ISOMERISM, l'io-marizm (Gr., °having equal parts°). Chemists formerly assumed that two bodies must be identical in chemical nature, in all respects, provided they consist of the same elements, combined in the same propor tions. This view was long ago found to be untenable and many substances (mostly com pounds of carbon or nitrogen) are now known, which exhibit widely different properties, al though possessing the same empirical formula. Bodies which possess this peculiarity are said to be °isomeric° with each other, and the prop erty itself is called °isomerism.° In its broad-I est sense, isomerism may be regarded as em bracing (1) polymerism; (2) metamerism; (3) isomerism in the narrower sense; and (4) geo, metrical isomerism.
Bodies are when they have the same percentage composition, but have differ ent molecular weights. Acetic acid, and grape sugar, C.1-12208, for example, are polymeric with each other, because they consist of the same elements, combined in the same proportions, and yet the molecular weight of grape sugar is three times as great as that of acetic acid. In this particitlar case there is no specially close relation between the polymeric substances, and the polymerism is therefore said to be °accidental.° When a close relation does exist between the bodies compared, the, poly merism is said to be °generic.° Ordinary acetic aldehyde affords a good example of generic polytnerism. Aldehyde has the formula C.H40, but when treated with a mineral acid it becomes transformed into paraldehyde, which has the formula C41-1,,,0•; and the reverse transforma tion (of paraldehyde into aldehyde) may be effected by the application of heat.
Compounds are said to be "metameric° when they have the same empirical formula, but dif fer structurally by containing different radicals, joined by a polyvalent element such as oxygen, nitrogen, or sulphur. Ethyl ether and propyl
methyl ether, for example, both have the em pirical formula C411,o0; but ethyl ether con• tains two ethyl radicals, united by an oxygen atom, and propyl-methyl ether contains a propyi radical and a methyl radical united by oxygen in the same manner. Thus these two meta meric bodies have the structural formulae 0 respectively CH, Metamerism is manifested, most commonly, by the ethers, esters and amines.
Isomerism in its narrower sense or "true isomerism," embraces those cases in which the bodies compared have the same empirical form ulae but have different structural formulae and do not (like metameric bodies) consist of defi nite carbon radicals united by oxygen sulphur, or nitrogen. True isomerism may be of two kinds: (1) "nucleus isomerism," and (2) "isom erism of position." The hydrocarbons afford good examples of both kinds of true isomerism. The paraffin known as propane, for example, has the empirical formula C4112, and the struc tural formula CH,—CH2—CH.. Propane may be converted into butane by replacing one of its hydrogen atoms by the methyl radical, CH.; but the substitution may be made in two essen,-, different ways, according as the hydrogen that is replaced is attached to the interior car bon atom, or to one of the terminal ones.. In the latter case the structural formula of the new substance is CH,—CH2—CH2—CH2, and the substance itself is known as "normal" bu tane. If the hydrogen that is replaced is at tached to the interior carbon atom, a different substance known as "isobutane" and having dif ferent properties from normal butane, is formed ;