Disproportionation

Meaning: Disproportionation is a redox reaction in which one species is both oxidized and reduced. An element in an intermediate oxidation state forms two products, one with a higher oxidation number and one with a lower. In 2H₂O₂ → 2H₂O + O₂, oxygen goes from −1 to −2 (reduced) and to 0 (oxidized). In Cl₂ + 2OH⁻ → Cl⁻ + OCl⁻ + H₂O, chlorine goes from 0 to −1 and +1. The species serves as its own oxidizing agent and reducing agent. The reverse, in which two forms of an element give one intermediate form, is comproportionation.

On the MCAT: Assign oxidation numbers on both sides. If one element from a single reactant appears in the products at both a higher and a lower oxidation number, the reaction is a disproportionation. Passages may give standard reduction potentials and ask whether an intermediate species will disproportionate. It does so spontaneously when E°cell = E°(reduction half) − E°(oxidation half) is positive, which means ΔG° is negative.

Common confusion: Students label any redox reaction with two products a disproportionation. It requires the same element, from the same reactant, to be oxidized and reduced. An element at its highest possible oxidation number can only be reduced, and one at its lowest can only be oxidized, so neither can disproportionate.