The property of compounds (isomers) which have the same molecular formula but different physical and chemical properties. The difference in properties is caused by a difference in molecular structure (that is, molecular architecture). A typical example is dimethyl ether, CH3OCH3, a chemically quite inert gas which condenses at −24°C, and ethyl alcohol, CH3CH2OH, a liquid of substantial chemical reactivity which boils at 78°C; both compounds have the molecular formula C2H6O.
Isomers may be classified as constitutional isomers or stereoisomers. Constitutional isomers differ in constitution or connectedness, relating to the question as to which atoms are linked to which others and how. Dimethyl ether and ethanol (Fig. 1) are constitutional isomers. In dimethyl ether each carbon is connected to three hydrogen atoms and the one oxygen atom; the two carbon atoms are thus equivalent. In ethyl alcohol (ethanol) one carbon is linked to three hydrogen atoms and the other carbon; the second carbon is linked to the first carbon, two hydrogens, and the oxygen atom which, in turn, is linked to the sixth hydrogen atom; the two carbon atoms are not equivalent. Stereoisomers, in contrast, have the same constitution but differ in the three-dimensional array of the atoms in space, called configuration.
Functional isomers.
Constitutional isomers
Constitutional isomers have been subdivided into functional isomers, positional isomers, and chain isomers.
Functional isomers (Fig. 1) differ in functional group, that is, the group (or groups) most material in determining chemical behavior. In the third example shown in Fig. 1 (propionaldehyde) the three compounds all correspond to the molecular formula C3H6O, but the first one has an aldehyde function, the second combines a double bond with an alcohol function, and the third one has an epoxide function.
Positional isomers (Fig. 2) have the same functional group but differ in its position along a chain or in a ring. Closely related are chain isomers which also have the same functional group or groups but differ in the shape of the carbon chain (Fig. 3 a); quite similar are ring isomers (Fig. 3 b) which differ in the size of one or more rings. Ring and chain isomers together are sometimes called skeletal isomers.
Positional isomers.
Skeletal isomers. (a) Chain isomers. (b) Ring isomers.
Stereoisomers
Compounds which have not only the same molecular formula but also the same constitution (connectivity of atoms) but which differ in the disposition of the atoms in space are called stereoisomers. Stereoisomers, in turn, are subdivided into two types: those that are mirror images of each other, called enantiomers, and those which are not mirror images, called diastereomers or diastereoisomers.
Enantiomers are unique in that they always come in pairs. Either a molecule is superposable with its mirror image, in which case it does not have an enantiomer, or it is not superposable with its mirror image, in which case it has one and only one enantiomer (since an object can have only one mirror image). Molecules which are not superposable with their mirror images are called chiral; those which are so superposable are called achiral. Enantiomers are much more alike than are other sets of isomers (constitutional isomers or diastereomers); thus they have the same melting point, boiling point, free energy, spectral properties, x-ray diffraction pattern, and so on.
Diastereomers have the same constitution but different spatial arrangement and are not mirror images. They resemble constitutional isomers in that there may be more than two isomers in a set and that their physical, energetic, and spectral properties are generally quite distinct. See also Optical activity; Stereochemistry.
