Isomerism

ISOMERISM (from the Greek isomeros—composed of equal parts), a phenomenon consisting in the fact that substances having the same molecular formula differ from one another in their physical and chemical properties. The first case of this kind was described by Liebig in 1823, and the term isomerism was proposed by Berzelius in 1830. The study of the phenomena of isomerism has shown that the difference in physical and chemical properties depends systematically not only on the difference in the elements entering into the composition of the substance, but also on the difference in the mutual arrangement of atoms in the molecule. Although isomerism is also encountered among inorganic compounds, incomparably more numerous and diverse cases of it are observed among organic compounds, and isomerism in organic chemistry has extremely important theoretical and practical significance. The theory of the structure of organic substances assumes all four valencies of carbon to be equivalent. Therefore, in the hydrocarbon methane, CH₄, all four H atoms are identical, and it is indifferent which of these atoms is replaced, for example, by the radical CH₃ or CH₂.CH₃. Thus, neither ethane CH₃.CH₃ nor propane CH₃.CH₂.CH₃ have isomers. In propane, however, an H atom can be replaced by a CH₃ group in two ways: either 1) at one of the terminal C atoms, or 2) at the middle one. Consequently, for butane C₄H₁₀, there can be two different structural formulas: 1) CH₃.CH₂.CH₂.CH₃,

2) CH₃.CH(CH₃).CH₃. Indeed, there exist 2 butanes, differing from each other both in chemical structure and in physical and chemical properties. In one, the chain of carbon atoms is straight, unbranched; in the other, it is branched. Isomerism depending on this cause is called chain isomerism. Compounds having an unbranched chain of carbon atoms are called normal (e.g., normal butane), and compounds with a branched chain of carbon atoms are called iso-compounds (e.g., isobutane).

If, in the presence of one and the same chain of carbon atoms, the difference in structure depends on the fact that other atoms or radicals are situated in different places of this chain, then such isomerism is called positional isomerism. Thus, for propane, there can be and indeed exist 2 isomeric monochloride derivatives: CH₃.CH₂.CH₂Cl (propyl chloride) and CH₃.CHCl.CH₃ (isopropyl chloride).

There can be phenomena of combined isomerism—simultaneously both chain isomerism and positional isomerism, as for example for the following two monochloride derivatives of butane: CH₃.CH₂.CHCl.CH₃ and CH₃.CH(CH₃).CH₂Cl.

As the number of carbon atoms in the molecule of organic compounds increases, the number of chain isomers grows rapidly. For the hydrocarbon C₅H₁₂ there are 3 isomers; for the hydrocarbon C₁₃H₂₈, 802 isomers are possible, etc. Positional isomers are more numerous and diverse than chain isomers. A special case of isomerism is encountered, depending on the fact that a divalent or polyvalent atom links radicals which are different in different isomers, but in sum contain the same number of atoms; such a case of isomerism bears the name of metamerism. For example, the ether C₄H₁₀O exists in two metamers: C₂H₅-O-C₂H₅ and CH₃-O-C₃H₇.

When in a molecule H passes from one atom to another, thereby causing a change in the character of the bonds, an isomeric compound is obtained; in this case, the substance reacts according to one structural formula in some reactions, and according to another in others, so that both formulas are as if equivalent. Such a phenomenon bears the name of tautomerism (equivalence) or desmotropism (change of bonds). One of the most typical examples of the phenomena of tautomerism can serve as the so-called "keto-enol" tautomerism, depending on the transition of the ketone form into the alcohol form and back: R-C-R (O) ketone form, R-C(OH)=C-R enol form. ("En" in the term "enol"—a prefix characterizing ethylene hydrocarbons—indicates a double bond; "ol"—the ending characterizing the name of alcohols.) Since the process of tautomerization is reversible, a state of equilibrium is established for a mixture of 2 tautomeric forms, depending on temperature, solvent, and other conditions. Both forms can be isolated in pure form.

The above-mentioned types of isomerism are observed both in compounds with an open chain of C atoms (acyclic, fatty series) and in cyclic compounds. The latter have, in addition, special types of isomerism: 1) Isomerism depending on the number of atoms entering into the construction of the ring itself, for example: hexamethylene and methylpentamethylene.

2) Isomerism caused by the unequal arrangement of those atoms or radicals which replace H atoms inside the ring (positional isomerism in the nucleus), e.g.: ortho-di-, meta-di-, para-dichlorobenzene, chlorobenzene, alpha-chloropyridine, beta-chloropyridine, gamma-chloropyridine.

All the cited types of isomerism depend on the unequal mutual distribution of atoms in the molecule, on the different chemical structure of the molecules, and therefore bear the name of structural isomers. But besides this, numerous cases are known where substances having undoubtedly one and the same structural formula nevertheless turn out to be different in physical and chemical properties. Structural formulas cannot provide an explanation for this phenomenon, because chemical molecules are corporeal formations having extension in 3 dimensions, whereas structural formulas are depicted on a plane and consequently have extension only in 2 dimensions. If, however, one gives structural formulas extension in the third dimension, i.e., depicts them in the form of corporeal models of molecules or in the form of projections of such models onto a plane, then the existence of this kind of seemingly superfluous isomers becomes fully understandable and necessary, as shown in 1874 by van't Hoff and Le Bel. Isomers of this kind are called stereoisomers.

The concept of isomerism lies at the foundation of all modern organic chemistry and has great significance in biology. Various isomers, both structural and stereoisomers, turn out to be different in their effect on living cells and, in turn, undergo unequal changes under the influence of life processes. Thus, for example, mustard gas Cl.CH₂.CH₂.S.CH₂.CH₂Cl is one of the most terrible chemical warfare agents, whereas its isomer CH₃.CHCl.S.CHCl.CH₃ does not possess such properties; only certain stereoisomers of hexoses are capable of undergoing alcoholic fermentation; different stereoisomers often have a different taste and possess unequal pharmacological action. A large number of facts of this kind are already known at the present time.