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.<br />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 .
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, Chainâ represents the already formed polymer chain, and <sup>â¢</sup> indicates a reactive free radical.
In 1937, Hans Adolf Krebs, who discovered the citric acid cycle bearing his name, confirmed the anaerobic dismutation of pyruvic acid into lactic acid, acetic acid, and CO<sub>2</sub>, by certain bacteria according to the global reaction:
The dismutation of pyruvic acid into other small organic molecules (ethanol + CO<sub>2</sub>, or lactate and acetate, depending on the environmental conditions) is also a key step in fermentation reactions. Fermentation reactions can also be considered as disproportionation or dismutation biochemical reactions. Indeed, the donor and acceptor of electrons in the redox reactions supplying the chemical energy in these complex biochemical systems are the same organic molecules simultaneously acting as reductant or oxidant.
Another example of biochemical dismutation reaction is the disproportionation of acetaldehyde into ethanol and acetic acid.
In cellular respiration, electrons are transferred from substrate (electron donor) to an electron acceptor; in fermentation, part of the substrate molecule itself accepts the electrons. Fermentation is therefore a type of disproportionation, and does not involve an overall change in oxidation state of the substrate. Most of the fermentative substrates are organic molecules.
Another example is microbial sulfur disproportionation (MSD), a type of energy metabolism involving the disproportionation of inorganic sulfur compounds, that exists often alongside reduction of sulfate in certain bacteria.
Sulfur isotopes of sediments are often measured for studying environments in the Earth's past (paleoenvironment). Disproportionation of sulfur intermediates, being one of the processes affecting sulfur isotopes of sediments, has drawn attention from geoscientists for studying the redox conditions in the oceans in the past.
Sulfate-reducing bacteria fractionate sulfur isotopes as they take in sulfate and produce sulfide. Prior to 2010s, it was thought that sulfate reduction could fractionate sulfur isotopes up to 46 â° and fractionation larger than 46 â° recorded in sediments must be due to disproportionation of sulfur intermediates in the sediment. This view has changed since the 2010s. As substrates for disproportionation are limited by the product of sulfate reduction, the isotopic effect of disproportionation should be less than 16 â° in most sedimentary settings.
Disproportionation can be carried out by obligate disproportionators or, more often, by microorganisms that can carry out sulfate reduction or sulfur oxidation as well. Common substrates for disproportionation include elemental sulfur (), thiosulfate () and sulfite ().
The Claus reaction is an example of comproportionation reaction (the inverse of disproportionation) involving hydrogen sulfide () and sulfur dioxide () to produce elemental sulfur and water as follows:
The Claus reaction is one of the chemical reactions involved in the Claus process used for the desulfurization of gases in the oil refinery plants and leading to the formation of solid elemental sulfur (), which is easier to store, transport, reuse when possible, and dispose of.