disproportionaatio

Disproportionation, sometimes referred to as disproportionation in certain languages (for example Finnish disproportionaatio), is a redox reaction in which a single chemical species is simultaneously oxidized and reduced to form two different species with distinct oxidation states. The element involved thus appears in two oxidation states in the products. The reverse process, where two oxidation states combine to form a single intermediate, is called comproportionation (synproportionation).

Disproportionation occurs when the redox potentials of the species favor both oxidation and reduction under the given conditions, often in solution. Factors such as pH, solvent, temperature, and catalysts influence its rate and outcome. It is common for reactive species such as hydrogen peroxide, hypohalous acids, or certain metal-oxo or halogen species to undergo disproportionation.

Examples include the decomposition of hydrogen peroxide: 2 H2O2 → 2 H2O + O2, where the oxygen ends up in different oxidation states in the products. In aqueous chlorine chemistry, chlorine gas disproportionates according to Cl2 + H2O → HCl + HClO, distributing chlorine between oxidation states 0, −1, and +1 in the products.

Disproportionation has relevance in environmental chemistry, electrochemistry, and synthesis, where it can be exploited to generate species with distinct redox properties or may occur as a competing pathway affecting product yields.

See also: comproportionation, redox reaction, oxidation state.