Disproportionation

In chemistry, disproportionation, sometimes called dismutation (the French word), is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation state. The reverse of disproportionation, such as when a compound in an intermediate oxidation state is formed from precursors of lower and higher oxidation states, is called comproportionation, also known as symproportionation.

More generally, the term can be applied to any desymmetrizing reaction where two molecules of one type react to give one each of two different types:

This expanded definition is not limited to redox reactions, but also includes some molecular autoionization reactions, such as the self-ionization of water. In contrast, some authors use the term redistribution to refer to reactions of this type (in either direction) when only ligand exchange but no redox is involved and distinguish such processes from disproportionation and comproportionation.
For example, the Schlenk equilibrium

is an example of a redistribution reaction.

The first disproportionation reaction to be studied in detail was:

This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it söndring.

In free-radical chain-growth polymerization, chain termination can occur by a disproportionation step in which a hydrogen atom is transferred from one growing chain molecule to another one, which produces two dead (non-growing) chains.

in which, Chain— represents the already formed polymer chain, and ^• indicates a reactive free radical.