Disproportionation
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.
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
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "disproportionation". doi:10.1351/goldbook.D01799
- ^ Charlie Harding, David Arthur Johnson, Rob Janes, (2002), Elements of the P Block, Published by Royal Society of Chemistry, ISBN 0854046909