In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can be applied to any desymmetrizing reaction of the following type, regardless of whether it is a redox or some other type of process: 2A -> A' + A'' Examples Mercury(I) chloride disproportionates upon UV-irradiation: Hg2Cl2 → Hg + HgCl2 Phosphorous acid disproportionates upon heating to give phosphoric acid and phosphine: 4 → 3 H3PO4 + PH3 Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate: 2 → + H2CO3 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 alkene and an alkane. {2CH3-\underset{^\bullet}CH2 -> {H2C=CH2} + H3C-CH3} Reverse reaction 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 synproportionation. History The first disproportionation reaction to be studied in detail was: 2 Sn2+ → Sn4+ + Sn This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it 'söndring'. Further examples Chlorine gas reacts with dilute sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows:3Cl2 + 6 OH- -> 5 Cl- + ClO3- + 3 H2O The chlorine reactant is in oxidation state 0. In the products, the chlorine in the Cl− ion has an oxidation number of −1, having been reduced, whereas the oxidation number of the chlorine in the ClO3− ion is +5, indicating that it has been oxidized. Decompositions of numerous interhalogen compounds involve disproportionation. Bromine fluoride undergoes disproportionation reaction to form bromine trifluoride and bromine:3 BrF -> BrF3 + Br2 The dismutation of superoxide free radical to hydrogen peroxide and oxygen, catalysed in living systems by the enzyme superoxide dismutase: 2 O2- + 2H+ -> H2O2 + O2 The oxidation state of oxygen is −1/2 in the superoxide free radical anion, −1 in hydrogen peroxide and 0 in dioxygen. In the Cannizzaro reaction, an aldehyde is converted into an alcohol and a carboxylic acid. In the related Tishchenko reaction, the organic redox reaction product is