[known in English as the Wolff-Kishner reduction], the reduction of a carbonyl group () into a methylene group (). An aldehyde or ketone is converted into the hydrazone, which is decomposed when heated under pressure in the presence of bases:
Subsequently, another version of the Kizhner-Wolff reaction was frequently used: an aldehyde or ketone, a hydrazine excess, and potassium hydroxide are heated in a high-boiling solvent (di- or tri-ethylene glycol) at 180°-200°C. The Kizhner-Wolff reaction may be used to reduce acid-sensitive carbonyl compounds. This method was developed by N. M. Kizhner [Kishner] in 1911 and by the German scientist L. Wolff (1857–1919) in 1912.