Semidione

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 R\sC(\sO-)\dC(\sO•)\sR', 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 (Ph\sC(\sO-)\dC(\sO•)\sPh), 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.