Disproportionation reactions

Disproportionation reactions are a special type of redox reaction where the same element is simultaneously oxidised and reduced. One substance acts as both the oxidising agent and the reducing agent.

For disproportionation to occur, the reacting substance must contain an element that can exist in at least three oxidation states. The element starts in an intermediate oxidation state and produces both a higher and a lower oxidation state.

Example — Decomposition of hydrogen peroxide:

Oxygen in H$_2$O$_2$ is in the –1 oxidation state. In the products, some oxygen is reduced to –2 (in H$_2$O) and some is oxidised to 0 (in O$_2$).

Disproportionation in alkaline medium — Phosphorus:

Phosphorus (0) is reduced to –3 (in PH$_3$) and oxidised to +1 (in H$_2$PO$_2^-$).

Disproportionation in alkaline medium — Sulphur:

Sulphur (0) is reduced to –2 (in S$^{2-}$) and oxidised to +2 (in S$_2$O$_3^{2-}$).

Disproportionation in alkaline medium — Chlorine (formation of household bleach):

Chlorine (0) is oxidised to +1 (in ClO$^-$) and reduced to –1 (in Cl$^-$). The hypochlorite ion (ClO$^-$) is the active bleaching agent — it oxidises coloured stains to colourless compounds.