Disproportionation

Disproportionation or dismutation is used to describe two particular types of chemical reaction:

• 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.

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:

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.

References

de:Disproportionierung