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Disproportionation, sometimes called dismutation, is a redox reaction in which a compound of intermediate oxidation state converts to two different compounds, one of higher and one of lower oxidation states. Although not widely accepted, disproportionation is sometimes used to describe any desymmetrizing reaction of the following type: 2 A → A' + A", regardless of any redox process.
Mercury(I) chloride disproportionates upon UV-irradiation:
As mentioned above, desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:
The oxidation numbers remain constant in this acid-base reaction. This process is also called autoionization.
Another variant on disproportionation is radical disproportionation, in which two radicals form an alkane and alkene.
The reverse of disproportionation, when a compound in an intermediate oxidation state is formed from precursors of lower and higher oxidation states, is called comproportionation, also known as synproportionation.
The first disproportionation reaction to be studied in detail was:
In 1937, Hans Adolf Krebs, who discovered the citric acid cycle bearing his name, confirmed the anaerobic dismutation of pyruvic acid in lactic acid, acetic acid and CO2 by certain bacteria according to the global reaction:
The dismutation of pyruvic acid in other small organic molecules (ethanol + CO2, or lactate and acetate, depending on the environmental conditions) is also an important 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.
While in 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. However, a rare type of fermentation may also involve the disproportionation of inorganic sulfur compounds in certain sulfate-reducing bacteria.