Disproportionation

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Disproportionation, also called dismutation, is a redox reaction in which a single compound in an intermediate oxidation state splits into two products: one is more oxidized and the other is more reduced. The reverse process is comproportionation (or symproportionation), where two different oxidation states combine to form the intermediate state. More broadly, disproportionation can describe any desymmetrizing reaction in which two molecules of one type give two molecules of two different types; this broader use isn’t limited to redox and includes some non-redox processes like the self-ionization of water.

Some authors use the term redistribution for similar ligand-exchange reactions that don’t involve a change in oxidation state (such as the Schlenk equilibrium).

Historical and classic examples
- The first well-studied disproportionation was examined with tartrates by Johan Gadolin in 1788.
- In free-radical polymerization, termination can occur by disproportionation: a hydrogen atom moves from one growing chain to another, producing two non-growing (dead) chains.

Biochemical and ecological examples
- In biology, pyruvic acid can disproportionate under anaerobic conditions to lactic acid, acetic acid, and carbon dioxide. Fermentation involves electron transfer within the substrate itself, so it’s a form of disproportionation without a net change in overall oxidation state.
- Acetaldehyde can also disproportionate to ethanol and acetic acid.
- Disproportionation also occurs with inorganic sulfur compounds in some bacteria. This helps scientists study ancient ocean conditions by looking at sulfur isotopes in sediments. Modern understanding shows that isotope fractionation from disproportionation is typically smaller than previously thought and often less than about 16 per thousand.

Comproportionation (the opposite process)
- The Claus reaction is a notable example of comproportionation: hydrogen sulfide reacts with sulfur dioxide to form elemental sulfur and water. This reaction is used in refining to produce solid sulfur.

Overall
Disproportionation covers both redox and some non-redox desymmetrizing reactions, with a range of important examples in chemistry, biology, and the environment.