Disproportionation

In chemistry, disproportionation, sometimes called dismutation (the French word), is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation state. The reverse of disproportionation, such as when a compound in an intermediate oxidation state is formed from precursors of lower and higher oxidation states, is called comproportionation, also known as symproportionation.

More generally, the term can be applied to any desymmetrizing reaction where two molecules of one type react to give one each of two different types:

2 A → A' + A"

This expanded definition is not limited to redox reactions, but also includes some molecular autoionization reactions, such as the self-ionization of water. In contrast, some authors use the term redistribution to refer to reactions of this type (in either direction) when only ligand exchange but no redox is involved and distinguish such processes from disproportionation and comproportionation.For example, the Schlenk equilibrium

2 RMgX → R2Mg + MgX2

is an example of a redistribution reaction.

History

The first disproportionation reaction to be studied in detail was:

2 Sn2+ → Sn4+ + Sn

This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it söndring.

Examples

Mercury(I) chloride disproportionates upon UV-irradiation:

Hg2Cl2 → HgCl2 + Hg

Phosphorous acid disproportionates upon heating to 200°C to give phosphoric acid and phosphine:

4 H3PO3 → 3 H3PO4 + PH3

Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:

2 HCO−3 → CO2−3 + H2CO3

The oxidation numbers remain constant in this acid-base reaction.

Another variant on disproportionation is radical disproportionation, in which two radicals form an alkene and an alkane.

2

CH

3

−

C

∙

H

2

⟶

H

2

C

=

CH

2

+

H

3

C

−

CH

3

{\displaystyle {\ce {2CH3-{\underset {^{\bullet }}{C}}H2->{H2C=CH2}+H3C-CH3}}}

Disproportionation of sulfur intermediates by microorganisms is widely observed in sediments.

4 S0 + 4 H2O → 3 H2S + SO2−4 + 2 H+

3 S0 + 2 FeOOH → SO2−4 + 2 FeS + 2 H+

4 SO2−3 + 2 H+ → H2S + SO2−4

Chlorine gas reacts with concentrated sodium hydroxide to form sodium chloride, sodium chlorate and water.