In chemistry, isomers (Greek isos = "equal", méros = "part") are compounds with the same molecular formula but different structural formulae. Isomers do not necessarily share similar properties unless they also have the same functional groups. This should not be confused with a nuclear isomer, which involves a nucleus at different states of excitement. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical isomers, et cetera (see graph below). __TOC__

Isomerism

A simple example of isomerism is given by propanol: it has the formula C₃H₈O (or C₃H₇OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)

Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first isomer, and to the center carbon in the second.

There is, however, another isomer of C₃H₈O which has significantly different properties: methoxyethane (methyl-ethyl-ether; III). Unlike the isomers of propanol, methoxyethane has an oxygen atom that is connected to two carbons rather than to one carbon and one hydrogen. This makes it an ether, not an alcohol, as it lacks a hydroxyl group, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.

Examples of isomers having different medical properties can be easily found. For example, in the placement of methyl groups. In substituted xanthines, Theobromine, found in chocolate, is a vasodilator with some effects in common with caffeine, but if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is theophylline, which has a variety of effects, including bronchodilation and anti-inflammatory action. Another example of this occurs in the phenethylamine-based stimulant drugs. Phentermine is a non-chiral compound with a weaker effect than amphetamine. It is used as an appetite reducing medication and has mild or no stimulant properties. However, a different rearrangement of the same atoms gives dextromethamphetamine, which is more potent than amphetamine; it is a very strong stimulant.

Allene and propyne are examples of isomers containing different bond types. Allene contains two double bonds, while propyne contains one triple bond.

Classification

There are two main forms of isomerism: structural isomerism and stereoisomerism.