Isomerism — Chambers's Encyclopaedia

Isomerism (from the Greek word isomērēs, 'composed of equal parts'), the relation between chemical compounds which are identical in their ultimate or percentage composition, but present differences in their chemical properties. Isomeric compounds, or isomerides, are divisible into metameric compounds, or metamerides, and polymeric compounds, or polymerides.

In all metameric compounds the molecular weight is the same, while in all polymeric compounds the molecular weights are simple multiples of the molecular weight of the lowest member of the group. As an illustration of metamerides, propionic acid, $C_3H_5CO\cdot OH$ , acetate of methyl, $CH_3\cdot CO\cdot OCH_3$ , and formic ether, $H\cdot CO\cdot OC_2H_5$ , may be taken. Their rational formulæ, which express their probable constitution, are perfectly distinct, yet they all have the same percentage composition, the same empirical formula, $C_3H_6O_2$ , and the same molecular weight (74).

As an illustration of polymerides, the hydrocarbons homologous with olefiant gas may be taken. Olefiant gas is represented by the formula $C_2H_4$ , propylene by $C_3H_6$ , butylene by $C_4H_8$ , amylene by $C_5H_{10}$ . These substances have the same percentage composition, but different molecular weights.

The carbohydrates, which are represented by the general formula $C_xH_{2n}O_n$ , present well-marked examples of isomerism. Thus, cellulose, $C_6H_{10}O_5$ , starch, $C_6H_{10}O_5$ , and gum, $C_6H_{10}O_5$ , are metameric; while grape-sugar, $C_6H_{12}O_6$ , possesses the same percentage composition, but twice as high a molecular weight, as lactic acid, $C_3H_6O_3$ , and the same percentage composition, but three times as high a molecular weight, as acetic acid, $C_2H_4O_2$ ; hence the three last-named substances are polymeric.

The most recent researches have brought to light the existence of several special varieties of isomerism. A tautomeric body is one in which the reaction to some reagents is as if certain hydrogen atoms were in one place in the molecule, while to others it is as if the hydrogen occupied a different position; and a tautomeric body may be desmomeric when it can be prepared in recognisably different forms, differing from one another in the position of these wandering hydrogens. Alloisomeric bodies have a similar chemical structure, but the geometrical symmetry is different, as in the following case (in which the symbol X stands for the group $\cdot CO \cdot OH$ ):

Fumaric Acid.	Maleic Acid.
$H : C : C : X$	$X : C : C : X$
$X : C : C : H$	$H : C : C : H$

The question of geometrically symmetrical or asymmetrical arrangement of atoms in a molecule has become, in the hands of Wislicenus and others, one of considerable importance in reference to isomerism. Quite possibly the allotropic modifications of some of the elements (see ALLOTROPY) are really isomeric differences of arrangement of the atoms within the Molecule (q.v.). See also AROMATIC SERIES AND CHEMISTRY (Vol. III. p. 152).