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.

Contents 1 History 2 Examples 3 See also 4 References

[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 ^[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

[edit] See also

• Dismutase

[edit] References