Disproportionation Reaction

the redistribution of atoms or their groups among molecules of the same substance, as a result of which the substance becomes two or more different substances, some of which have more—and others fewer—of those atoms or groups than the original substance. Most disproportionation reactions are oxidation-reduction processes and are associated with the redistribution of atoms of oxygen or hydrogen. For example, upon heating, mercurous oxide yields mercury and mercuric oxide:

In the presence of alkalies, aldehydes yield the corresponding alcohols and carboxylic acids (the Cannizzaro reaction):

Irreversible Zelinskii catalysis belongs to the same type of disproportionation reaction:

as does the conversion of free alkyl radicals into alkenes and alkanes:

2CH₃–CH₂ CH₂=CH₂ + Ch₃–CH₃

An example of a disproportionation reaction that is accompanied by redistribution of alkyl groups is the conversion of alkyl benzenes into a mixture of benzene and polyalkyl benzenes when treated with aluminum chloride:

The disproportionation reaction is extensively used in chemistry and often has great preparative significance.