na 2,5 2,1 2,1 1,8 1,2 1,0 0,9 The electronegativity of the carbon atom depends on its hybrid state and is determined by the proportion of the s-orbital in the hybrid orbital of the C atom: sp3 - 2.51; sp2 - 2.59; sp - 2.75. The electronegativity difference (REO) of the bound atoms of different elements serves as a measure of the polarity of the bond between them (Fig. 6).

Chapter 2

30

The main types of chemical bond

Ionic bonding

A non-polar covalent bond

Covalent bond

Weak polar bond Polar covalent bond

Figure 6. Difference in electronegativity of related atoms

Covalent C-C bonds are non-polar. In addition to them, C-H bonds in hydrocarbons are also considered non-polar, although the latter should be considered less polar (REO for bonds): C-H 2.5 (C) - 2.1 (H) =0.4; dipole moment 0.2 D). Bonds formed by atoms with different electronegativity will be polar. Symbols are used to indicate the polarity of a bond.

įį-

CH3ĺ&O The uneven distribution of the electron density of the polar covalent ERQG FUHDWHV D FKDUJH VHSDUDWLRQ FKDUDFWHUL]HG E\ D GLSROH PRPHQW ȝ H[SUHVVHG LQ 'HE\HV  '  Â-30 NOÂP  7KH GLSROH PRPHQW ȝ LV D function of the bond length (r) and charge (q) of the bonded atoms.

ȝ=q· r The calculated values of the dipole moments of some compounds are presented in Table 7. The polarizability of a compound is its ability to change polarity when subjected to an external field (usually a reagent). The polarizability of

Chemical bonds

31

bonds in a molecule is estimated from the experimentally determined value of molecular refraction (MRD, cm3), based on the physical constants of organic compounds, such as the refractive index nD20 and density d:

n2- 1 M MRD = ——— —— n2 + 2 d Where n = nD20 is the refractive index, d is the density, and M is the molecular weight. The MRD is the sum of atomic breaks (AR) or bond breaks (R), which is an additive quantity and is measured in cm3. The AR values are summarized in lookup tables and can be used to calculate the MRD.

MRD = Ȉ Ⱥ5 The polarizability of a bond increases with an increase in the size of the atoms (electron shells) that form the bond (Table 7). Bonds are more easily polarized; their maximum electron density is located farther from WKHQXFOHLRIWKHERXQGDWRPVʌERQGVDUHPRUHSRODUL]DEOHWKDQıERQGV Polarizability basically determines the reactivity of molecules. Hydrogen bond. A hydrogen atom bonded to a strongly electronegative atom (F, O, N, Cl) can form an additional weak bond, called a hydrogen bond, with the lone pair of another strongly electronegative atom of another or the same molecule. The energy of this interaction is in the range of 10-40 kC/mol. In organic substances, these bonds are formed between functional groups containing acidic protons and atoms with unshared electron pairs. Examples of intermolecular hydrogen bonds:

R O

R H... .. O O

......

R H.... O

H O

R

R O H.... O

H

Chapter 2

32

The hydrogen bond plays an important role in the manifestation of many physical and chemical properties of molecules. Intermolecular hydrogen bonds are responsible for the abnormally high boiling points of many compounds—water, alcohols, and carbonic acids. Hydrogen bonds increase the solubility of a number of substances when solvated through a solvent. Hydrogen bonds also help stabilize any ions that form in solutions. If the interacting functional groups are located in such a way that the closure of a six-membered or five-membered ring is possible, then an intramolecular hydrogen bond is formed. Examples of molecular (intramolecular) hydrogen bonds are shown in the following formulas.

O

O

C O

N O

O H

O H

Particular attention should be paid to the spectral determinations of acid A. The H1 NMR spectrum of the acid shows an anomalously lowfield proton resonance of the COOH group (CDCl3įSSP WKHYDOXH of the chemical VKLIWį LV DQLQGLFDWRURI WKH FKHODWHQDWXUH RI WKH SURWRQ and does not depend on the degree of dilution of the solution concentration. Interestingly, the N-dimethyl groups are anisochronous (i.e. the 2 dimethyl groups do not rotate around the C-N bond) and resonate as WZRVLQJOHWVDWįDQGSSP7KHGLDVWHUHRJURXSRIWKH1-dimethyl groups is clearly visible in the C13 105VSHFWUXPįc 42.45 g and 42.29 g, ppm). In the IR spectrum there is no absorption in the region of 3000-3600 cm-1, but a strongly broadened absorption band at 1536 cm-1 is visible, which is characteristic of the enol double bond. All these data prove that acid A exists in a favorable unconventional resonance structure (B) stabilized by an H-bond:

Chemical bonds O

33 H

COOH

O

O O

Cl Cl H3C

N

MeO

H3C

OMe

CH3

OMe

N MeO CH3

A

B

Hydrogen bonds play an important role in the formation of the spatial structure of proteins, nucleic acids, polysaccharides, as well as in a number of biochemical processes.

3. Delocalized chemical bonds  7\SHV RI FRQMXJDWH V\VWHPV ʌ ʌ- S ʌ-). Covalently bonded molecular orbitals (MOs) between two atoms are called localized MOs with two centers and two electrons. If an orbital closed by one or more bonds covers three or more nuclei, then we are talking about a delocalized bond. A delocalized bond, MO, is a covalent bond with more than two atoms. Delocalized communication can take place on open or closed connections. Conjugation is the formation of a single delocalized electron cloud in a molecule as a result of the overlap of unhybridized p-orbitals of several atoms. :KHQ D GHORFDOL]HG 02 LQFOXGHV WZR RU PRUH ʌ ERQGV WKLV W\SH RI FRQMXJDWLRQ LV FDOOHG D ʌʌ FRQMXJDWLRQ ([DPSOHV RI ʌʌ-conjugated systems are presented below.

H2C= CH –CH= CH2

H2C= CH –CH= CH– CH= CH2

H2C =CH– &Ł&+

H2C= CH– &Ł1

$PRQJ FRPSRXQGV FRQWDLQLQJ D FORVHG ʌʌ-conjugated system, aromatic compounds, especially benzene, are of the greatest interest. If the FRQMXJDWLRQ LQYROYHV D ʌ-bond and an atom with a p-orbital on the neighboring atom, WKLVW\SHRIFRPELQDWLRQLVFDOOHGSʌ-conjugation.

Chapter 2

34

H2C HC O CH3 H2C

HC Cl

-

H2C= CH– CH2 Allyl-anion

O H3C C NH2 ·+

H2C= CH –CH2 H2C= CH –CH2 allyl-radical

allyl-cation

3.2. Electronic displacements in molecules of organic compounds. Inductive and mesomeric effects. The atoms that make up the molecule are subject to mutual influence, transmitted by electronic and spatial effects. For example, if a C atom is bonded to a more electronegative atom, nitrogen, oxygen, or halogen, then an electron density deficit, GHQRWHG³į´DQGDQH[FHVVHOHFWURQGHQVLW\GHQRWHG³į-”, appear on the heteroatom.

į &ĺ%U

į- į &ĺ2+

į-įį&ĺ1+2

The polarity of the bond is WUDQVPLWWHGDORQJWKHFKDLQRIı-bonds by the mechanism of electrostatic induction, which causes an inductive effect (I-effect). The transmission occurs with a gradual weakening, it is no longer visible after three or four connections.

į¶¶į¶įįC4–C3–C2–C1ĺ;į¶¶į¶į *UDSKLFDOO\WKHLQGXFWLYHHIIHFWLVUHSUHVHQWHGE\DĺVLJQDORQJWKH covalent bond line, the tip of which points to the more electronegative atom. The direction of the inductive action of the substituent is qualitatively assessed by comparing its effect on the electron cloud of the bond with the effect of an almost low-polarity C-H bond and assuming the I-effect of the hydrogen atom to be zero. If substituents shift the electron GHQVLW\RIı-bonds towards themselves more than a hydrogen atom, then they have a negative inductive effect (-I-effect). 6XFKVXEVWLWXHQWVUHGXFHWKHHOHFWURQGHQVLW\LQWKHV\VWHPRIı-bonds and are called electron-withdrawing substituents (electron acceptors).

Chemical bonds

35

įį-

&ĺ; Demonstrates -I-effect: a) electronegative atoms and groups: F > Cl > Br > I; OH, NH2 and their bonds; b) groups with semipolar compounds (semipolar):

+ R3N O

O N + Oc) Onium and cationic substituents:

—NH3;

—NR3;

—OR2;

d) Unsaturated and aromatic substituents:

O C OH

O C H

O C R

C N

CH CH Substituents that repel electrons exhibit a positive inductive effect (+Ieffect) and are called electron-withdrawing substituents.

įį-

;ĺ& +I-effect indicates: a) alkyl groups:

Chapter 2

36

CH3 —CH2CH3 —CH (CH3)2 —C(CH3)3; b) anionic substituents:

—O ;

—NR.

The mesomeric effect, or conjugation effect (M-effect), is the GLVSODFHPHQW RI DQ HOHFWURQ DORQJ D V\VWHP RI GHSRODUL]HG ʌ-bonds. The mesomeric effect, in contrast to the inductive one, is transmitted without attenuation through the conjugated system. Substituents that change the electron density exhibit a negative mesomeric effect (-M effect) in the conjugated system and withdraw electrons. í7KH0-HIIHFWLVLOOXVWUDWHGE\WKHFXUYHGD[LVIURPWKHʌ-bond to the atom as follows. O H2C

HC

O

C

H2C HC

OH

C

OH - The M-HIIHFW PDQLIHVWV LWVHOI LQ DV\PPHWULF ʌʌ-conjugated conjugated systems.

C C C X

C C C X

C C C X C C C X

Groups showing the -M effect:

O ; C OH

O ; C H

O C ; R

C N;

O N + ; O -

O S OH O

Chemical bonds

H C

H2C

37

O C

N

C OH

Substituents pulling electron density away from them exhibit a positive mesomeric effect (+M-effect) with respect to the conjugated system. These include substituents containing a lone pair of electrons or a heteroatom with a negative charge. These substituents donate electrons and create an excess electron density in the conjugated chain by introducing electron pairs into the conjugated system. 7KH0HIIHFWPDQLIHVWVLWVHOILQSʌ-conjugated systems.

C

C

X

C

X

X

Groups with +M effects:

ÂÂ

R2N—;

ÂÂ

ÂÂ

RO—;

Hal—

For example:

H2C

CH

OCH3

NH2

In order to determine whether the M-effect exists, it is necessary to isolate the system of combined bonds in the molecule. If the system is symmetrical, there is no M-HIIHFW,IWKHUHLVDSʌ-conjugate system, then there is a + M-HIIHFWLIWKHUHLVDQDV\PPHWULFʌʌ-conjugate system, then WKHUHLVDí0HIIHFW Superconjugation (hyperconjugation) The inductive action of the alkyl groups, as expected, should decrease in the following order:

Chapter 2

38

H3C H3C

H3C C

H3C

CH

>

> H3C CH2

>

CH3

H3C

However, if the alkyl groups are linked to an unsaturated system, such as a double bond or a benzene ring, this order is reversed in some conjugated systems. Thus, it turns out that alkyl groups can, under certain conditions, induce electron transfer by a mechanism different from the inductive effect. This is explained by the extension of the concept of conjugation or the mesomeric effect and the assumption that electron GHORFDOL]DWLRQRFFXUVZLWKWKHSDUWLFLSDWLRQRIHOHFWURQVRIQHLJKERULQJıbonds: H H H C CH CH2 H H CH2

H CH2

H H C C H H CH2

CH2

H CH2

This effect is called superconjugation (hyperconjugation); with its help, it was possible to explain a number of incomprehensible events. It should be noted that, in fact, proton separation does not occur in the above compounds, since when it moves from its initial position, one of the conditions necessary for delocalization will be violated. The reversal of the expected (inductive) series of electron donor properties (CH3 > CH3CH2 > CH3CH3CH > CH3CH3CH3C) can be explained by the hyperconjugation effect. 7KHERQGZLWKWKHĮ-carbon atom of an unsaturated system depends on the presence of hydrogen atoms. It is clear that the effect of CH3 hyperconjugation should be maximal in compound A, and not in compound B CH3CH3CH3C:

Chemical bonds

H H C CH CH2 H

CH3 H C CH CH2 H

39

CH3 CH3 H3C C CH CH2 H3C C CH CH2 H CH3 B

A

Steric effect. Since each of the molecules that chemically interact with each other has a free spatial structure, they may encounter a process called the steric effect, i.e., in certain cases, bulky groups or radicals involved in the connection may interfere with the intended reaction. The most common steric effect is classical steric hindrance, when sufficiently bulky groups directly effect the reactivity of a certain region of the molecule, prevent the reagent from approaching the reaction center, or create a voltage in the transition state, effecting the availability of electrons. For example, complex C, formed with triethylamine, dissociates very easily, while complex D is very stable, since in it three ethyl groups attached to the nitrogen atom are “removed”, which excludes this possibility a conformation is formed in which steric hindrance may arise for attacking the nitrogen atom:

CH3 H2C

CH3 CH3

N B H2C H2C

CH3

H2C

CH2

HC

N B

H2C

CH2

CH3 C CH3

CH2

H2C D

CH3 CH3 CH3

CHAPTER 3 CHEMICAL REACTIONS

1. Basic concepts, classification of reagents A chemical reaction is an interaction of molecules in which atoms and groups of atoms are redistributed between molecules in the system under consideration, that is, old bonds are broken and new bonds are formed. A reaction center is an atom or a group of atoms involved in breaking or forming bonds. The ability of a substance to enter into a certain chemical reaction and to react more or less quickly characterizes its reactivity, and this is always considered in relation to a particular reaction partner. The starting compounds of organic reactions are called reactants, and the resulting substances are called products. For convenience, one of the reagents is called the attack reagent and the other is called the substrate. The substrate is one of the reactants, as a rule, has a more complex structure, is considered the object of transformation, and the reaction center in most cases contains a carbon atom. Typically, a substrate refers to a molecule that provides a carbon atom for a new bond. A reagent is a particle (radical, ion, molecule) that attacks the reaction center of a substrate. The concepts of “substrate” and “reagent” are conditional and in many cases are interchangeable. The classification of the reagents is given in Table 8. In heterolytic bond cleavage reactions, the reagent usually either donates an electron pair to the substrate or accepts one. Reagents that donate an electron pair are called nucleophiles, and their reactions are called nucleophilic reactions. Electrophilic reagents (electrophiles) are reagents that attach an electron pair to a substrate. Their reactions are called electrophilic reactions.

Chemical reactions

41

Table 8 Classification of reagents R-radicals particles with free electrons 1. Atoms FÂ, ClÂ, BrÂ, IÂ, HÂ 2. Atomic groups (particles) with an unpaired electron: ÂÂ NO2 , CH3, ÂÂ Â OOR, OH, R

Electrophiles E + particles with vacant orbitals. 1. Cations Cl+ , Br+ , +NO2 v 2. Neutral electrophiles: AlCl3, BF3, SnCl4,SO3

Nu Nucleophiles particles with free electron pairs 1. Anions RO-, Br-, Cl-, OH2. Neutral nucleophiles: NH3, H2O, CH3OH 3. Donors of ʌelectrons: CH2=CH2 ,

When a substrate molecule is cleaved, the part that does not contain a carbon atom is called a leaving group. A group separated by an electron pair is called a nucleofuge, while a group not separated by an electron pair is called an electrofuge. In a homolytic bond cleavage, the reagent supplies the substrate with an unpaired electron to form a bond, i.e., it acts as a radical or an atom.

2. Types of organic reactions and their mechanisms 2.1. Classification of organic reactions. It is characterized by various signs: the nature of the communication failure; type of substrate conversion; type of activation; the number of particles involved in determining the speed. According to the nature of the rupture of bonds, the reactions are divided into radical, ionic and concert (concert). Homolytic (radical) reaction - a reaction involving radicals formed during the homolytic cleavage of bonds. A free radical is a particle with an unpaired valence electron.

Chapter 3

42

ÂÂ Xʟ 7 do not change the color of neutral indicator paper, and acids with pKa > 10 do not have a sour taste.

Chemical reactions

47

Table 9 pKa values of some Brønsted acids in relation to water Acid Oxalate Ant Benzoin Asetic Propion Phenol Ethanol Acetamide Ammonia Nitromethane Acetone Acetylene Benzene Methane Perchlorate Sulfate Chloride Phosphate Carbonate Water

Formula OH- WXUúXODU (COOH)2 HCOOH C6H5COOH CH3COOH C2H5COOH C6H5OH C2H5OH NH- acids CH3CONH2 NH3 CH- acids CH3NO2 CH3COCH3 C2H2 C6H6 CH4 Inorganic acids HClO4 H2SO4 HCl H3PO4 H2CO3 H2O

pKa 1,23 3,75 4,19 4,76 4,87 10,0 16,0 15,1 33 10,6 20 25 43 48 -10 -9 -7 2,1 6,4 15,7

The strength of an acid depends on the strength of the conjugate base (anion) formed from that acid. The more stable the anion, the stronger the acid. The acidity depends on several factors that contribute to the stabilization of the anion: - electronegativity and polarity of the atom attached to the proton: CHacid < NH-acid < OH-acid; is the degree of charge delocalization in the anion (-); - solubility (interaction with a solvent). The smaller the ion and the more localized the charge, the better the resolution.

Chapter 3

48

Electron-withdrawing substituents (electron acceptors) help to delocalize the negative charge, stabilize the anion and thereby increase the acidity. On the other hand, electron-donating substituents reduce it. The basicity of compounds in aqueous solution is characterized by the constant pKb, which is related to pKa through the ionic product of water:

pKb = 14 – ɪKɚ CH3COOH + OHCH3COO + H2O -

ɪKb í  However, the acid value BH + pKa of the conjugate base (:B), more commonly referred to as pKBH+, is used to characterize basicity. This allows us to use the same scale to characterize the ionization of both acids and bases. The higher the pKBH+ value, the stronger the foundation should be (Table 10).

C6H5NH2 + H3O+

+ C6H5N+H3 + H2O pKBH = 4,6

Table 10 pKBH+ values of some bases of different classes Name of the compound Ammonia Methylamine Dimethylamine Trimethylamine Aniline Water Methanol Phenol Asetic acid

Base

Conyugate acid N- bases + NH3 NH4 CH3NH2 CH3NH3+ (CH3)2NH (CH3)2NH2+ (CH3)3NH2+ (CH3)3NH C6H5NH3+ C6H5NH2 O- bases H2O H 3O + CH3OH CH3OH2+ C6H5OH C6H5OH2+ CH3COOH CH3C(OH)OH+

pKBH +

9,25 10,6 10,7 9,8 4,6 -1,7 -2 -6 -6

Chemical reactions

49

Among Brensted bases, anions usually have more basic properties than neutral molecules: NH2¯ > NH 3; HO¯ > H 2O; RO¯ > RO. All of these anions are very strong bases. Basicity decreases in the order of anions > nEDVHV!ʌ-bases. When a proton binds to a neutral base, salt-like onium compounds (ammonium, oxonium, sulfonium) are formed.

+

B base

AH acid

BH +A

-

onium salt

The strength of the base is determined by the electron availability of the heteroatom and the stability of the resulting cation. The more stable the cation, the stronger the base. The strength of n-bases with the same substituents at the heteroatom decreases sequentially:

R NH2 > R OH > R SH Electron-donating substituents on the (R) radical increase basicity, while electron-withdrawing substituents decrease basicity. 3.2. Lewis electronic theory. Almost simultaneously with Brensted, G. Lewis put forward a broader theory of acids and bases, according to which the acid-base properties of compounds are determined by their ability to accept or donate an electron pair, forming a new bond. Lewis acids are electron pair acceptors and Lewis bases are electron pair donors. In both of these theories, the basic concept is expressed as an electron pair. In the Lewis theory, the concept of an acid has a broader meaning. An acid is any particle with an empty orbital that can add a couple of electrons to its shell. Acids in the Lewis theory are capable of accepting the H + proton, halides of elements of the second and third groups of the periodic system with an unoccupied orbital in the outer shell (BF3, AlCl3, FeCl3, FeBr3, ZnCl2), tin and sulfur compounds (SnCl4, SO3), electron pair.

Chapter 3

50

-

+

C2H5)2O

+ BF3

Base

Acid

C2H5)2O BF3 Donor-acceptor complex

Lewis acids also include cations, such as Ag+, Cu+, Hg2+, R3C+ carbocations, NO2+, Br+ cations, etc. For Lewis acids, fewer quantitative measurements of relative acid strength have been made and, unlike Brensted acids, there are no general tables. A qualitative assessment RI WKH DSSUR[LPDWH DFLGLW\ RI 0;n-type /HZLVDFLGV;-halogen) is proposed.

BX3 > AlX3 > FeX3 > GaX3 > SbX5 > SnX4 > AsX5 > ZnX2 > HgX2. Lewis acids act as electrophilic reagents in heterocyclic reactions: complex p-Lewis base

H3C H

H+

Lewis acids

H

+

H+

CH3 + SO3

H3C H

H CH3 SO3

4. Methods for the study of organic compounds. The chemist is most interested in one thought: what substance and with what molecular structure was obtained as a result of the reaction he is studying? To answer this question, several factors must be taken into account. First, you need to make sure that the substance is completely pure. To do this, there are various methods of analysis - gas-liquid chromatography, thin-layer chromatography, etc.

Chemical reactions

51

Gas-liquid chromatography is the separation of a gas mixture due to the different solubility of the sample components in a liquid or the different stability of the resulting complexes. Separation is based on differences in volatility and solubility (or adsorption) of the components of the mixture being separated. This method can be used to analyze gases, liquids and solids with a molecular weight of less than 400, which must meet certain requirements, the main ones being volatility, heat stability, inertness and ease of preparation. Organic substances, as a rule, fully meet these requirements, therefore gas chromatography is widely used as an excellent method for the analysis of organic compounds. Thin layer chromatography is a chromatographic method in which a thin layer of adsorbent is used as the stationary phase. The method consists in the fact that the substances to be separated are differently distributed between the sorbent (adsorbent) layer and the eluent flowing through it (a mixture of two or more solvents), as a result of which the displacement distance of these substances along the layer simultaneously changes. Thin layer chromatography opens up great opportunities for the analysis and separation of substances, since both the sorbent and the eluent can vary over a wide range. Plates with various sorbents are commercially available, which allows the method to be used quickly and regularly. One variation of thin layer chromatography is the more reliable and reproducible high performance thin layer chromatography, which uses special plates and sophisticated equipment. Secondly, after making sure of the individual purity of the compound, it is necessary to analyze it by physicochemical methods. For this, a number of physical tools are used, such as infrared (IR), ultraviolet (UV), nuclear magnetic resonance NMR, electron paramagnetic resonance, and EPR methods. In IR (infrared) spectroscopy, the experimental result is the infrared spectrum, which is a function of the intensity of the transmitted infrared radiation as a function of its frequency. Usually in the infrared spectrum there are a number of absorption bands, the position and relative intensity of which can be used to judge the structure of the sample under study. This approach became possible due to the large amount of experimental data collected: there are special tables that relate the absorption frequency to the presence of certain molecular fragments in the sample. A database of IR spectra of some classes of compounds has also been created, which allows you to automatically compare the spectrum of an unknown substance with those already known and thereby identify this substance.

52

Chapter 3

Using infrared spectroscopy, you can quickly and reliably identify various functional groups - hydrogen compounds, carbonyl, carboxyl, hydroxyl, amide, and amine and many unsaturated solutions - double and triple carbon-carbon compounds, and aromatic systems.

Figure 7. IR spectrum of acetone.

As can be seen from Figure 7, the main functional group of acetone C=O corresponds to a wavelength of 1715 cm-1 in the IR spectrum. Ultraviolet (UV) (electron) spectroscopy is one of the most common areas of research in the field of optical spectroscopy. Ultraviolet spectroscopy involves obtaining, studying and using absorption and reflection spectra in the ultraviolet region. It is designed to transfer the electrons of organic molecules from the ground state to the excited state. The energy of photons in the ultraviolet and visible regions of the spectrum is quite high (1.7-100 eV, or about 100 to 730 nm). The range of ultraviolet radiation for analytical studies is within (2000 - 4000A). To study the structure and composition of hydrocarbons, their absorption spectra in the ultraviolet and infrared regions are determined. Ultraviolet spectroscopy is more important for the analysis of aromatic compounds. Benzene, toluene, paraffins, naphthenes and olefins can be determined in this way. The field of application of UV spectroscopy was limited to basic aromatic hydrocarbons, but in recent years has expanded due to the development of the synthesis of new aromatic polymers and polymers containing double bonds. When solving analytical problems, the main advantages of the ultraviolet spectroscopy method are its high sensitivity, accuracy and high speed of analysis in the identification of hydrocarbons, as well as the simplicity of the experimental technique and equipment, the small amount of substance required for the study is

Chemical reactions

53

sufficient. The disadvantage of this method in some cases is that its use in analytical studies is limited, which can sometimes be explained by the overlap of the spectra and their insufficient selectivity. For qualitative and quantitative analysis of the identification of mixtures of hydrocarbons, the range of application of ultraviolet absorption spectra is limited to basic aromatic hydrocarbons, since only they have sufficiently characteristic spectra. Using ultraviolet spectroscopy, individual aromatic hydrocarbons, the amount of naphthalene hydrocarbons, naphthalene and methylnaphthalene isomers are determined. This analytical method is used to determine both phenols and thiophenols in gasoline (in the 290 and 265 nm wavelength range). Using ultraviolet spectroscopy, one can obtain information about one-, two- and multi-ring aromatic hydrocarbons.

Figure 8. UV spectrum of cyclopentane

Since nuclear magnetic resonance (NMR) is a complex physical process, the mathematical apparatus of quantum mechanics is used to describe it. We will use a simple “classical” model to understand the physical basis of NMR. If we imagine the nucleus of an atom as a spinning ball with a “+” charge, we will see that the charge moves in a circular orbit, creating a microscopic ring current. Since the ring current, in turn, induces a magnetic field, such a core behaves like a microscopic magnet. In this case, the magnetic moment of the core is directed along the axis of rotation, precessing the axis. Such a nucleus can be compared to a small spindle-shaped magnet with its own spin and magnetic moment. In the absence of an external magnetic field, the spin states are energetically

54

Chapter 3

degenerate. When the nucleus is placed in an external magnetic field, entanglement disappears, i.e., the magnetic moments directed towards the field and against it have different energies. As a result, there is a transition from one energy level to another energy level. The splitting of energy levels in a magnetic field is usually called the Zeeman effect. It is this transition, i.e., the absorption of electromagnetic rays by the nuclei of a sample placed in a magnetic field, that forms the physical basis of NMR spectroscopy. Simply put, NMR spectroscopy can be described as follows: a compound or molecule under study is placed in an ampoule, placed in a magnetic field H0, and irradiated with electromagnetic beams with a frequeQF\Ȧ:KHQDZHDNUDGLRIUHTXHQF\PDJQHWLFILHOG+٣ is applied to the magnetic field H0 (H٣H0 ) and the energy is absorbed at a IUHTXHQF\YDOXHȦFRUUHVSRQGLQJWRǻ( KȦLH105VSHFWURVFRS\LV PDLQO\REVHUYHGDWDIUHTXHQF\Ȧ Ȗ+DQGWKHQDWXUHRIthe resonance LV PRPHQWXP 3  ƫO DQG WKH PDJQHWLF PRPHQW LV GHWHUPLQHG E\ — Ȗ O where P is the spin of the l-QXFOHXV Ȗ-FKLURPDJQHWLF QXPEHU ƫ-Planck constant. At present, 80 magnetic nuclei can be simultaneously studied using modern NMR facilities. Since NMR spectroscopy is the main physicochemical method, its results can be used to obtain information about both intermediate and final products of chemical reactions. NMR conformational equilibrium, diffusion of atoms and molecules in solids, internal motions, hydrogen bonds, etc. NMR is based on the quantum mechanical magnetic properties of the atomic nucleus. When a sample is placed in a strong magnetic field, the magnetic energy levels of the nuclei split (similar to the splitting of electronic levels in a magnetic field). Energy levels are created as a result of the interaction of the magnetic moments of nuclei with an external magnetic field. Nuclear magnetic resonance developed in two directions; proton magnetic resonance and C13 nuclear magnetic resonance based on C13 isotope. In the first case, the spectrometer provides information on the state of protons in the compound under study, and in the second case, on the state of the C atoms that make up the compound.

Chemical reactions

55

Figure 9. NMR spectrum of acetic acid

Figure 10. C13 NMR spectrum of vinyl acetate ether.

Electron paramagnetic resonance (EPR) is a phenomenon of resonant absorption of electromagnetic radiation by paramagnetic particles placed in a constant magnetic field, and is one of the methods of radio spectroscopy. It is used to study systems with a spin magnetic moment (that is, having one or more unpaired electrons): atoms, free radicals in gas, liquid and solid phases, systems in the triplet state, transition metal ions. In the absence of a stable magnetic field H, the magnetic moments of unpaired electrons are arbitrarily directed, and the state of the system of such particles becomes energetically degenerate. When a field H is applied, the projections of the magnetic moments in the direction of the

56

Chapter 3

field take on certain values and the degeneracy is lifted (see the Zeeman effect), i.e., the electronic energy level E0 splits.

CHAPTER 4 ALKANES

1. Acyclic saturated hydrocarbons Compounds consisting of two elements - carbon and hydrogen - are called hydrocarbons. Alkanes are straight or branched carbon compounds without double or triple bonds. Alkanes are the simplest organic compounds; their molecules are built only from carbon and hydrogen atoms connected to each other by simple (single) bonds. Saturated hydrocarbons are widely distributed in nature and are used in various practical applications. All other, more complex organic substances are derivatives of alkanes and can be obtained from them by replacing hydrogen atoms with other atoms or groups of atoms. The opposite is also possible, i.e., the synthesis of hydrocarbons from other compounds by chemical transformations. Saturated hydrocarbons are less reactive under normal conditions and are therefore called paraffins (lat. parumaffinis - “inert”). The general formula for the homologues of the methane series is CnH2n+2, where n = 1, 2, 3, etc. The first representative of saturated hydrocarbons is methane CH4.

Chapter 4

58

H H C H H H H H C C H H H H H H H C C C H H H H H H H H H C C C C H H H H H H H H H H H C C C C C H H H H H H

structural formula of methane

structural formula of ethane

structural formula of propane

structural formula of butane

structural formula pentane

Each subsequent member of this series differs from the previous one in the CH2 group. Such a series of organic compounds similar in structure, each subsequent member of which differs from the previous one by the CH2 group, is called a homologous series. The members of the series are called homologues, and CH2 is called the homologous difference. For example, methane and decane, propane and butane are homologous to each other. If the number of carbon atoms is taken as n, then the general formula for alkanes is CnH2n+2. Methane, ethane and propane do not have isomers. Two structural formulas can be written for butane and three for pentane:

Alkanes

59

H H H H H H H C C C C H H H H H

H H H C C C H H H H H

H H H H H H C C C C C H H H H H H

H C

H C

H

H H H H C C C C H H H H H

H H H C H H

H C

H

C

H C H H

C H

H

This type of isomerism is called carbon skeletal isomerism. As the number of carbon atoms in a molecule increases, the number of structural isomers increases. Consider the rules for the nomenclature of alkanes. The most complete set of rules currently in use has been developed at international conferences and is called the “Regulations on the International Union of Pure and Applied Chemistry (IUPAC nomenclature)”. Unfortunately, in practice, no one strictly follows these rules, so you need to familiarize yourself with other naming systems. The first four representatives of the homologous series of straight chain alkanes have non-systematic names. The subsequent members of the series, starting with pentane, are called according to a certain system: the names consist of a prefix indicating the number of carbon atoms (pent-, hex-, hept-, etc.), and the ending an, indicating that the compound refers to paraffin class. The names may refer to branched or straight hydrocarbons, and the prefix n- (normal) is usually added to denote a straight chain. The replacement of hydrogen atoms in a normal hydrocarbon by hydrocarbon radicals is necessary to form the name of a branched hydrocarbon chain. A hydrocarbon radical is a neutral particle; an unpaired electron is formed during the symmetrical (homolytic) splitting of bonds of a hydrocarbon molecule:

Chapter 4

60

H

H

H

C

C

H

H

H

H

H

H

C

C

H

H

H

When a hydrogen atom is removed from an alkane molecule, monovalent radicals with one unpaired electron are formed. The names of such radicals are formed from the names of the corresponding ones: replacing the suffix -an in hydrocarbons with -il in radicals: methyl

CH3

CH3

CH3 CH2

butyl

CH2 CH2 CH2

ethyl

CH3 CH2 CH2

CH3 CH2 CH CH2

propyl

secondary butyl

CH3 CH3

CH CH2

isobutyl

CH3

CH3

C

tret-butyl

CH3

Positively and negatively charged particles - carbocations and carbanions are formed when an asymmetric (heterolytic) bond of a hydrocarbon molecule is broken:

H

H

H

H

C

C

H

H

H

H

C

C

H

H

H

H

H

H

C

C+

+ H-

H H karbkation

H

H

H

H

C

C

H H karbanion

-

+ H+

Alkanes

61

Chemical reactions involving hydrocarbons and their derivatives are called radical if they proceed with the formation of radicals. If the reactions proceed with the formation of carbocations and carbanions, they are called ionic reactions. According to the IUPAC nomenclature rules, to designate alkanes: a) select the longest chain of carbon atoms in the structural formula and number these atoms, starting from the end closest to the branch; b) the number in the name of the substance must indicate on which carbon atom the substituent group (radical) is located; c) if there are several groups of substituents, each of them must be designated by a number; d) when the branching starts from carbon atoms, at an equal distance from the ends of the carbon chain, the numbering is carried out from the end closer to the radical with a simpler structure. CH3 H3C

HC H2C CH3

H3C

CH3

3 HC

CH3

CH3 2,2-dimethylpropane

2-methylbutan 1 2 H3C H2C

C

4 5 6 HC CH2 CH3

1

2

H3C

HC

CH3 C2H5

3 4 HC CH3

CH3 CH3

3-methyl-4-ethylheksane

2,3-dimethylbutane

A carbon atom bonded to only one neighboring carbon atom is called a singlet (singlet carbon atoms are highlighted in the formula 2,2,3trimethylpentane): CH3 H3C C

HC H2C CH3

CH3 CH3

A carbon atom connected to two carbon atoms is called binary (in the formula of 2,2,3-trimethylpentane, the binary carbon atom is highlighted):

Chapter 4

62

CH3 H3C C

HC H2C CH3

CH3 CH3

The triplet carbon is bonded to three carbons:

CH3 H3C C

HC H2C CH3

CH3 CH3

Finally, an example of a quadruple carbon (attached to four carbons) is the second carbon of 2,2,3-trimethylpentane: CH3 H3C C

HC H2C CH3

CH3 CH3

In addition to the IUPAC nomenclature, replacement nomenclature is used less frequently. According to the latter, any alkane is considered to be methane in which hydrogen atoms are partially or completely replaced by radicals: CH3 H3C C H3C CH3 tetramethylmetane

H H3C C H3C CH3 trimethylmetane

1.1. Physical properties of saturated hydrocarbons. Saturated hydrocarbons are colorless substances, practically insoluble in water and having a density less than 1. Depending on the composition, they are gaseous, liquid or solid.

Alkanes

63

As the mass of a molecule increases, the boiling point, melting point, and density increase. Methane, ethane, propane and butane are gases under normal conditions; they are almost odorless. Pentane and subsequent hydrocarbons (up to C16H34) are liquids with a characteristic gasoline smell and gradually decreasing volatility. Higher saturated hydrocarbons are odorless solid non-volatile substances. The boiling and melting points of hydrocarbons depend not only on the number of carbon atoms in the molecule, but also on their structure. Normal hydrocarbons boil higher than isostructural hydrocarbons and vice versa, with the most branched isomer having the highest melting point. Table 11 Structural dependence of the physical properties of isomeric hydrocarbons (pentanes). Name

Consist

Compound

n-pentane

C5H12

H3C H2C H2C H2C CH3

Tqay. 0 C 36,1

2-methylbutane (isopentane)

C5H12

H3C

27,9

2,2-dimethylpropane

C5H12

HC H2C CH3 CH3 CH3

9,5

TΩU0C – 129,8 – 159,9 -16,6

H3C C CH3 CH3

The differences noted in the properties of hydrocarbons of different composition and structure is used in the separation of their mixtures, for example, in the refining of oil. Alkanes have an anesthetic effect. Because of their low water and blood solubility, very high air concentrations are required to produce toxic blood levels. Therefore, under normal conditions, lower alkanes are physiologically inactive. Hydrocarbons C5H12 - C8H18 moderately irritate the respiratory tract. The higher representatives of the homologous series are more dangerous when it comes into contact with the skin.

Chapter 4

64

1.2. Chemical properties of saturated hydrocarbons. Common features. Saturated hydrocarbons are substances that react little at ordinary temperatures. Alkanes are not capable of addition reactions, but under certain conditions they enter into substitution reactions with the breaking of CH bonds, as a result of which hydrogen atoms are replaced by other atoms or groups of atoms that form hydrocarbon derivatives. When exposed to high temperatures or chemicals, alkane molecules can split, destroying the carbon skeleton. Since C-H bonds are less polar, their cleavage occurs by a homolytic mechanism with the formation of free radicals. This is facilitated by high temperatures, catalysts or ultraviolet (UV) radiation. Don’t burn. However, all saturated hydrocarbons react with oxygen at high temperatures, and in the presence of excess oxygen, carbon 4-oxide (IV) and water are formed. A large amount of petroleum hydrocarbons is used as a fuel for the production of heat and electricity by combustion: ɋ+4 Ɉ2 ĺ&22 + 2H2O + 880 kC ɋnɇ2n+2 Q Ɉ2 ĺQ&22 + (n+1) H2O +Q A mixture of methane with oxygen or air can explode if ignited. The most powerful explosion is achieved by mixing methane and oxygen in a volume ratio of 1:2. Therefore, mixtures of methane with air in coal mines, factory boilers and apartments are dangerous. During the combustion of solid alkanes, there is not enough oxygen to burn all the carbon, and carbon is released in free form, which can be seen when burning a paraffin candle - a lot of soot is formed. When burning gaseous alkanes, soot usually does not form, since they mix well with air and burn completely. Thermal decomposition. With strong heating without access to oxygen, hydrocarbons decompose into simple substances - carbon and hydrogen:

CH4 ĺ&+2 – Q C2H6 ĺ&+2 – Q Isomerization H3C H2C H2C CH3

AlCl3 t

H3C

HC H3C CH3

Alkanes

65

Reactions of radical substitution. Interaction with halogens. Under normal conditions, chlorine and bromine react very slowly with saturated hydrocarbons. If a liquid saturated hydrocarbon is shaken with bromine water, the hydrocarbon layer becomes yellow or brown after separation of the liquids, since bromine is more soluble in the hydrocarbon and passes into it through the water layer; but the brown color of bromine does not disappear because it hardly reacts. The interaction of saturated hydrocarbons with chlorine or bromine is accelerated by heating and especially in the light. Halogen gradually replaces hydrogen atoms in alkane molecules, forming mixtures of halogen derivatives of hydrocarbons, and hydrogen halides are released. For example, when chlorine acts on methane, hydrogen atoms are gradually replaced by chlorine atoms and a mixture of chlorine derivatives is formed.

CH3Cl + HCl

CH4 + Cl2

chloromethane

CH3Cl + Cl2

CH2Cl2 + HCl methylenchloride

CHCl3 chloroform

CH2Cl2 + Cl2 CHCl3 + Cl2

+ HCl

CCl4 + HCl tetrachloromethane

This reaction, identified by the Nobel laureate N. N. Semenov, has a radical chain character and is denoted by the symbol SR. At the first stage of the reaction, the chlorine molecule is split into two free radicals (atoms), which is called chain nucleation or initiation.

Cl

Cl

2 Cl

Then, when chlorine atoms collide with a hydrocarbon molecule, they abstract an H atom to form alkyl radicals, which abstract a chlorine atom from a chlorine molecule to form a haloalkane and a newly released Cl &OÂ UDGLFDOZKLFKUHDFWVDJDLQZLWKDQRWKHUK\GURFDUERQPROHFXOHWKXV also continuing the kinetic chain. This phase, called the growth of the chain, lasts long enough until it breaks.

Chapter 4

66

H

CH3 + Cl

weak

CH3 + HCl

CH3 + Cl Cl

CH3Cl + Cl

chain growt

CH3Cl

CH3 + Cl Cl + Cl

Cl2

CH3 + CH3

CH3 CH3

chain break

With direct halogenation of more complex hydrocarbons, hydrogen substitution can occur on different carbon atoms. For example, when excess propane is chlorinated, the reaction proceeds in two directions - two mixtures of alkyl halides are formed: H3C H2C CH3

+ Cl2

hv

H3C

-HCl

HC CH3 Cl

+

H3C H2C CH2 Cl

2-chloropropane 1-chloropropane

In these reactions, it is easier to replace the H-atom attached to the quaternary C-atom than to replace the H-atom attached to the double and single C-atoms, which is explained by the different degree of stability of the resulting alkyl radicals. The third R+ is more stable, and hence the reaction rate substitutions above. Bromination proceeds more selectively than chlorination, with the formation of almost one product.

H3C H2C CH2 CH3 n-butane

+ Br2

hv -HBr, 98%

H3C HC CH2 CH3 Br 2-brombutane

Reaction with thick nitric acid (HNO3) (nitration reaction). Does not interact with alkanes at 20°C, but oxidizes them when heated. In 1889 M.I. Konovalov showed that alkanes can react with dilute nitric acid when heated and under pressure. Under these conditions, hydrogen in the

Alkanes

67

hydrocarbon molecule is replaced by a nitric acid residue - NO2 (nitro group).

H3C H2C H2C CH2 NO2 t, p

H3C H2C H2C CH3 + HNO3 n-butane

1-nitrobutane

-H2O

H3C HC H2C CH3 NO2

2-nitrobutane

As with halogenation, the nitration reaction proceeds regioselectively the H atom at the double C atom is more easily replaced, which leads to a higher yield of 2-nitrobutane. Later, a method was developed for the interaction of alkane vapors with nitric acid vapors for the nitration of saturated hydrocarbons in the gas phase. The reaction is accompanied by cracking of hydrocarbons, resulting in the formation of a mixture of mononitro derivatives. Thus, nitration of ethane gives nitroethane and nitromethane:

CH3 CH3 + HNO3

420 0C -H2O

CH3 CH2 NO2 + CH3 NO2

Alkane nitro compounds are colorless liquids, insoluble in water, with a slight ethereal odor. Reaction with sulfuric acid (sulfonation reaction). In frost, even fuming sulfuric acid (oleum) does not affect saturated hydrocarbons, but can oxidize them at high temperatures. At 100 °C, branched alkanes react with oleum to form a triple carbon hydrogen substitution product. Hydrocarbons combined with a sulfuric acid residue SO2OH (sulfo group) are called alkane sulfonic acids.

Chapter 4

68

CH3 CH CH2 CH3 + H2SO4 CH3

t0 SO3

SO3H CH3 C CH2 CH3 + H2O CH3 2-Methylbutane-2-sulfonic acid

Sulfonic acids of saturated hydrocarbons with a chain of 8-20 carbon atoms are used for the production of detergents - synthetic detergents. The action of oxidizing agents. At low temperatures, saturated hydrocarbons are resistant even to strong oxidizing agents. So, for example, a solution of KMnO4 or a mixture of chromium (K2Cr2O7 + H2SO4) does not oxidize saturated hydrocarbons at 20 °C. At high temperatures, under the action of atmospheric oxygen, alkanes ignite and burn with the formation of CO2 and H2O, releasing a large amount of heat. This is due to their use as fuel.

CH4 +2O2—ĺ&22 + 2H2O 891 kC/mol The combustion of 1 kg of methane releases 55,000 kC of heat. At present, methods have been developed for the low-temperature oxidation of hydrocarbons with atmospheric oxygen in catalysts. By oxidation from the kerosene fraction of oil, mixtures of high-fatty acids can be obtained, shown in the diagram: H3C CH2)mCH3

O2

H3C CH2)n COOH + H3C CH2)p COOH n, p < m

Oxidation is accompanied by the breaking of carbon chains, so the resulting acids contain fewer carbon atoms than the original hydrocarbons. This process is of great importance because the higher fatty acids are used to make candles and various detergents. Cracking of hydrocarbons. At high temperatures (450-550 °C) in the absence of air, hydrocarbons decompose, breaking carbon chains and forming simpler alkanes and unsaturated hydrocarbons. This process is called cracking. The decomposition of hydrocarbons at temperatures

Alkanes

69

above 800 °C leads to the formation of the simplest gaseous hydrocarbons; in addition, at this time, carbon chains are converted into cycles and aromatic hydrocarbons are formed. This process is called pyrolysis. The use of special catalysts in the processes of cracking and pyrolysis and pressure leads to the regulation of these processes and the production of the necessary products. Dehydrogenation reaction. It consists in the separation of hydrogen molecules from alkanes and leads to the formation of unsaturated and cyclic hydrocarbons. It is of great industrial importance. The dehydrogenation of alkanes requires the presence of a heterophase catalyst - Cr2O3, Pt, Pd, Ni. Propylene formation: 400-600 0C

CH3

CH2

CH3

Cr2O3

CH3

CH

CH2

+ H2

2. Cyclic saturated hydrocarbons Ring hydrocarbons, consisting of carbon atoms connected by simple bonds, are called cycloalkanes or cycloparaffins. Homologous sequence. Nomenclature. Isomerism. According to IUPAC rules, the name cycloalkane comes from the name of an n-alkane with the same number of carbon atoms and the prefix cyclo-. For example:

cyclobutane

cyclohexane

cyclododecane

Chapter 4

70

Homologous order of cycloparaffins:

cyclopropane

cyclobutane

cyclohexane

cyclononane

cyclopentane

cyclodekane

The general formula of cycloalkanes is CnH2n. In addition to saturated monocyclic hydrocarbons with the general formula CnH2n, cycloalkanes in the narrow sense are also known, saturated hydrocarbons whose molecules include two, three or more rings (general formulas CnH2n-2, CnH2n-4, etc.). Since the number of carbon atoms simultaneously belonging to two neighboring rings is not the same, they are considered different classes of compounds. If two cycloalkanes are connected by a carbon atom, such a compound is called a spiran (from the Latin spira - gyrus, a bun that looks like a figure eight). If two rings share two carbon atoms, then a condensed ring system is formed. In bridged ring systems, the rings always share at least three carbon atoms. Spirants:

spiro-[3,3]-heptane

spiro-[2,4]-heptane

Condensed cycle systems:

bicyclo-[4,1,0]-heptan norkaran)

bicyclo[3,2,0]-heptane

Alkanes

71

Bridge cyclic systems:

bicyclo[3,1.1] heptane

bicyclo[2,2,1] heptane norbornene)

According to the rules of IUPAC nomenclature, substituents are numbered according to their position in the cycle, so that the sum of the numbers is minimal.

CH3

CH3

C2H5 CH3 1,4-dimethylcyclohexane

1-methyl-3-ethylcyclopentane

Cycloalkanes are characterized by several types of isomerism. 1. Isomerism of the carbon skeleton:

CH3

CH3 CH3 cyclopentane

methylcyclobutane

1,2-dimethylcyclopropane

2. Isomerism of substituent positions: CH3 H3C

CH3

C2H5

CH3 1,2-dimethylcyclopropane

1,1-dimethylcyclopropane 2-ethylcyclopropane

Chapter 4

72

3. Interclass isomerism: Cycloalkanes are isomers of alkenes with the corresponding number of carbon atoms:

H2C HC

CH3

propene

cyclopropane C3H6 4. Geometric (cis-, trans-isomerism):

H3C

CH3

H3C

H

H

H

sis-1,2-dimethylcyclopropane

H

CH3 trans-1,2-dimethylcyclopropane

Physical properties. The physical properties of cycloalkanes are similar to other hydrocarbons. The first two, cyclopropane and cyclobutane, are gases at room temperature; those with a ring size of C5 to C11 are liquids; higher cycloalkanes are solids. With an increase in the cycle size T increases. Cycloalkanes are immiscible with water, but are miscible in all proportions with most non-polar solvents. Chemical properties. Coupling reactions: Let us analyze the schemes of chemical reactions characteristic of cycloalkanes. For comparison, let’s start with the reactions of propylene:

Alkanes H2, Ni 20 0C Br2

H2C HC CH3

73

H3C H2C CH3

H2C Br

HBr

H C CH3 Br

H3C HC CH3 Br

H2, Ni 20 0C Br2

H3C H2C CH3 H2C H2C CH2 Br

HBr

Br

H2C H2C CH3 Br

H2, Ni 120 0C

H3C H2C CH2 CH3

Other coupling reactions with cyclobutane proceed under more severe conditions. Cyclopentane and cyclohexane do not enter into coupling reactions. Coupling reactions of cycloalkanes with substituents follow the Markovnikov rule:

Br CH2 + H Br H2C

CH

CH3

CH2

CH

CH3

CH3

Substitution reactions: Medium and large cycles are similar in their chemical properties to alkanes. Substitution reactions, such as the chlorination of cyclopentane and larger cycloalkanes, are simpler than the corresponding alkanes, since the formation of isomeric substitution products is less likely in this case:

Chapter 4

74

Cl hv

+ Cl2

one isomer

-HCl chlorcyclohexane

Reactions with cycle size change:

CH2 CH3 AlCl3

CH3

t ethylcyclobutane

methylcycpentane

AlCl3

CH3

t cycloheptane

increase in cycle size

methylcyclohexane

cycle size reduction

Alkanes

75

Preparation methods: Dehydrogenation of alkyl halides.

CH2

Cl

Zn

H2C

-ZnCl2

CH2 Cl

H2C

CH2

Br

2Na

H2C

CH2

Br

-2NaBr

Dimerization of alkenes:

CH2

CH2

C

CH2

CH2

C

CH2

C CH2

CH2 C

CH2

From oil: In nature, some types of oil (for example, Baku oil) contain significant amounts of cycloparaffins. Therefore, they are called naphthenes. Therefore, its value is highly valued in the world market. Hydrogenation of benzene: 3H2, Pt 200 0C

Chapter 4

76

Applied and exclusive representatives: Cyclopropane is a colorless gas used in medicine as a narcotic that does not damage the respiratory tract. Cyclohexane is a colorless liquid with a pleasant odor. It is widely used as a solvent, but is primarily important as a raw material for the production of polyamides. Its catalytic oxidation with oxygen can produce cyclohexanol, cyclohexanone or adipic acid, depending on the conditions:

OH O2

O

HOOC

CH2)4

cyclohexanol

cyclohexanone

COOH

adipic acid

Adamantane was isolated from oil in 1933. Chemists are especially interested in adamantane because the skeleton of this compound is the building block of diamond. A diamond crystal is a polymer built on the basis of an adamantane skeleton in which all C-H bonds are replaced by C-C bonds.

H

H

H

H adamantane

3. Detonation properties of hydrocarbons 3.1. Octane number. Alkanes are part of gasoline used as fuel for internal combustion engines. In the engine cylinder, fuel vapors are subjected to maximum compression; when it is ignited; its constituent

Alkanes

77

hydrocarbons immediately explode, forming products of complete combustion (CO2, H2O vapors). However, this process may be accompanied by detonation; premature explosion of the fuel before reaching maximum compression. During detonation, incomplete combustion occurs (with the formation of C, CO, H2 and lower hydrocarbons), the fuel is not used completely, and the rhythm of the engine is disrupted. It has been established that the detonation properties of hydrocarbons depend on their structure: the more branched the hydrocarbon chain, i.e., the more triple and quadruple carbon atoms in its molecule, the less prone to detonation and the higher its quality as a fuel; normal hydrocarbons are the most prone to detonation. So, the hydrocarbon 2,2,4-trimethylpentane (isooctane), which is part of gasoline, has high anti-knock properties; n-heptane is extremely prone to detonation. Standard mixtures are prepared from isooctane and n-heptane; knocking properties are compared with similar properties of gasolines of different brands, characterized by octane number (r.u.). For example, if the fuel has an ON of 85, this means that a mixture of 85% isooctane and 15% n-heptane is suitable for detonation properties.

CH3 CH3 C CH2 CH CH3 CH3

CH3

H3C H2C CH2 CH2 CH2 CH3 CH3 n-heptane

isooctane

For aviation and automobile engines, high-quality fuel must be high. That is, above 90 high-octane gasoline must be rich in hydrocarbons with a branched carbon chain. The antiknock properties of gasolines can be improved, for example, by adding various substances (antiknock agents) to them - WHWUDHWK\OOHDGɋ2ɇ5)4Pb, manganese salts. 3.2. Cetane number. Completely different requirements apply to diesel fuel. In this case, the fuel is injected into the chamber with heated air and should burn out with an explosion. Therefore, flammability plays a positive role here. The ideal diesel fuel is n-cetane (n-hexadecane) with a FHWDQH QXPEHU RI  WKH ZRUVW VWDQGDUG GLHVHO IXHO LV ȕmethylnaphthalene (0 cetane number).

78

Chapter 4

CH3 ȕ-Methylnaphthalene As diesel fuel, medium oil fractions boiling in the range of 230-290 °C are particularly suitable. To increase the flammability of diesel fuels, organic nitro compounds are added as accelerators.

4. Natural sources of saturated hydrocarbons Each class of organic substances, including saturated hydrocarbons, is characterized by a number of general methods of synthesis. The latter makes it possible to judge the relationship of compounds of one class with substances of other classes and the ways of their mutual transformation. In addition, the synthesis of substances from other compounds whose structure is known is one of the best ways to prove the structure of this substance. The main amount of alkanes is used in the form of their mixtures and is extracted from natural resources - oil, gas, shale, mountain wax. Natural combustible gases are mixtures of gaseous hydrocarbons; they are found in the earth’s crust, sometimes forming huge gas fields. In addition, combustible gases accompany oil (natural petroleum gas) and are often released in large quantities from wells during oil production. The main component of natural gases is methane. In addition to methane, petroleum gas also contains ethane, propane, butane and isobutane. Natural gas is a cheap and efficient fuel used both in industry and in everyday life, as well as a valuable chemical raw material. Especially promising is the use of natural petroleum gas for the chemical industry: the hydrocarbons contained in it are the feedstock for the production of synthetic rubber, plastics and other materials. Oil is a natural resource, a complex mixture of organic substances, mainly hydrocarbons. This is a valuable product that is associated with use in various areas of the economy. The composition of oil in different fields is not the same. Some oils contain significant amounts of aromatic hydrocarbons. Oil consists of liquid, solid and gaseous hydrocarbons dissolved in it. With high gas content, oil sometimes gushes from wells under their pressure. Oil is an efficient and cheap fuel. In addition, this is the most valuable chemical raw material, on the basis of which synthetic rubber, plastic

Alkanes

79

substances, etc. are obtained. Distilling oil, you can buy products for various purposes. The main method of oil refining is fractionation (distillation) after the primary removal of gases, as a result of which the following main oil products are isolated: – Gasoline FUXGH ɋ–ɋ Ɍb. = 40-ƒɋGHQVLW\ -0.780 g/cm3. - Naphtha (mixture of (C7–&  K\GURFDUERQV  Ɍb. = 120- ƒɋ density = 0.785-0.795 g/cm3. It is used as a filler and extractant for liquid devices. – Kerosene ɋ–ɋ  7b = 200- ƒɋ GHQVLW\  -0.846 g / cm3. – Gas oil ɋ–ɋ 7b = 220–ƒɋGHQVLW\ –0.919 g/cm3. – Oil residues (fuel oil). The gasoline fraction contains hydrocarbons with 5-12 carbon atoms. Oil or petroleum ether (Tb = 40-70 ° C), gasolines for various purposes aviation, automobile (Tb = 70-120 ° C), etc. are isolated from it by distillation. Hydrocarbons with 9-16 carbon atoms in the fraction, and fuel oil (fuel oil) is a mixture of higher hydrocarbons. At temperatures above 300 °C, a certain amount of products that do not decompose at this temperature, which are called solar oils, are distilled from fuel oil and are used as various types of lubricants. . In addition, valuable products such as petroleum jelly and paraffin (the latter is a mixture of solid hydrocarbons rich in certain types of oil) are obtained from fuel oil by distillation, low pressure distillation or steam distillation. After the processing of fuel oil, the residue, called tar, is used to cover roads. Diesel fuel is also used as a direct fuel. For modern technology, the most valuable products of oil refining are gasoline. However, during direct distillation, no more than 20% of the gasoline fraction is obtained from oil. By cracking the higher fractions of oil, its yield can be increased up to 6080%. The first oil cracking unit was built in Russia in 1891 by engineer V. G. Shukhov. There are the following main types of hacks: - liquid phase, in which the raw material (fuel oil) is fed into the cracking furnace in liquid form; - vapor phase, when the raw material is supplied in vapor form; - catalytic, where the raw material is split in special catalysts. Depending on the type of cracking, cracked gasolines of different composition and for different purposes are purchased. During cracking, along with liquid gasoline hydrocarbons, simpler gaseous hydrocarbons, mostly unsaturated, are obtained. They produce

80

Chapter 4

gases called cracking gases (up to 25% cracked oil) and are a valuable industrial source of unsaturated hydrocarbons. A certain amount of light gasoline can be obtained by compressing petroleum gas; the vapors of gasoline hydrocarbons contained in it are condensed to form natural gasoline. Mountain wax or ozokerite is a mixture of solid hydrocarbons. Its resources are located in the Caspian Sea, Central Asia, Krasnodar Territory, Cheleken Island in Poland. Ceresin, a solid substitute for wax, is isolated from ozocerite.

CHAPTER 5 UNSATURATED HYDROCARBONS

Unsaturated hydrocarbons are hydrocarbons that have lower elemental hydrogen content than saturated hydrocarbons with the same number of carbon atoms. In unsaturated hydrocarbons, the C atom is also tetravalent, but in addition to single bonds, it can form multiple (double or triple) bonds with each other. Due to the presence of multiple bonds, unsaturated carbon atoms in hydrocarbons are not completely saturated with hydrogen; therefore - they are called unsaturated. Unlike alkanes, unsaturated hydrocarbons are highly active in chemical reactions and are especially prone to coupling reactions.

1. Alkenes (olefins) 1.1. Homologous order, isomerization and nomenclature of alkenes. Alkenes (ethylene hydrocarbons or olefins) are unsaturated hydrocarbons, in the molecules of which there is one double bond between C atoms, i.e., the >C=C< group. Ethylene hydrocarbons form a homologous series, the composition of each member of which is expressed by the general empirical formula CnH2n. The first representative of this series is a hydrocarbon containing C2H4 - ethylene:

C C

H2C

CH2

Both carbon atoms and all H atoms in ethylene are equivalent. Thus, ethylene corresponds to a univalent radical called vinyl CH2=CH– (see Table 2). As in the alkanes series, ethylene hydrocarbons, starting with homologues with four carbon atoms, exhibit isomerism. However, alkenes have more isomers than alkanes with the same number of carbon atoms because, in addition to carbon skeleton isomerism, alkenes are

Chapter 5

82

characterized by double bond position isomerism and geometric isomerism. Thus, there are only two isomeric alkanes of C4H10 composition - butane and isobutane. There are four unsaturated hydrocarbons with the composition C4H8. CH3

H3C H2C HC CH2 but-1-en

H3C

C CH2

2-methylpropene

H

CH3 CH3

CH3

C

C

H sis-but-2-en

C H

C

H

CH3

trans-but-2-en

A double bond prevents the atoms attached to it from rotating around the bond line. Therefore, the structure of cis- and trans-but-2-ene molecules is spatially stable; the distance between the methyl groups in each is different, which means that the sizes of these molecules are not the same. Such a difference in spatial structure also leads to unequal properties: for example, the cis-isomer corresponds to Tm = -ƒɋDQG 7E ƒɋDQGWUDQV-isomer: -ƒɋDQGƒɋ*HRPHWULFLVRPHULVP is possible only when both alkene substituents are different at each carbon atom of the ethylene group. For example, geometric isomerism is impossible for butylene, which has a double bond between the first and second carbon, since its molecule has two identical substituents - hydrogen - at one of the unsaturated carbon atoms. 1.2. Physical properties of alkenes. The C=C double bond is often called an olefin bond, and alkenes are called olefins (they form oils). These terms are due to the fact that gaseous low molecular weight alkenes under the action of chlorine or bromine give fatty products. Ethylene hydrocarbons are colorless. The boiling and melting points of normal ethylene homologues increase with an increase in the number of carbon atoms in their composition. The first three homologues (C2 – C4) are gases, amylenes (C5H10) – C16H32 are liquids, and higher ethylene hydrocarbons are solids. Alkenes are insoluble in water, their density is less than 1, but slightly higher than that of alkanes. Naming. Branched-chain alkenes are named according to the IUPAC nomenclature as follows: 1. The longest carbon chain is selected and numbered starting from the end closest to the double bond. If the double bond is equidistant from both ends, the substituents are numbered from the lowest numbered position. 2. Position and names of substituents (radicals) are indicated. 3. The position of the double bond is indicated by a number and the name of the alkene is given.

Unsaturated hydrocarbons

H3C C CH2

3

4

H2C HC CH2 CH3

CH3

1-butene

2-methyl-1-propene

4 3

2

1

3 2 1

83

1 H3C

2 1

H3C HC C CH3

2 C

3 4 C CH3

CH3 CH3

CH3

2,3-dimethyl-2-butene

2-methyl-2-butene 1 2 3 4 5 H2C C CH CH2 CH3 C2H5 CH3 2-methyl-3-ethyl-1-pentene

According to the Geneva nomenclature, after the name of the alkene, a number is placed indicating the position of the double bond:

1 H3C

2 C

3 4 C CH3

CH3 CH3 2,3-dimethylbutene-2 There are three types of isomerism in alkenes: 1. Isomerism of the carbon skeleton. 2. Isomerism of the double bond state. 3. Cis-, trans-isomerism. The first two types are types of structural isomerism and cis-trans- is spatial isomerism.

Chapter 5

84

H2C

CH CH2

CH3

1-butene H2C

C

CH3

CH3 2-methyl-1-propene 1-butene and 2-methyl-1-propene have the same molecular formula C4H8, but differ in structure and properties, so these compounds are isomers. The first isomer is straight chain, while the second is branched, which is an example of carbon skeletal isomerism. 1-butene differs from 2-butene only in the position of the double bond, but this difference in structure is reflected in the chemical properties. Here, let’s get acquainted with the isomerism of the position of the double bond. H

H C H3C

H C

C CH3

sis-2-butene

H3C

C

CH3 H

trans-2-butene

If the hydrogen atoms standing on different carbon atoms are located on the same side of the plane of the double bond, then such an isomer is called a cis isomer, and if they are on opposite sides, a trans isomer. 1.3. Chemical properties. Unlike alkanes, hydrocarbons of the ethylene series are characterized by various reactions of addition of a double bond, which are easier than the reactions of substitution of the H atom. All specific properties of alkenes are determined by the nature of the double bond (see Chap. 1 and 2,). The ʌ-bond is weaker than the ı-bond; when it is broken, two atoms or groups of atoms are attached to the Catoms. ʌ- Electrons are farther from the plane of the molecule, therefore more accessible to attack than the nucleus of the C atom, so only electrophiles can attack these bonds. In this regard, the most typical reactions for unsaturated hydrocarbons are electrophilic coupling (AE) reactions. Electrophilic coupling reactions. Classical reactions of this type are the reactions of addition of hydrogen halides, halogens, and sulfuric and

Unsaturated hydrocarbons

85

hydrochloric acids to alkenes. The AE reaction mechanism is shown in the following diagram: C

C

+ E

C

zeif

E

C

electrophile

carbcation substrate elektrophilic attack

C E

E C

+Nu

C

C

complex

adduct

Nu

nukleophilic attack

First, the electrophilic particle is attracted by the bond electrons and forms a molecular complex in which the reactants are directed in a certain way. When the activation energy is reached, the electrophile breaks off a SDLURIʌ-electrons, thereby forming a new bond with the C-atom, and the neighboring carbon atom acquires a “+” charge and becomes a carbocation. Since this step consumes the most energy, it proceeds slowly and is the limiting step for the entire reaction. The carbocation is very reactive and readily reacts with any nucleophile present in the reaction zone to form a reaction product. Hydrogenation (hydrogenation reaction). This reaction proceeds at high temperature and pressure in the presence of a catalyst (nickel, platinum or palladium):

Ni H2C CH2

+ H2

t

H3C

CH3 H

H3C C CH2 + H2 CH3

Ni t

H3C C CH3 CH3

It is assumed that the reaction proceeds on the surface of a solid catalyst (nickel, platinum, or palladium). In this case, alkene and hydrogen PROHFXOHVDUHDGVRUEHGRQWKHFDWDO\VWWKHʌ-bond in the alkene is broken, and a hydrogen molecule is added at WKH VLWH RI WKH ʌ-bond break. The alkane formed on the catalyst surface is desorbed (torn off from the catalyst surface) into the space above the catalyst. To determine which side of the double bond (cis- or trans-) is attacked by an electrophilic and

Chapter 5

86

nucleophilic species (for example, Br+, Br-), the following experiment was carried out:

+ Br2 cyclohexene

H Br Br H trans-1,2-dibromocyclohexane

This example shows that the connection is transoidal. Otherwise cis1,2-dibromocyclohexane will be obtained: H H Br Br cis-1,2-dibromocyclohexane

In this reaction, two stages can be distinguished: the addition of a bromine cation and the addition of a bromine anion. The rate of a stepwise reaction is known to be determined by the slowest of the steps. In the case of the addition of bromine to alkenes, the slowest, i.e., limiting, step is the addition of the bromine cation. A positive particle (or particle without electron density) in organic chemistry is called an electrophile. If the reaction rate is limited by the addition of an electrophile, the reaction mechanism is called electrophilic bonding. The addition of HY-type reagents to unsymmetrical hydrocarbons is carried out in such a way that the hydrogen atom of the reagent is mainly connected to the most hydrogenated atom C by multiple bonds (Markovnikov’s rule).

Unsaturated hydrocarbons Br2

87

H2C HC CH3

H 2O

1,2-dibromopropane

Br Br H3C HC CH3

HBr

H2C HC CH3

2-bromopropane

Br

propylene H OSO3H

H3C HC CH3

isopropyl sulfuric acid

OSO3H

This direction of binding is determined by the relative stability of the carbocations formed in the rate-limiting step of the reaction. 1

H3C HC CH3 1

binary, more stable

2

H2C HC CH3

Y

H3C

H C CH3 Y

+H 2

X

H2C H2C CH3 single, less stable

The double carbocation is more stable than the first, so this direction of reaction is formed and carried out faster. Markovnikov’s rule obeys the general rule: at equilibrium, the reaction proceeds in the direction of the formation of a more stable intermediate at the limiting stage. The reaction with bromine is convenient for the qualitative and quantitative determination of unsaturated compounds; when interacting with bromine or its solutions (usually bromine water is used - a 1-3% solution of bromine in water), the brown color of the reagents immediately disappears. During the interaction of ethylene hydrocarbons with molecular hydrogen in the presence of catalysts (Ni, Pt), hydrogen atoms easily FRPELQHZLWKFDUERQDWRPVERXQGE\GRXEOHERQGVWKHʌ-bond is broken DQGWKHı-bond is retained.

Chapter 5

88

H2C CH2 H2C CH CH3

H2 Ni, Pt H2 Ni, Pt

H3C CH3

ethane

H3C CH2 CH3 propane

This reaction is of great practical importance for the conversion of unsaturated compounds of various classes into alkanes. Hydrogenation is used, for example, in the food industry to obtain solid fats (solid margarine fats) from liquid vegetable oils. Under normal conditions, ethylene hydrocarbons do not react with water, but when heated in the presence of catalysts (zinc chloride, sulfuric acid), water combines with carbon atoms of the double bond to form alcohols. Ethylene homologues bind water with a double bond according to Markovnikov’s rule:

H2C CH2 etylene

H2C CH CH3 propylene

H2O H2SO4 H2O ZnCl2

H3C CH2 OH

ethanol

H3C CH CH3 OH

propan-2-ol

Alkene polymerization reactions are of great practical importance. Polymerization is the process of formation of a giant molecule (polymer macromolecule) by successively connecting several simple molecules (monomers): R n H2C CH R

H2C CH

monomer

n polymer

where n is the degree of polymerization and indicates the number of monomer units in the polymer molecule. The polymerization of alkenes proceeds both according to the homolytic (radical-chain) and ionic mechanisms (cationic, coordinationionic polymerization). Radical chain polymerization requires harsh

Unsaturated hydrocarbons

89

conditions (high pressure and temperature) and the participation of initiators (peroxides) in the reaction, but gives a low-quality product: low molecular weight and low melting point, high polydispersity and low mechanical properties. Polymers with better physical and mechanical properties are obtained by carrying out coordination polymerization on Ziegler-Natta catalysts (Al(C2H5)3ÂTiCl4 + Li complex). According to the ionic mechanism, polymerization proceeds under mild conditions, and for asymmetric monomers of the CH2=CH–R type, isotactic and syndiotactic stereoregular polymers with high density and high mechanical strength are obtained. Oxidation reactions proceed more easily in unsaturated hydrocarbons WKDQLQDONDQHVGXHWRWKHLUʌERQGV Depending on the activity of the oxidizing agent and the conditions, unsaturated hydrocarbons are oxidized to various products (epoxides, glycols, aldehydes and ketones, carboxylic acids). At high temperatures, they burn to form CO2. One of the most characteristic oxidation reactions is the interaction of unsaturated hydrocarbons with a neutral or slightly alkaline solution of KMnO4 (E. E. Wagner’s reaction, 1888). At this time, cis-diols are formed: R1

HC CH R2 alkene

KMnO4 H2O

R1

H C

CH

OH OH

R2

+ MnO2 brown

glycol

The reaction proceeds in an aqueous solution, so an oxygen atom from the oxidizing agent KMnO4 is added to the place of the double bond. The reaction under consideration is very sensitive and is used for the qualitative determination of unsaturated compounds: already at 20 °C, the color of a neutral or alkaline solution of violet KMnO4 disappears and a brown precipitate of MnO2 forms. In an acidic environment, complete discoloration of the reagent occurs, since Mn+7 is reduced to a colorless Mn+2 ion. Under the action of strong oxidizing agents, ethylene hydrocarbon molecules are broken by breaking the carbon chain in a double bond with the formation of carbonic acids and ketones.

Chapter 5

90

O

H3C H2C HC CH CH3 pent-2-ene

H3C H2C COOH + H3C COOH

K2Cr2O7, H+

propionic asid

acetic acid

O

H3C

HC

C CH3 CH3

K2Cr2O7, H+

+ CH3 C CH3

H3C COOH

O

acetic acid

acetone

2-methylbut-2-ene

The direct catalytic oxidation of ethylene with air oxygen is of great practical interest, resulting in the formation of ethylene oxide, a valuable starting material for many syntheses: H2C

CH2

O2

H2C

Ag2O , t

CH2 O

Ozonolysis (Garry’s reaction). K. Garries (1910) found that even at low temperatures, depending on the conditions, ozone decomposes and quickly combines with the formation of ozonides, forming carbonyl compounds, into a C=C double bond: CH3COOR

R1

+ O

R2 +

R

H

alkene

O O ozone

Zn

R1

C

R2

+ R

O H2O2

C

H

O O

R1

C

R2

+ R C

OH

O

The KMnO4 oxidation reaction and ozonolysis can be used to prove the structure of alkenes. So, in the complete oxidation (reaction with KMnO4 when heated in an acidic medium and ozonation, then treatment with hydrogen peroxide), of isomeric alkenes of the formula C4H8, various reaction products are determined by determining the structure of the primary alkene.

Unsaturated hydrocarbons

O

O H2C

HC H2C CH3

HC HC CH3

+

H2CO3

H3C H2C C OH propionic acid

H2O

CO2

H3C

91

O

O

H3C

C

OH acetic acid H2C C CH3

CH3

O

H2CO3 CO2

H2O

+

H3C C CH3 O acetone

Nucleophilic compound. The introduction of electron-withdrawing groups into alkenes weakens the electrophilic substitution. It has been shown that the introduction of electron-withdrawing groups inhibits the coupling reaction initiated by the electrophile, but the same groups activate the coupling reaction initiated by the nucleophile. In the following series of substituents, the efficiency of both processes decreases: CHO > COR > CO2R > CN > NO2

7KHVH VXEVWLWXHQWV DFW E\ UHGXFLQJ WKH GHQVLW\ RI ʌ-electrons on the carbon atoms of the alkene, thereby facilitating the approach of the Ynucleophile and, most importantly, promoting its delocalization. As a result, the negative charge of the intermediate carbanion is delocalized. This delocalization is generally more efficient when it includes mesomeric delocalization:

Chapter 5

92

N

CN C Ph

Ph

CN

C

C

NC

C

Cl

HCN

Ph

CH3O

H3CO

H

Ph

F F C

F CH OH 3

H3CO

C H

C

C Cl Cl

+ CN H

F F

Ph

C

NC C

H

F

C

Ph

C H

Cl

NC

C

+ OCH3

Cl Cl

The dirHFWLRQRIDGGLWLRQRIDQXQV\PPHWULFDOUHDJHQW+ FeCl3 > BF3 > TiCl3 > ZnCl2 > SnCl4 In addition to alkyl halides, alkenes and alcohols can also be used to alkylate aromatic compounds. This requires the presence of a protic acid to protonate the alkene or alcohol. BF3 is often used as a Lewis acid catalyst in the following reactions:

MeCH CH2

H+

+ MeCH

CH3

-H2O MeCH OH

CH3

H+

MeCH +

OH2

CH3

C6H6 BF3

Me2CHPh

Chapter 6

132

Lewis acid type catalysts can also carry out dealkylation, which means that the alkylation reaction is reversible. For example, ethylbenzene undergoes disproportionation in the presence of BF3 and HF:

Et

Et HF

Et +

+

BF3

Et 45%

ethylbenzene

Acylation. Friedel-Crafts acylation follows the same general kinetic law as alkylation under the same conditions, where the kinetics can be easily controlled: Speed = k [ArH] [RCOCl ] [AlCl3] In this case, a similar general dilemma arises: does the acyl cation act as an acyl cation and effective electrophile in an ion pair (a) or in a polar complex (b)?

RC O AlCl4 a)

RC Cl

O---AlCl3 b)

Acyl cations have been found in a number of solid complexes, in the liquid complex of MeCOCl and AlCl3 (by IR spectroscopy), in polar solvents, and in some cases when R is a bulky group. In less polar solvents and in a number of other cases, acyl cations were not found, but in this case, a polarized complex should be present, acting as an electrophile. The acylation reaction can be represented by the following scheme:

Aromatic hydrocarbons

133

a) O H

b)

H

O C R

C R

+ HCl + AlCl3

O AlCl3 C R Cl

AlCl4

c)

An important difference between an acylation reaction and an alkylation reaction is that the former requires more than 1 mole of Lewis acid, while the latter requires only a catalytic amount. This is due to the fact that the Lewis acid forms a complex (d) with the ketone (c) - the acylation product, thereby excluding it from further participation in the reaction: C

O AlCl3

R d)

2.4. Action of oxidants. Benzene is more resistant to oxidizing agents than saturated hydrocarbons. A weak solution of HNO3, KMnO4, etc. is not oxidized with benzene homologues are more easily oxidized. However, their benzene ring is inactive towards oxidizing agents. But the radicals attached to the benzene ring (side chains) easily react with oxidizing agents.

O + 4,5 O2

V2O5 450 0C

O

+ 2 CO2 + 2 H2O

O maleic anhydride

Chapter 6

134

Complete oxidation (combustion) of benzene leads to the formation of carbon dioxide and water, in the absence of oxygen and in the presence of a V2O5 catalyst, partial oxidation breaks the benzene ring and leads to the formation of valuable industrial raw materials - maleic anhydride. Maleic anhydride easily reacts with 1 mole of water to form dibasic unsaturated maleic acid: O O

HC

+ H2O

COOH maleic acid

HC O

COOH

In benzene homologues, the side chains are oxidized first, the benzene core remains unchanged. No matter how complex the side target is, it is destroyed under the action of strong oxidizing agents, and only the carbon directly attached to the nucleus does not break away from it and turns into a carboxyl group (COOH). Thus, any benzene homologue with one side chain is oxidized to a monobasic aromatic (benzoic) acid, and with two or more radicals, to polybasic aromatic acids. R

COOH

R1 R2 O

O

COOH COOH

; benzoic acid

1,2,4-benzenetricarbonic acid R3

COOH

3. Industrial methods for obtaining aromatic hydrocarbons In industry, aromatic hydrocarbons are obtained by dry distillation of coal, as well as from oil. 3.1. Getting from coal. Coal is used both as a fuel and as a raw material for the production of many important products. In the process of dry distillation, that is, when heated to 1000 °C and above in furnaces without an air intake, coal decomposes with the formation of the following main products: a) coke (75-80%); b) coke oven gas (up to 3.5%); c) ammonia water (mainly contains inorganic substances, mainly ammonia).

Aromatic hydrocarbons

135

Coke is a solid product containing carbon with a small amount of ash. In metallurgy, it is used as a reducing agent for the separation of metals, mainly iron, from ores (in the blast furnace process). Until the beginning of the second half of the 19th century, the dry distillation of hard coal was carried out almost exclusively for the production of coke, so this process was called the coal coking process. Coke oven gas is a by-product of coal coking. Otherwise, it is called lighting gas, because earlier it was used mainly for lighting, and not just as a fuel. The main components of coke oven gas are methane (30-50%) and hydrogen (30-50%). In addition to them, it contains a significant amount of vapors of aromatic hydrocarbons. Raw benzene (up to 1.5% by weight of coal) is isolated from coke oven gas - a mixture of aromatic hydrocarbons with a boiling point of up to 160 °C; upon distillation of the latter, a mixture of pure benzene, toluene and xylenes is obtained. Thus, at present, coke gas is not only a fuel, but also a valuable gas, which is a source of aromatic compounds. Coal tar is a dark viscous liquid with an unpleasant odor. By distillation, the resin is separated into fractions (as a percentage of the mass of the resin): light oil, Tb XSWRƒɋ(up to 2%); average oil, Tb. = 160-ƒɋXSWR KHDY\RUFUHRVRWHRLO7b. = 230–ƒɋXSWR 10%); anthracene or green oil, Tb. = 270-360°C (up to 23%); fat is the residue after distillation. Benzene, toluene, xylenes are obtained from light oil (significantly less than coke oven gas); from medium oil - naphthalene, phenols, pyridine; heavy - mainly naphthalene; from anthracene - anthracene, phenanthrene, etc. Peck is a black solid mass that softens when heated, used as a building material, road surface, etc. used in fields. 3.2. Getting from oil. The composition of the oil mainly includes acyclic (fatty) and alicyclic hydrocarbons. Aromatic hydrocarbons are present in small quantities. Only in some fields is the oil relatively rich in aromatic hydrocarbons. At present, various methods of oil aromatization have been developed and are being used on an industrial scale. The main ones are cracking and pyrolysis (catalytic reforming), during which dehydrocyclization and dehydrogenation of oil and alicyclic hydrocarbons occur at a certain temperature in the presence of catalysts. 3.3. Arena representatives. Benzene C6H6: colorless flammable liquid with a characteristic odor (Tb = 80.1 °C, Tm = 5.53 °C; d420 = 0.8790). It is slightly soluble in water. Inhalation of benzene vapor has a

Chapter 6

136

harmful effect on the body. It serves as a staple in the manufacture of dyes and pharmaceuticals, synthetic fibers and many plastics. Phenol, nitrobenzene, aniline and many other aromatic compounds are obtained from benzene. It is one of the best solvents for organic substances. Toluene (methylbenzene): C6H5–CH3. A colorless, flammable liquid that smells slightly different from benzene. Tb ƒɋ7m = -ƒɋ d420=0.8669. Trinitrotoluene (TNT) CH3C6H2(NO2)3 is mainly used as an explosive and also in the production of dyes, benzaldehyde and saccharin. Xylene (1,2-, 1,3- and 1,4-dimethylbenzenes) - C6H4 (CH3)2. They are Tk. but close to toluene. They are colorless liquids, varying greatly in T. o-xylenem-xylenep-xylene

Tq. 0C144,4 139,1138,4 Tԥ. 0C-25,2-47,9+13,3 d4200,8802 0,8642 0,8611 C6H5-CH(CH3)2 cumene is a flammable colorless liquid, insoluble in water. In industry, the alkylation of benzene with propylene is also obtained by the decomposition of terpenes and camphor. Used in the production of phenol.

CHAPTER 7 HC=O) in their molecules. Aldehydes are compounds in which the carbonyl group is bonded to one or two hydrogen atoms. Ketones have two hydrocarbon residues at the C atom of the carbonyl group. The structure of aldehydes and ketones can be represented by the following general formulas:

O

O C H aldehyde group

R C H aldehyd

C O keto-group

R

R C O

keton

According to international nomenclature, the carbonyl group is called an oxo group, and aldehydes and ketones, respectively, oxo compounds. General formula CnH2nO. 1.1. Nomenclature and isomerism of aldehydes. The most commonly used are the trivial names of aldehydes, derived from the corresponding names of acids into which aldehydes are converted during oxidation. The simplest aldehyde, having one carbon atom, is called formaldehyde or formaldehyde (Latin formiga - “anthill”). It is the only aldehyde in which the carbonyl group is bonded to two hydrogen atoms. Trivial and international names are formed in the same way as other aldehydes. Its structure is expressed by the following formulas:

Chapter 8

162

O H C

or H CHO H formic aldehyde formaldehyde, methanal) O O H3C C H3C H2C C H H acetic aldehyd ethanal) propion aldehyde propanal)

or

H2CO

O H3C H2C H2C C H fatty aldehyde butanal)

According to the IUPAC nomenclature, the names of aldehydes are formed by adding the suffix “-al” to the names of normal hydrocarbons. In the formula, the general structure (the main carbon chain) is chosen and the C atom of the aldehyde group is inserted into it. Chain numbering starts from this C atom, omitting the number 1 in the name of the compound, because the aldehyde group is always at the beginning of the chain. O H3C H2C

HC C CH3 H

2-methylbutanal

CH3

O H3C HC H2C C CH3 3-mehyilbutanal

H

H3C

O

C H2C H2C C H

CH3 4,4-dimethylpentanal

The names of ketones according to the radical-functional nomenclature: the radicals are chosen in order of magnitude and end with the word “ketone”, and according to the international nomenclature, the names of ketones are formed from the names of the corresponding hydrocarbons, the suffix “-on” is added to them. CH3 C

CH3

O dimethylketone propanone acetone

CH3 C

CH2

CH3

O methylethylketone butanone

H3C CH2 C O

CH

CH3

CH3

ethylisopropylketone 2-methylpentan-3-one

Oxocompounds

163

1.2. Physical properties. The first representative of aldehydes is formaldehyde in the form of a gaseous substance, acetaldehyde is already volatile liquid, higher homologues are liquids, Tb. naturally increases as the number of carbon atoms in the molecules increases. Aldehydes with the largest number of carbon atoms are solids. Formaldehyde and acetaldehyde are highly soluble in water; the solubility of later homologues gradually decreases. Lower aldehydes have a strong characteristic odor. Under normal conditions, there are no gaseous substances among ketones: excess acetone is liquid; higher ketones are solids. Ketones are characterized by a not very strong and even pleasant smell, which is the reason for the use of some of them in perfumery. Lower ketones are miscible with water. 1.3. Chemical properties. Due to the presence of a carbonyl group in the molecules of aldehydes and ketones, these substances are very active in chemical reactions. Moreover, aldehydes are more active than ketones. The oxygen of the carbonyl group is more electronegative than the FDUERQ VR WKH ʌ HOHFWURQV RI WKH GRXEOH ERQG DUH VKLIWHG WRZDUGV WKH oxygen (1). This bond is significantly polar, which indicates the high reactivity of the carbonyl group in reactions with nucleophilic reagents.

C

O

0,552 C

1,448

R

O

R C

1 H

2

O

C R

O 3

Alkyl radicals have electron-donating properties (+I-effect), so they transfer electrons to the carbonyl C-atom (2), reducing the partial positive FKDUJH į  RQ LW 7KHUHIRUH WKH DFWLYLW\ RI WKH FDUERQ\O JURXS LQ acetaldehyde CH3–CH=O is slightly less than that of formaldehyde (CH2O), since the carbonyl carbon is not bonded to the alkyl. As the alkyl radicals in the aldehyde series become more complex, the reactivity decreases. In ketones, under the influence of two radicals, the spatial availability and positive charge of the carbonyl atom C decrease even more, as a result of which the activity of the carbonyl group will be lower than that of aldehydes (3).

Chapter 8

164

Carbonyl compounds are characterized by the following chemical properties: – nucleophilic coupling reactions (AdN-reactions); – coupling-dissociation reactions with N-nucleophiles; – CĮ–H-acidity and aldol-crotonic condensation; - redox reactions; - substitution reactions of H-atoms in the radical. Nucleophilic addition reactions (AdN) at the carbonyl group easily proceed with both anions (HSO3–, –&1+&Ł&–) and neutral nucleophiles (HOH, ROH, RNH2) and can be represented by the scheme: mild

R C

O

R substrate

+

NuH

O

R C R dipolar ion

C

-H NuH

OH

R

fast

R

Nu

tetraedral adduct

Oxocompounds

165

OH HSO3Na

HOH, 20 0C

R CH

hydrosulphyte derivative of aldehyde

SO3Na OH

gem diol

R CH OH

O

C2H5OH

R C

OH R CH

polyacetal

OC2H5

H 2C2H5OH/H

HCN/HO

OC2H5

OH R CH

NaC

CH

diacetal of aldehyde

R CH OC2H5

oxynitrile (cyanohydrin)

C N

ONa R CH

H3O

C N

NH3, maye

OH R CH

C N

Reactions with S-, O- and C-nucleophiles are usually reversible and require acid-base catalysis with weak nucleophiles:

R R

C

NuH

O

+

+ H

OH

R R Nu

C

OH + H2O

The acid activates the substrate and the base as a reagent converts it into a more active anionic form. The reaction with sodium hydrosulfite does not require a catalyst, it proceeds quickly at 20 °C; aldehydes and ketones are easily detected by the appearance of a precipitate. This method is used to detect aldehydes and methyl ketones, as well as to separate mixtures containing carbonyl compounds. When dissolved in water, aldehydes pass into a partially hydrated form and form unstable gem diols. When aldehydes react with alcohols, polyacetals (alcohol esters) are formed, which, after acidification, turn into full acetals. Under the

Chapter 8

166

influence of water in an acidic environment (but not in an alkaline one), they are destroyed and again form aldehydes. Used to protect the aldehyde group from oxidation. To react with oxo compounds, cyanide acid is activated, making it slightly alkaline. The obtained cyanohydrins are used IRUWKHV\QWKHVLVRIĮ-hydroxycarboxylic acids. Sodium acetylenide reacts with aldehydes in liquid ammonia to form alkynol alcohol, while unsaturated alcohol reacts with acid. It is used to grow the carbon skeleton in the synthesis of complex organic compounds. Addition-cleavage reactions with N-nucleophiles proceed according to a two-stage scheme: 1st stage - AdN-reaction, 2nd stage - elimination of water, which leads to the formation of a product with a double carbonnitrogen bond (>C): R R

C

O

+ H2N X AdN

OH R C NH X R

R E

R

C

N X

+ H2O

These reactions are accelerated by both acidic and basic catalysis, but require an optimal pH value of the medium, since the nucleophilic reagent itself reacts with the acid. Unsubstituted imines formed by the interaction of aldehydes with ammonia are unstable and tend to polymerize. Ketone oximes are used to obtain acid amides, for example, F\FORKH[DQRQH R[LPH LV FRQYHUWHG LQWR İ-caprolactam, from which a synthetic fiber, capron, is obtained. Hydrazones are used to reduce carbonyl compounds to hydrocarbons, and phenylhydrazones are used to determine the melting point of carbonyl compounds. The C-H acidity appears in aldehydes and ketones, which have at least one H atom in posLWLRQĮWRWKHFDUERQ\OJURXS8QGHUWKHDFWLRQRIEDVHV a proton is separated from the Ca atom and a stabilized anion is formed, which exhibits dual properties: a carbanion and an enolate ion. The ambidextrous anion can convert to its original ketone or enol form by accepting a proton.

Oxocompounds

H R CH C O R

R R CH C O R

OH

R

R R CH C O

R CH C

O

R C

R C

-HOH karbanion

R C

OH R C R

H

NH3

enolyat

NH2OH

R

H

O

R C OH

-H2O

R CH N OH

C

OH

enol form, less stable

R

R CH NH2 OH

O R C H

167

R CH NH aldimine

oxime

-H2O NH2 NH2

R CH N NH2

hydrazone

-H2O NH2 NH R CH N NH

-H2O PCl5 -POCl3

Cl RCH

phenylhydrazone

gem-dichchlorohydrocarbon

Cl

The reaction of carbonyl compounds with phosphorus pentachloride leads to geminal dichloro derivatives of hydrocarbons. The ketone and enol forms are called tautomers, and the mutual conversion is called ketoenol tautomerism. Due to CH-acidity in the presence of bases, aldehydes and ketones can self-FRQGHQVH IRUPLQJ ILUVW ȕ-hydroxycarbonyl FRPSRXQGV DOGROV RU NHWROV  DQG ZKHQ KHDWHG WKH\ WXUQ LQWR Į ȕunsaturated carbonyl compounds due to the removal of water.

O

Chapter 8

168

O H2C

C

O H2C

+ HO H

H

H

+ HOH H O

O H2C

C

O

C

H2C

C

H

O

weak H3C

HC H2C

C

H

H

HOH quickly

OH O H3C

HC H2C

3-hydroxybutanal

C H

O

t -HOH

H3C

HC

HC

C

croton aldehyde

+ HO H

Redox reactions. Reduction (hydrogen addition) of aldehydes and ketones is possible under the action of molecular hydrogen, atomic hydrogen ([H]) and hydride ion (H¯) as in a solid-phase catalyst (Ni, Pt, Pd). Depending on the reagent and its concentration, reduction can go either to alcohols or to hydrocarbons, aldehydes to simple alcohols, and ketones to double alcohols. NaBH4 LiAlH4 R

O

H2/Ni

R

Zn/HCl

C

H2/Pt/Fe

R CH R OH

R

CH2 R

The oxidation reactions of aldehydes and ketones proceed differently: aldehydes are easily oxidized under the action of light oxidizing substances (even atmospheric oxygen), forming carbonic acids without changing the number of carbon atoms (formaldehyde - formic acid carbonic acid):

Oxocompounds O R

O

O

C

R

C

H

OH

O H

169

O

C

O

O H

H

O OH

C

C

H2O + CO2 OH

OH

Ketones are more difficult to oxidize because the carbonyl group has no hydrogen in its molecules with the carbon already oxidized. In relation to oxidizing agents, ketones are similar to ternary alcohols: they are resistant to the action of weak oxidizing agents and are oxidized by breaking the C-C bonds on both sides of the carbonyl group under the action of hard ones (Popov’s rule), resulting in the formation of acids and ketones with fewer carbon atoms. 1

O

C

H3C

C

2

1 H3C H2C

H3C

+

CH3 O ethylisopropylketon 2-methylpentan-3-one

C

H3C

OH

OH 2-methylpropan acid

acetic acid

HC CH3

O HC

O

2

H3C H2C

C

+ OH

propan acid

H3C

C

CH3

O propanone

The action of weak oxidizing agents is used to analyze aldehydes and separate them from ketones. Qualitative reaction to aldehydes with a solution of silver oxide in ammonia: O R C H

2 AgNH3)2OH

O + 2 Ag

R C ONH4

+ 3 NH3

+ H2O

Chapter 8

170

As a reagent, a colorless solution of silver oxide in aqueous ammonia containing the complex compound [Ag(NH3)2]OH is obtained. When heated with aldehyde, and sometimes in the cold, a silver mirror layer forms on the surface of the glass container (silver mirror reaction). Ketones are not oxidized by silver oxide and do not enter into this reaction. Another widely used reagent for the determination of aldehydes is blue copper (II) hydroxide - Cu(OH)2, which, when heated, oxidizes aldehydes, turning into copper-1 oxide in the form of a red brick precipitate:

O R C H

O R C OH

2 CuOH)2 0

100 C

Cu2O

+

+ 2 H2O

Usually, Fehling’s liquid is taken for the reaction, which is prepared by mixing a solution of copper sulfate with an alkaline solution of a salt of tartaric acid. When heated with aldehyde, the dark blue color of the reagent disappears, a brick-red precipitate of copper (I) oxide precipitates from the solution. Ketones do not give these reactions. Formaldehyde with ammonia gives urotropin: N

O H C

3 NH2

NH2 3 H2C

H

OH

-3H2O

HN H2C

H2 C N H

NH2 3 H2C OH NH CH2 -3H2O -3NH3

H2C H2C N

C N H2

H2C

CH2 N C H2

urotropin

Urotropin is used as a diuretic and in the treatment of rheumatism and gout. The substance obtained by the reaction of solid urotropine with nitric acid - hexogen (or cyclonite) - is used in the production of explosives: N H2C H2C N

C N H2

H2C

CH2 N

O2N HNO3, thick

N

N N

C H2

NO2 hexogen

NO2

Oxocompounds

171

The redox reaction (disproportionation), discovered by S. Cannizzaro in 1853, is possible only for aldehydes that do not contain hydrogen at the CĮ-atom. The reaction proceeds when the aldehyde layer is treated with alkali solutions to form alcohol (reduction product) and carbonic acid salt (oxidation product):

CH2OH

COOK O 2

C

+

+ KOH H

potassium benzoate

benzyl alcohol

Two different aldehydes can participate in the Cannizzaro reaction, in AdN reactions the more active one is oxidized to an acid, and the less active one to an alcohol.

H

O C

CH2 OH O +

O

KOH

+

H C

H C

HOH

H

OK benzyl alcohol

potassium salt of formic acid

Substitution reactions of H-atoms in the radical. Halogenation of saturated oxo compounds proceeds more easily than alkanes, and occurs only in the Į state due to the carbonyl group:

Cl

Cl2

R

C O

CH2 CH3

R H OH)

C

CH

CH3

+ HCl

O

Halogenation in an alkaline environment occurs 12,000 times faster than in an acidic environment, and leads to polyhalogenation. Trihalosubstituted methyl ketones and ethanal are unstable in an alkaline

Chapter 8

172

environment, and the C-C bond is cleaved to form carbonic acid and the halo form (HCHal3). R

C

CH3

O

3 I2

R

NaOH

C O

I C I I

NaOH

R COONa

HOH

+ CHI3

The iodoform reaction serves to detect methyl ketones. Aromatic aldehydes and ketones participate in the reactions of electrophilic substitution of H-atoms of the ring, inhibit them and direct the electrophile to the carbonyl group in the meta position: O

H C Br2

O

H

FeBr3

C

+ HBr Br m-bromobenzaldehyde O

H C KNO3

H2O

+

H2SO4