Disproportionation or dismutation is used to describe two particular types of chemical reaction:[1]
- A chemical reaction of the type: 2A → A' + A" where A, A' and A" are different chemical species. Most but not all are redox reactions. For example: 2H2O → H3O+ + OH- is a disproportionation but is not a redox reaction.
- A chemical reaction (reversible or irreversible) in which a species is simultaneously reduced and oxidized so as to form two different products.
The reverse of disproportionation is called comproportionation.
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History
The first disproportionation reaction to be studied in detail was:
- 2 Sn2+ → Sn + Sn4+
This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it 'söndring'. (K. Sv. Vet. Acad. Handl. 1788, 186-197; Crells chem. Annalen 1790, I, 260-273).
Examples
- Chlorine gas reacts with sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows [2]:
- 3Cl2 + 6OH− → 5Cl− + ClO3− + 3H2O
- As a reactant, the oxidation number of the elemental chlorine is 0. In some of the product, Cl− has an oxidation number of −1, having been reduced; whereas the oxidation number of chlorine in the chlorate ion is +5, indicating that it has been oxidized.
- The dismutation of superoxide free radical to hydrogen peroxide and oxygen, catalysed in living systems by superoxide dismutase:
- 2O2− + 2H+ → H2O2 + O2
- The O2 "species" has an oxidation state of -1 in the superoxide free radical anion, -2 in hydrogen peroxide and zero 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 the corresponding ester. In the Kornblum–DeLaMare rearrangement, a peroxide is converted to a ketone and an alcohol.
- The disproportionation of hydrogen peroxide into water and oxygen catalysed by the enzyme catalase:
- 2H2O2 → 2H2O + O2
- The Boudouard reaction is for example used in the HiPco method for producing carbon nanotubes, high pressure carbon monoxide disproportionates when catalysed on the surface of an iron particle:
- 2CO → C + CO2
See also
References
- ^ International Union of Pure and Applied Chemistry. "disproportionation". Compendium of Chemical Terminology Internet edition.
- ^ Charlie Harding, David Arthur Johnson, Rob Janes, (2002), Elements of the P Block, Published by Royal Society of Chemistry, ISBN 0854046909
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