Disproportionation

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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 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 catalysis of hydrogen peroxide to water and oxygen by the enzyme catalase:

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