Organic chemistry an introductory course

It shows that organic molecules are often * It is customary to represent the molecular formula of a substance of high, but not exactly known, molecular weight by V times its empirical si

it shall not, by way of trade, be lent, re-sold, hired out, or otherwise disposed of without the publisher's consent, in any form of binding or cover other than that in which it is published

PERGAMON PRESS LTD

Headington Hill Hall, Oxford

4 & 5 Fitzroy Square, London WA

PERGAMON PRESS (SCOTLAND) LTD

2 & 3 Teviot Place, Edinburgh 1

THE MACMILLAN COMPANY

60 Fifth Avenue, New York 11, New York

Library of Congress Catalog Card No 63-21100

Set in 10 on 12 pt Times Roman and printed in Northern Ireland at The Universities Press, Belfast

Preface

THIS book, which does not assume any previous knowledge of Organic Chemistry, is suitable for use throughout the Advanced level course of the General Certificate and in other courses of similar standard

During his thirty-two years as a teacher, the author has seen significant changes in the style and content of introductory text­books of Organic Chemistry Modern developments, especially

in industry; greater emphasis on physico-chemical principles; a widening of scope to satisfy the requirements of the Universities and of a variety of examination syllabuses—these, and other considerations have contributed towards the wealth of information now contained in the introductory text The large numbers of students now taking advanced courses represent a considerable ability range, however; with the result that modern texts are often too diffuse, in the author's experience, for the majority of those who must use them It seems, therefore, that there is a need for books which, whilst keeping new developments in view, concentrate on stating the basic principles and reactions clearly and concisely This is the author's present aim

In writing a book of limited size, many problems of selection and treatment arise, if scope and clarity are not to be sacrificed Nevertheless, this book is in no sense a collection of notes On the contrary, there is ample material herein to satisfy the needs of any student undertaking a first course of Organic Chemistry Nothing of importance is consciously omitted Industrial appli­cations are mentioned and there is a separate chapter about some of the more important of these To make the book a

vii

viii PREFACE

self-contained unit without disrupting the continuity of the text, relevant physico-chemical principles are discussed in the last chapter The exercises illustrate the text, most sets covering several chapters each to co-ordinate the subject matter

In all, these comparatively few pages carry the essential features

of a modern first course, approached with a purposeful directness which, the author feels, will be advantageous to the reader

Acknowledgements

THE AUTHOR wishes to thank the following individuals and bodies for supplying material for this book and for giving their kind permission for its inclusion:

Mr M Schofield, M.A., F.R.I.C., author of the article

"Chemistry in the Forest" and Miss Jane O'Malley of the

Ě and  Publicity Department for the block illustrating wood distillation in the Forest of Dean

Mr A G Hervey of the Petroleum Information Bureau and

Mr K Hutchinson of the National Coal Board for information concerning petroleum and natural gas

Mr N Kirkland and Mr R Senior of the Public Relations Department of the Esso Petroleum Company for the photograph and the diagram illustrating petroleum distillation

Mr E Webb, B.Sc, for his photographs of atomic models The author is indebted to his wife and to Mr H G Burks, A.R.S.M for their help in the reading of the proofs

ix

1 Characteristic Features

F O R T H O U S A N D S of years chemistry has been employed as the means to an end in, for example, metallurgy, pottery, perfumery, brewing, dyeing and pharmacy Thus, when the era of chemistry

as a science began some 300 years ago, a large number of useful

substances of unknown composition awaited classification This was carried out on the basis of origin—animal, vegetable or mineral In the course of time compounds derived from living

organisms, animal or vegetable, came to be called "organic" and

those of mineral origin "inorganic"

This classification persisted until the early years of the 19th century when it was thought that the living source of organic compounds was of fundamental importance because (a) newly developed analytical techniques showed that they all contained carbon combined with a few other non-metallic elements, in contrast with the wide range of composition of mineral derivatives; (b) it had not yet been found possible to synthesise an organic compound in the laboratory

In 1828, however, Wöhler synthesised urea, a typical product of animal metabolism, from mineral reagents (potassium cyanide, lead oxide and ammonium sulphate) Other syntheses of a similar nature followed, not only of carbon compounds which had already been obtained from living sources, but of an even larger number of hitherto unknown carbon compounds which had

no counterpart in nature In this way the "living source" theory

1

2 O R G A N I C CHEMISTRY

was abandoned, although the term "organic" was retained with

a new meaning, viz "containing carbon"

According to the modern system of classification by com­

position, then, organic compounds are those containing the element

carbon, irrespective of their source Inorganic compounds are

compounds of all the other elements, although it is convenient to include the oxides of carbon and the carbonates in this category

The elements of organic chemistry

The metabolism of our bodies depends on regular supplies of organic materials These include:

(a) Carbohydrates: starch* {C % \i 1{ ß^ x from cereals, potatoes and bread, and the common sweetening agent sucrose (cane sugar) C1 2 H 2 2 0 n , both of which are digested into glucose

C6H1 206

(b) Proteins, chiefly from animal cells There are many proteins, all having approximately the same composition, viz 50% carbon, 25% oxygen, 15% nitrogen, 7% hydrogen and up to 5% sulphur

(c) Fats, e.g stearin C5 7H1 1 0O6 in beef and mutton fats

(d) Vitamins, e.g vitamin A, C2 0H3 0O and vitamin D, C2 8H4 40

In addition to these food materials, most people are familiar with acetic acid C2H402 in vinegar; hydrocarbon petroleum products like petrol, paraffin oil and petroleum jelly ("Vaseline"); naphthalene C1 0H8 in firelighters and mothballs; alcohol C2HeO

in intoxicating drinks and "meths"; aspirin C9H804; nicotine

C1 0H1 4N2 in tobacco; anaesthetics, such as ether C4H1 0O and chloroform CHC13; explosives, e.g nitroglycerine C3H5N3 0 9 (in

dynamite) and TNT C7H5N306; plastics, e.g Perspex ( €5Η802)Χ; polythene ( C H ^ and PVC (QHgCl)^ DDT insecticide C1 4H9C15 This very short list is nevertheless representative of organic compounds as a whole It shows that organic molecules are often

* It is customary to represent the molecular formula of a substance of high, but not exactly known, molecular weight by V times its empirical (simplest) formula In this case, V probably lies between 200 and 350

CHARACTERISTIC FEATURES 3

big and that the elements normally present in them, in addition

to carbon, are few, viz: hydrogen and oxygen (very common), together with nitrogen, sulphur and halogens

These few elements are essential for plant and animal meta­bolism and may be regarded as the basic materials of many aspects of modern civilisation Not the least important of their applications is in the process of education, which would be seriously hampered by the lack of writing ink (derived from gallic acid C7H605), paper and books For example, this book is

chiefly composed of cellulose* (C e K 10 O 5 ) x in the form of paper, cotton thread and cloth; printer's ink, which is amorphous carbon suspended in a drying oil, like linseed oil (derived from glycerine C3H803 and linolic acid C1 8H3 202) ; glue (a mixture of peptones from animal proteins) and starch paste

The carbon atom

The idea of valency as a number denoting the combining capacity of an element was first put forward by Frankland in

1852 Six years later, Kekulé postulated the quadrivalency of carbon and in 1865 Crum Brown introduced the method of

representing each valency separately Thus the carbon atom was thought to have four bonds, of unknown nature, through the agency of which it could combine with four monovalent, or two divalent, atoms

Η Η—C—Η 0 = C = 0

I

Η

Methane Carbon dioxide

The four bonds were tacitly assumed to be in one plane until

1874, when van't Hoff and Le Bel independently put forward a space theory According to this theory, the carbon atom was represented as being at the centre of a regular tetrahedron with the valency bonds directed towards the corners (Fig 1, p 4)

* Compare starch In this case, * V * is probably in the region of 2000

CHARACTERISTIC FEATURES 5

These formulae are, in fact, identical because the two chlorine atoms are always next to each other in the space model (Fig 2) For 75 years, then, the symmetrical distribution in space of the

four valency bonds about the carbon atom has been accepted The only modification is that the modern electronic theory has explained the nature of the bond.*

The atomic number of carbon is 6 and its atomic weight is 12 Thus the carbon atom has a nucleus of 6 protons and 6 neutrons,

surrounded by 6 planetary electrons in two shells The first shell contains the stable duplet of helium leaving an incomplete second shell of 4 valency electrons This atom combines by directional covalency with other non-metallic atoms, sharing a further 4 electrons to gain the stable electronic arrangement of neon Thus the modern formula for methane is

χ = 5 C valency electrons , · = Η electrons

Note that (a) it is customary to show only valency electrons; (b) the formula has to be represented in one plane, although the

molecule is, in fact, three-dimensional; (c) a bond represents a shared pair of electrons, one from each atom

The structure of carbon compounds

There are literally hundreds of thousands of carbon compounds

so that they easily outnumber the compounds of all the other

elements put together Yet carbon combines with very few elements (p 3) The explanation of this paradox lies in the

* The modern theories of atomic structure and valency are briefly discussed

in Chapter 20

ORGANIC CHEMISTRY

FIG 3

FIG 4

Open chain or aliphatic compounds

C

c c

Closed ring or cyclic compounds

Atomic models readily show that a chain of more than two carbon atoms is not really straight When we speak of a "straight chain" we mean an "unbranched chain", such as that in normal butane, C4H10 (see Figs 3 and 4 opposite)

These space arrangements may appear to be different but, in

fact, they both represent the same molecule and they are both written on paper in the same way, thus:

Η Η—C—Η

Η ·

In all three representations there is a "straight" (i.e unbranched) chain of four carbon atoms, with three hydrogen atoms attached

to each of the end carbon atoms and two to each of the others

In other words, all three representations conform to the same rational formula of C H 3 C H 2 C H 2 C H 3 ( page 9) This use of the rational formula is worth remembering

With regard to closed rings, the strain theory of von Baeyer (1885) and its modern counterpart, the concept of directional covalency, are important The normal angle between the valency bonds is 109J° and distortion leads to an unstable molecule Now, of the regular figures, the pentagon has angles (108°) corresponding most closely to 109J°, so that a closed ring of five carbon atoms in one plane (as shown on p 7) requires very little distortion Rings of four or three carbon atoms are less stable, although they are known to exist Similarly, any number of carbon atoms over five produces distortion

Organic formulae

Every inorganic compound has a "formula", by which is normally meant its molecular formula In some cases, however, the formula is actually the empirical, or simplest, formula Such

CHARACTERISTIC FEATURES 9

is the case with water, the properties of which indicate that the

molecular formula should be (U 2 0) x—compare some of the organic compounds listed on page 2

In organic chemistry, five types of formula are in common use For example, acetic acid has

(a) an empirical formula H2CO, found by quantitative analysis,

(b) a molecular formula H4C202, corresponding to the observed molecular weight of 60,

(c) a rational formula CH3-COOH, showing the grouping of the atoms, as indicated by the observed chemical properties,

(d) a structural formula

Η O Η—C—C—Ο—Η

I

Η showing the individual linking of the atoms,

(e) a space formula

10 ORGANIC CHEMISTRY

Each of these series corresponds to a group of compounds which (a) all contain the same elements and can all be represented by a single general formula,

(b) all have similar chemical properties,

(c) have the common increment of CH2 between successive members of the series,

(d) show a gradual variation in physical properties with increasing molecular weight

These points are clearly illustrated by the homologous series

of the paraffin hydrocarbons (p 27)

Radicals and typical groups

Groups of elements (or "radicals") which may behave as single units are known in inorganic chemistry (e.g = S 04, = C 03,

— N 03 and NH4—), but compounds consisting entirely of such radicals (e.g ammonium nitrate N H4N 03) are comparatively rare The reverse is true in organic chemistry Single atoms rarely play a part in the constitution of organic molecules, which are usually composed of two or more radicals as shown in their rational formulae (p 9)

The commonest radicals are the alkyl groups, each containing one hydrogen atom less than the paraffin hydrocarbon from which

it is derived and named

Methane, CH 4 , gives methyl CH 3 — Ethane, C 2 H 6 , gives ethyl C 2 H 5 — Propane, C 3 H 8 , gives propyl C 3 H 7 —

Alkyl groups occur in all homologous series, associated with a

typical group which varies from series to series This is the group

which is assigned to all members of a given homologous series

in accordance with their common chemical properties Some typical groups are

Hydroxyl —OH Carbonyl =CO

Carboxyl (two associated groups) —COOH

Amino — N H Cyanide —CN

CHARACTERISTIC FEATURES 11

EXAMPLES

Alcohols, ( Cn H 2 n + l O H ) Amines, ( C „ H 2 n + i N H 2 )

Methyl alcohol C H 3 O H Methyl amine C H 3 N H 2

Ethyl alcohol C 2 H 5 O H Ethyl amine C 2 H 6 N H 2

Propyl alcohol C 3 H 7 O H Propyl amine C 3 H 7 N H 2

The commonest single atoms of organic compounds are

(a) chlorine, as in the alkyl chlorides CH3C1, C2H5C1, etc., (b) oxygen, as in the ethers (CH3)20, (C2H5)20, etc

Taking a simple example, two substances of molecular formula

C2H60 are known Substance A is a colourless, volatile liquid; substance Β is a gas If the valencies of the constituent atoms are

to be satisfied, there are only two possible arrangements (below) But which structure belongs to A and which to Β ? This is decided

by their chemical properties For example, the two substances behave towards the phosphorus chlorides (PC13 and PC15) as follows:

A—reacts readily with both chlorides, forming ethyl chloride;

C2H60 -* C2H5C1 Here, a monovalent chlorine atom has re­placed both an oxygen atom and a hydrogen atom, which must therefore together form a monovalent hydroxyl group —OH Β—has no simple reaction with the pentachloride under any

conditions, but forms methyl chloride CH3C1 with the aid

of heat and pressure when added to phosphorus trichloride

A—ethyl alcohol Β—dimethyl ether

It is important to remember that structural and rational

formulae, by means of which isomers are distinguished from each

other, depend on the observed chemical properties, as indicated

above The chemical properties of a substance cannot be changed

simply by rearranging its structure on paper!

Types of reaction

Substitution occurs when an atom or radical takes the place of

another atom or radical of the same valency For example, when

equal volumes of methane and chlorine react together, methyl

chloride and hydrogen chloride are formed as shown by the

equation

CH 4 + Cl 2 = CH3CI + HCl

Here, a monovalent chlorine atom has taken the place of a mono­

valent hydrogen atom Substances which behave exclusively in

this way are said to be "saturated"

Addition takes place when a molecule of substance A adds on a

molecule of substance Β to form one molecule AB only For

example, ethylene C2H4 reacts with chlorine in this way

C2H4 + Cl2 = C2H4C12 Similarly, acetaldehyde adds on ammonia

C2H40 + N H3 = C2H7NO For this reason, ethylene and acetaldehyde are described as

"unsaturated" compounds

Condensation is a reaction in which a number of simple mole­

cules (not necessarily the same) build up into a complicated

Polymerisation is a special type of condensation common

amongst unsaturated compounds, which, as we saw above, react

by addition All of the combining molecules must be the same and there must be only one product, having the same empirical formula as the original substance and a molecular weight which is

an exact multiple of the original molecular weight Thus, heat causes acetylene to polymerise to benzene (the empirical formula

Mechanism equations

The use of molecular formulae in an equation representing

an organic reaction supplies limited information only If struc­tural formulae are used, however, the mechanism of the reaction can be illustrated Take, for example, the substitution and additions of the last section

Substitution of chlorine for hydrogen in methane

CH 4 + Cl 2 = CH3CI + HCl

Η Η Η—C—ΙΗ + CÍ J CI = Η—C—CI + Η—CI

The first equation tells us that three of the four hydrogen atoms

of the acetic acid molecule have been replaced by chlorine atoms; the second makes it clear that three similarly placed hydrogen atoms are substituted, leaving the acidic —COOH group un­affected

The multiple bond

The structural formulae of the two unsaturated compounds mentioned above—ethylene and acetaldehyde—both include a

double bond (C=C and C=0) This is best explained on the

basis of the electronic theory (Chapter 20)

In the ethylene molecule, each carbon atom achieves its octet

by sharing two of its four valency electrons ( x ) with hydrogen atoms and two with the other carbon atom, as shown This

C2H402 + 3C12 - C2HC1302 + 3HC1

CH3COOH + 3C12 = CCI3COOH + 3HC1

Addition of ammonia to acetaldehyde

16 O R G A N I C CHEMISTRY

This explanation holds good for the C = 0 bond of acetaldehyde and for other multiple bonds, such as the treble (triple) bond of

acetylene HC=CH and the cyanide radical —G=N All three

types (CC, CO, CN) have the additive property in common and,

in fact, all of them add hydrogen In general, however, each has its own characteristic addition reactions Thus the addition of chlorine (above) is characteristic of CC bonds; it does not occur with CO and CN bonds

Nomenclature

If a pharmacist is asked for "spirit of salt" or "lunar caustic",

he will oblige with hydrochloric acid and silver nitrate respectively Similarly, old names still survive in organic chemistry Some

relate to the original source, like "formic acid" (Latin, formica—

the ant); some have no obvious meaning, like "alcohol" This

is a pity, because the systematic nomenclature is both simple and informative, as in inorganic chemistry

Substitution compounds may be named in two ways: (a) accord­

ing to the radicals they contain; (b) according to the reactants which produce them For example, when chlorine substitutes methane (CH4), the product CH3C1 may be called either methyl chloride (two words) or monochloromethane (one word) Notice that in the latter method, the name is, so to speak, in reverse The original substance comes last, preceded by the substituting substance, which is preceded in turn by the number of atoms of chlorine which have been introduced

Addition compounds are usually named after the reacting sub­

stances Thus the two addition compounds of page 12 are called ethylene dichloride and acetaldehyde ammonia (two words)

CHARACTERISTIC FEATURES 17

solutions of silver nitrate and sodium bromide at ordinary

In this case, the reacting "molecules" are better described as

"ionic aggregates", bound together by mutual attraction but ready to react in the right conditions, viz: in aqueous solution Organic compounds are covalent and there are two main types

to consider, as follows

Saturated substances, which form substitution compounds

(p 12), usually react very slowly at ordinary temperatures Even when the conditions are made more favourable by (e.g.) applying heat in the presence of a common solvent, the reaction may still take an appreciable time to complete For example, if ethyl bromide, which is insoluble in water, is shaken with aqueous silver nitrate, a slight precipitate is produced (contrast above)

If alcohol is used as the common solvent for both substances a better reaction is observed, but heat is still needed to increase the rate of precipitation

Unsaturated substances may well form their addition compounds

rapidly at ordinary temperatures, since the preliminary removal

of an atom or radical is not involved, as it is in substitution Nevertheless, some additions are more difficult than others, needing heat and/or the presence of a catalyst for quick reaction From the above general observations, it is clear that it is not sufficient to be able to name the substances used in a given

reaction The most satisfactory reaction conditions are just as

important It would be misleading to say that "sodium bromide and silver nitrate immediately form a precipitate" without mentioning water Similarly, the phrase "in hot alcoholic solu-tion" is essential in describing the formation of silver bromide from ethyl bromide and silver nitrate

Apparatus and experimental technique

Most of the organic compounds discussed in this book are liquids, many of them very volatile Further, heat is generally

18 O R G A N I C CHEMISTRY

needed to promote organic reactions These two points are illustrated by the apparatus in common use

The reflux apparatus (Fig 8) is very useful for prolonged reac­

tions involving hot liquids Vapour, which would otherwise escape, is condensed and returned to the reaction vessel In this way, both reactants and products which are volatile are confined, no matter how long the reaction takes to complete

The distillation apparatus (Fig 9) is used when the reaction takes

place more rapidly on heating and it is desired to remove the volatile product as it is formed The cork (C) may carry a dropping funnel (for adding more liquid as required) or a thermometer (if the temperature is important)—or possibly both The wire gauze (W) may be substituted by a water bath or a sand tray for controlled heating at various temperatures The adapter (A)

The separating funnel (Fig 10) is often used in the course of

purification Many organic liquids are insoluble in

water and impurities are usually extracted from them

by agitation with suitable aqueous solutions On

standing, the purified liquid and the aqueous solution

separate into two layers, the lower of which may be

run off by opening the tap (T) According to their

respective densities, this lower layer may be the aqueous

purifying solution (discarded) or the required organic

liquid

The fractioning column (Fig 11) facilitates the sepa­

ration of volatile liquids which are mutually soluble

The homogeneous mixture is heated and the vapour,

containing a high proportion of the most volatile FIG 10

20 O R G A N I C CHEMISTRY

component(s), enters the column Here it meets a large surface on which the less volatile components condense and return to the flask For example, the laboratory preparation of acetaldehyde (b.p 21°Q yields a product which is contaminated with alcohol (b.p 78°C) and water On heating in a fractionating apparatus, acetaldehyde vapour only passes through the column

uncondensed, as indicated by the constant temperature of 21°C

registered by the thermometer at the top

Steam distillation (Fig 12) may be used to separate liquids which

are insoluble (or only sparingly soluble) in water from the by­products of their preparation Steam is blown through the heated mixture, when the required liquid and water distil together into the receiver, separating into two layers on standing

This method is particularly useful when the impurities are solid and when the required liquid has a high boiling point (over 150°Q

C o n d e n s e r

F l a s k FIG 11

CHARACTERISTIC FEATURES 21

Iodobenzene (b.p 188°C), aniline (b.p 184°C) and bromobenzene (b.p 156°C) are extracted by this method, which is discussed further in Chapter 20

Ether extraction makes use of (a) the high solubility of many

organic substances in ether, (b) the immiscibility of ether and water and (c) the volatility of ether Other volatile organic solvents which are immiscible with water may be substituted for ether

Aqueous emulsions may take a considerable time to separate into two layers For example, the emulsion of aniline and water obtained by steam distillation behaves in this way because the specific gravity of aniline is 1-02 On shaking the emulsion with ether, however, all of the aniline immediately dissolves in it and the ethereal solution separates quickly on standing After discarding the aqueous layer, aniline is easily obtained from the ethereal solution by warming on a hot water bath, when the volatile ether (b.p 35°Q readily evaporates

FIG 12

22 O R G A N I C CHEMISTRY

Ether extraction may also be applied to aqueous solutions which are difficult to separate In this case, the required solute must be

much more soluble in ether than it is in water, when most of it is

transferred to the ethereal layer

Final purification If the required organic liquid has been in

contact with water or aqueous solutions during the course of its

FIG 1 3 The final distillation of a volatile liquid, such as ether (b.p 3 5 ° C )

CHARACTERISTIC FEATURES 23

and the liquid is gently heated over a gauze or water bath (according to its volatility) The vapour which distils when the

thermometer registers a constant temperature, which is the known

boiling point, is collected (Chapter 20)

Whenever the thermometer, the bulb being in the vapour, registers a constant temperature during straight distillation or fractionation, it means that the distillate contains only one liquid; i.e is pure

a

FIG 1 4 Recrystallisation is the recognised method of purifying a solid

For this purpose a suitable solvent is required, in which the solid

in question dissolves freely at high temperatures, but only spar­ingly at low temperatures In this way, most of the solute is recovered on cooling the hot solution Using a reflux apparatus, the solid is dissolved in the minimum of hot solvent and then allowed to cool in an open dish

Many organic solvents are very volatile (hence the use of the reflux in the dissolving process) and care must be taken that the

24 O R G A N I C CHEMISTRY

solvent does not evaporate completely on standing, leaving the impurities behind When sufficient crystallisation has taken place, the liquid is drained off and the crystals left in a warm place to dry

The purity of the crystals may be tested by the melting-point

method (Fig 14, p 23) A capillary tube, sealed at one end and containing a little of the solid under test, is fastened to a ther­mometer by a rubber band and heated very slowly in a bath

of suitable liquid If the solid is known to melt below 100°C, water should be used For general purposes, however, an inert liquid of high boiling-point (e.g "liquid paraffin") is preferable

If the opaque solid suddenly changes to transparent liquid the solid is pure and the thermometer reading when this happens is the required melting-point If it becomes "mushy" and remains

so over a temperature range, the solid is impure

2 Aliphatic Hydrocarbons

only, are the simplest of all organic substances The three logous series to be considered are related as follows:

The alternative names for these three series of hydrocarbons (alkanes, etc.) were recommended at an international conference

on systematic nomenclature in 1949 There is no alkene or alkyne

corresponding with η = 1 in their general formulae

Hydrocarbons burn in the air, when complete oxidation to carbon dioxide and water occurs in accordance with the general equation

C x U y + + ^ 02 = xC0 2 + I H20

When previously mixed with air or oxygen (within certain limits

25

26 O R G A N I C CHEMISTRY

of proportion) gaseous hydrocarbons and the vapours of volatile liquid hydrocarbons (e.g petrol) form explosive mixtures If sufficient oxygen is present in the mixtures, the explosive reaction

on ignition is in accordance with the above general equation If the supply of oxygen is limited, however, there is preferential oxidation of hydrogen to water and partial oxidation of carbon

to carbon monoxide This happens in the internal combustion engine and in mine disasters (see methane, p 27)

Apart from this similar behaviour on combustion, there are interesting and important differences in chemical properties between the homologous series The paraffins are saturated and inert*; the olefines and acetylenes are unsaturated and reactive (p 12)

Saturated Hydrocarbons

Homologous series of the paraffins (Alkanes), CnH2 n +2

The following table gives relevant information about the first twenty-four members of the paraffin series The general formula,

CH2 increment between successive members and gradual change

in physical nature with increasing molecular weight are clearly illustrated (p 9)

Chemically the paraffins are saturated and unreactive, the only characteristic laboratory reaction being the substitution of hydrogen atoms by chlorine or bromine atoms Nevertheless, these hydrocarbons are very important commercially The petroleum deposits of, for example, North America and the Middle East consist of a mixture of liquid and solid paraffins which can be separated into a variety of useful products, in­cluding petrol The natural gas associated with these deposits

is a mixture of gaseous paraffins, which is piped direct to con­

sumers as fuel (p 116, et seq.)

* This applies to the paraffins—a saturated substance is not necessarily inert

Methane, or "marsh gas", occurs naturally wherever vegetation

is rotting under water It is also the dangerously explosive gas

in coal mines When ignited, the comparative shortage of air in the explosion mixture results in the formation of the poisonous gas carbon monoxide

CH + 1 | 0 = CO + 2 H 0

2 8 O R G A N I C CHEMISTRY

When exploded with sufficient air, or burnt in the normal way, complete oxidation occurs

CH4 + 2 02 = C 02 + 2 H20 Methane also forms an explosive mixture with chlorine in the proportion of 1:2 by volume When this mixture is exposed

to strong sunlight, or ignited, the reaction is

CH4 + 2C12 = C + 4HC1

In ordinary daylight, however, mixtures of methane and chlorine react very slowly, the reaction being one of substitution For example, with a mixture of equal volumes, methyl chloride

CH3CI is formed eventually

Η Η

I _ , I

H—C—jH + CljCl = H—C—CI + HCl

Η Η With more chlorine, a second substitution may be slowly carried out under the same conditions, forming methylene dichloride (dichloromethane) CH2C12 The third and fourth substitutions are more difficult, but may be effected with the aid of heat and sunlight, forming chloroform, CHC13 and carbon tetrachloride, CC14 These two substances are normally prepared by other methods

Bromine also substitutes the hydrogen atoms of methane in a similar way to chlorine but with greater difficulty; the first substitution only is normally possible, forming methyl bromide

CH3Br Iodine does not react with methane (Chapter 20)

by the action of water on aluminium carbide

A14C3 + 12H20 = 4A1(0H)3 + 3CH4

The following two methods of large scale preparation are more important, however, because they can be adapted to the pre­paration of other members of the paraffin series

ALIPHATIC H Y D R O C A R B O N S 2 9

1 By heating a mixture of anhydrous sodium acetate* and soda

limef in a hard glass test tube, collecting over water

t Soda lime, made by slaking quicklime with sodium hydroxide solution,

is not deliquescent It may be represented in equations as "NaOH" or

"Ca(OH) ", whichever is the more convenient

Η

( C H 3 ) 3 C H Isobutane

These two varieties of butane have similar chemical properties but different physical properties, the normal (n-) form being easier to liquefy by cooling (b.p +1°C) than the iso form (b.p - 1 7 ° Q

2 By reducing ethyl iodide, using an aluminium-mercury couple and methyl alcohol

C2H5I + 2H = C2He + HI The properties of ethane are similar to those of methane; it is inflammable and forms explosive mixtures with air

C2H6 + 3 | 02 = 2 C 02 + 3 H20 and it is slowly substituted by chlorine in diffused light

Η Η Η Η

I I I I

Η—C—C—¡H + C1IC1 = H—C—C—CI + HCl

Η

C ( C H 3 ) 4

In this case there are two iso forms (with branched chains) These may be distinguished by naming them as substituted

methanes, the bold C being regarded as the carbon atom of

methane Thus the first isopentane C2H5CH(CH3)2 is dimethylmethane; the second is tetramethylmethane By the same system of nomenclature, isobutane is trimethylmethane (p 16)

ethyl-Unsaturated Hydrocarbons

Homologous series of the olefines {alheñes), Cr iH2 /t

Each member of this series contains two hydrogen atoms per molecule less than the corresponding paraffin from which it derives its name

32 ORGANIC CHEMISTRY

Η Η Η Η

oc-butylene ß-butylene

Η Η—C—Η

Three isomeric butylenes J-J

Iso-butylene

Physically, the olefines resemble the paraffins; the lower

members mentioned above are gases, followed by liquids and

waxy solids as the molecular weight increases

Chemically, the olefines are very reactive forming addition

compounds (p 12) and so differ markedly from the paraffins

This dehydration process may be carried out in several ways, e.g.:

(a) By passing ethyl alcohol vapour through a hot tube containing

aluminium oxide (catalyst)

Ethane C 2 H 6 Ethylene (Ethene) C 2 H 4

Propane C 3 H 8 Propylene (Propene) C 3 H 6

Butane C 4 H 10 Butylene (Butene) C 4 H 8

The typical group of this homologous series is the ethylenic

ALIPHATIC HYDROCARBONS 3 3

(b) By heating a mixture of ethyl alcohol and excess concentrated sulphuric acid until the mixture darkens and effervesces The wash bottle contains alkali solution to remove acidic by­products (C02, S02)

FIG 1 6

PROPERTIES AS a hydrocarbon, ethylene burns and forms

explosive mixtures with air

C2H4 + 3 02 = 2 C 02 + 2 H20 The paraffins burn with non-luminous flames; the flame of ethylene is smoky and luminous, indicating the higher carbon content

As an unsaturated substance, ethylene has additive properties,

its molecule adding on two monovalent atoms or groups (call these X and Y) to form a saturated substance, thus:

3 4 ORGANIC CHEMISTRY

For example, when mixed with hydrogen and passed over hot

nickel (catalyst), hydrogenation or addition of hydrogen, occurs

(Sabatier and Senderens, 1899)

This reaction is common to all unsaturated substances (e.g

pp 38, 61, 66, 82) Additions which are characteristic of the ethylenic bond are:

Halogens—chlorine (gas, cold), bromine (liquid, vapour or

aqueous solution, cold) and iodine (hot alcoholic solution) are decolorised

Hypochlorous acid—i.e a cold, dilute solution of chlorine in

water ("chlorine water"); Cl2 + H20 = HCl + HOC1

CH2—CH2 > CH2OHCH2Cl

t Τ Ethylene chlorhydrin

OH Cl