Disproportionation
From Wikipedia, the free encyclopedia
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. While the most common type is a redox reaction, other types are possible. For example: 2H2O → H3O+ + OH- is a disproportionation but is not a redox reaction.
- A chemical reaction in which an element is simultaneously reduced and oxidized to form two different products.
The reverse of disproportionation is called comproportionation.
[edit] 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).
[edit] Examples
- Chlorine gas reacts with sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows:
- 3Cl2 + 6OH− → 5Cl− + ClO3− + 3H2O
- As a reactant, the oxidation number of the elemental chlorine is 0. In the products, 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 catalysis of hydrogen peroxide to water and oxygen by the enzyme catalase (shown as Fe-E) occurs in two stages:
- 2H2O2 → 2H2O + O2
- 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
- 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.