Isomerism

ISOMERISM. In the period when chemical analysis was first introduced, the belief was generally held that the properties of every substance depended solely upon its chemical composition. Two substances which had the same properties were therefore supposed to be identical in composition; and, conversely, if a difference could be detected in the properties of two samples of a substance under the same conditions, it was held to be due to some difference in chemical composition.

The fundamental postulate that the properties of a substance are determined uniquely by its chemical composition is subject to certain well-defined types of exception. The general term "isomerism" which is sometimes assigned to these can be classified more accurately under three headings as, (i.) polymorphism, where a difference of properties is caused by a different arrangement of identical molecules in a crystal; (ii.) polymerism, where the molecules are of identical composition, but of different size; and (iii.) isomerism, where the molecules are of identical composi tion and size, but of different structure.

Polymorphism.

The fundamental postulate set out above was expressed in a precise form in 1801 by the Abbe Haily, who made it the basis of his classification of minerals. "His guiding principle was this : that every difference in the fundamental form of a crystal implied a difference in its chemical composition" (Armitage, A History of Chemistry, 1906, p. 104). Thus, although the two minerals calcite, or Iceland spar, and aragonite appear to be merely two crystalline modifications of ordinary chalk (calcium carbonate) (see fig. I), it is often possible to detect the presence of iron (as ferrous carbonate) in calcite, and of strontia Dalton was able to suggest that "almost probably the atoms [molecules] of the new gas consist of two of olefiant gas." This conclusion was subsequently confirmed by Faraday, who separated the same compound by condensation from oil-gas at o° F, and showed that it contained carbon and hydrogen in the same pro portion as olefiant gas, but had twice the density. Faraday cited this case as evidence of "the existence of bodies composed of the same elements and in the same proportion but differing in their qualities," and ventured to predict that "now we are taught to look for them they will probably multiply on us." This pre diction was abundantly fufilled ; but in the case cited by Faraday the two bodies (now known as ethylene, and butylene, C4118) differ in the number of atoms in the molecule; they are therefore polymeric and not isomeric in the strict usage of the word. These two phenomena give rise, however, to such similar changes of properties that it is often difficult to distinguish be tween them, unless the molecular weights of the two forms can be determined with ease and certainty, as in the case of ethylene and butylene, where they can be deduced at once from the densi ties of the two gases (see STOICHIOMETRY).

In the second case cited by Faraday, the fulminates prepared and analyzed by Liebig (1823) and Gay Lussac (1824) have the same composition as the cyanates prepared and analyzed by Wi5hler (1824), although these compounds have nothing in common but their composition. Whilst, however, the per centage composition of both silver salts can be represented by the formula AgCNO, it is not easy to prove whether these salts are isomeric or polymeric, since the ordinary methods of determining molecular weights are not applicable to salts such as these. An undoubted example of true isomerism was dis closed, however, when \Willer in 1828 converted ammonium cyanate into urea, since these two compounds are identical in molecular weight as well as in their percentage composition. Both compounds can, in fact, be represented by the formula CON2H4, but whereas ammonium cyanate is regarded as an aggregate of ammonium and cyanate ions, NH4 and CNO, urea is formulated as a compound in which two amino-groups, NH2, are united by a carbonyl group, CO, thus : Ammonium cyanate, (NH4) CNO; Urea (or carbamide), CO(NH2)2.

The structural isomerism of these two substances can therefore be summarized by saying that in one compound (ammonium cyanate) all four atoms of hydrogen are attached to one atom of nitrogen to form an ammonium radical, whereas in the other compound (carbamide) each atom of nitrogen carries two atoms of hydrogen. This contrast is shown more clearly in the following structural formulae, where the links or "bonds" be tween the atoms are represented by straight lines, and the plus and minus signs represent opposite electric charges: The double barbs pointing in opposite directions are used to in dicate the fact (discovered at a much later date) that the con version of one isomeride into the other can take place in either direction, i.e., that the process is reversible (Walker and Hambly, 1895). Wohler's discovery not only provided one of the first authentic cases of isomerism, but was hailed as the first example of the synthesis of an organic compound from inorganic materials by ordinary laboratory processes and without the intervention of any vital force. Since the urea which he had prepared from am monium cyanate proved to be identical in every respect with that which is found in urine, Wohler announced his discovery to Berzelius in the following terms: "I must tell you that I can prepare urea without requiring a kidney or an animal, either man or dog." Metameric Substances.—Wohler's synthesis, as affording the first recorded example of isomeric change, i.e., of the conversion of one isomer into another, is second in importance only to the fact of isomerism itself, and Berzelius therefore introduced a special term to describe it. Thus in 1831 he introduced the term

isomeric composed of equal parts) to describe bodies composed of an equal number of atoms of the same elements but arranged in an unlike manner, and therefore possessing different chemical properties and crystalline forms, and the term polymeric (roXbs, several) to describe those cases in which, although the relative number of the atoms is the same, the absolute is not. In the following year he introduced the further term metameric (Gr. per ot, change) to describe those isomeric substances which change easily into one another. Kekule, however, confused this simple nomenclature by using the term isomeric to describe all compounds which have the same percentage composition but different properties; these compounds were then classed, on the one hand as polymeric when they had the same composition but differed in molecular weight, and on the other hand as meta meric when the molecular weight was the same but the structure of the molecules was different; Kekule therefore applied the term "metameric" to ordinary cases of structural isomerism. The terms isomeric and polymeric have fortunately survived in the sense of Berzelius' original definitions ; but the term metameric, which was rendered ambiguous by Kekule's attempt to make it mean something quite different, has been generally abandoned.

Examples of Structural Isomerism.—When structural chemistry came into existence about 1858, large numbers of iso meric compounds were prepared by deliberate synthesis. For in stance, the following series of alcohols and acids can be regarded as fundamental units for the preparation of more complex carbon compounds : Methyl alcohol, CH3.0H Formic acid, H.00.0H Ethyl alcohol, C2H;>•OH Acetic acid, CHa•CO.OH Propyl alcohol, C3H7.0H Propionic acid, C2H5.00.0H Butyl alcohol, C4H9•OH Butyric acid, C3H7.00.0H Amyl alcohol, C5Hii.OH Valeric acid, C4H9 CO.OH In these abbreviated structural formulae, it will be seen that the alcohols contain an alkyl group or alkyl "radical," consisting of carbon and hydrogen in the ratio linked to the charac teristic hydroxyl group, OH, of the alcohols; while the acids con tain the same alkyl groups linked to the characteristic carboxyl group, CO.OH, of the acids. By eliminating a molecule of water from an alcohol and an acid, it is possible to produce a member of the series of esters (q.v.), which afford innumerable examples of structural isomerism. Thus the three compounds: Ethyl formate, Methyl acetate, Propionic acid, C2H6.CO-OH, are obviously isomeric, since they all have the formula C3H602; but, as they cannot be converted into one another by any simple process, they are not metameric in the sense cf Berzelius' defini tion (although Kekule cited the fatty acids and their esters as examples of his proposed use of this term).

Closer inspection shows, however, that isomerism can also occur in the alcohols and acids themselves, except in the simplest mem bers of the two homologous series set out above. Thus we find that two different models of the propyl group, C3H7, can be con structed as follows: the spare bond by which the group is attached to hydroxyl or to carboxyl being at the end or at the centre of the chain of carbon atoms. We can therefore write the formulae of propyl alcohol, and of butyric acid, each in two ways: Similarly, the higher members of this series of alcohols and acids give rise to increasing numbers of isomers. Thus the butyl radical, can exist in four forms, and there are, therefore, four isomeric butyl alcohols and four isomeric valeric acids. Only a few further steps in the series are necessary in order to reach compounds of which hundreds of isomers are theoretically possible. There is therefore no limit to the extent to which structural isomers could be multiplied in organic chemistry if necessary.

Optical Isomerism.

If the two-dimensional formulae which have been used in the preceding paragraphs were correct, it would be possible to construct two alternative models to represent the molecule of ethyl alcohol, thus In one model, which we may call the trans-form, the two hydrogen atoms are at opposite corners of the cross, whilst in the other, the cis-form, the two hydrogen atoms are adjacent. Since ethyl alcohol is known only in one form, we conclude that these planar formulae are incorrect. We theref ore follow Pasteur in supposing that the atoms are arranged in three dimensions in space, and con clude that the conventional formulae in two dimensions are mis leading. Isomerism in compounds of this type then becomes possible for the first time when an asymmetric carbon atom, i.e., a carbon atom linked to four different radicals, is present in the molecule. Two isomers are then possible, as in the formulae : the two acids in 1831 as illustrations of the phenomenon which he proposed to describe as isomerism. In 1849 Pasteur made a crystallographic study of the salts of the two acids, the results of which are described in the article STEREOCHEMISTRY, since the phenomena are instances of stereoisomerism. As a result of these researches, and of the later theories of von 't Hoff and Le Bel (1874), it became possible to attribute the optical activity to the presence of an asymmetric carbon atom whose four different groups were situated at the corners of a tetrahedron, the carbon atom being at the centre. Since tartaric acid has two such asym metric carbon atoms—those in heavy type in the formula