Semidiones are radical anions analogous to semiquinones, obtained from the one-electron reduction of non-quinone conjugated dicarbonyls.
The simplest possible semidiones are derived from 1,2-dicarbonyls and have structure , making them the second member of a homologous series starting with ketyl radicals.
They are often transient intermediates, appearing in reactions such as the final reduction step of the acyloin condensation.
Benzil semidione (), synthesized by Auguste Laurent in 1836, is believed to have been the first radical ion ever characterized.
Semidehydroascorbate is a stable semidione produced by the one-electron oxidation of Vitamin C.