disproportionated

Disproportionated is an adjective used in chemistry to describe a substance that has undergone a disproportionation reaction. In a disproportionation, a single species is simultaneously oxidized and reduced to form two different species, each with a different oxidation state. The term emphasizes that the original substance splits into products that share the same chemical element but differ in oxidation number.

Disproportionation reactions are a type of redox process and are common in inorganic chemistry, certain organic radicals, and in aqueous solutions where specific conditions favor electron rearrangement. Conditions such as acidity, temperature, and the presence of catalysts or other reactive partners can influence the thermodynamics and kinetics of the process. The reverse process, where two species in different oxidation states react to form a single intermediate-valence species, is called comproportionation.

Examples illustrate the variety of disproportionation pathways. Chlorine in water can disproportionate according to Cl2 + H2O → HCl + HClO, producing chloride and hypochlorous acid. Hydrogen peroxide can disproportionate as 2 H2O2 → 2 H2O + O2, yielding water and oxygen. Nitrous acid can disproportionate via 3 HNO2 → HNO3 + 2 NO + H2O, forming nitric acid and nitric oxide. In some cases, sulfurous acid disproportionates to sulfuric acid and hydrogen sulfide: 2 HSO3− → HSO4− + S2− under appropriate conditions.

Disproportionated species may be observed in solution chemistry, solid-state systems, or in catalytic cycles. The related concept of disproportionation is contrasted with comproportionation, where reactants contain the same element in different oxidation states and form a compound with an intermediate oxidation state. The term “disproportionated” thus conveys a history of redox restructuring leading to distinct products.