定义Definition
克莱门森还原是用锌汞齐和浓盐酸把醛、酮的羰基直接还原为亚甲基（C=O → CH₂）的反应，适合对酸稳定的底物，尤其是芳基烷基酮。
The Clemmensen reduction converts the carbonyl of aldehydes and ketones directly into a methylene group (C=O → CH₂) with zinc amalgam and concentrated hydrochloric acid; it suits acid-stable substrates, especially aryl alkyl ketones.
机理与特点Mechanism and features
反应在锌表面进行，机理尚不完全清楚，一般认为经有机锌卡宾类中间体，而不经过醇（醇在该条件下不被还原为烷烃）。与傅–克酰基化联用可把酰基引入后还原为烷基，避免傅–克烷基化的重排和多烷基化问题。
It occurs on the zinc surface by a mechanism still debated, probably via zinc carbenoids rather than alcohols (which are not reduced under these conditions). Paired with Friedel–Crafts acylation it installs straight alkyl chains without the rearrangement and polyalkylation of Friedel–Crafts alkylation.
由来Origin
丹麦化学家埃里克·克莱门森于 1913 年报道。
Reported by the Danish chemist Erik Clemmensen in 1913.
与沃尔夫–基斯纳还原互补Complement to Wolff–Kishner
克莱门森在强酸中进行，沃尔夫–基斯纳在强碱中进行，可根据底物对酸碱的耐受性选择。对酸敏感（缩醛、叔醇）的底物不宜用克莱门森还原。
Clemmensen runs in strong acid and Wolff–Kishner in strong base, so the choice depends on the substrate’s tolerance; acid-sensitive groups (acetals, tertiary alcohols) rule out Clemmensen.
反应Reactions
锌汞齐、浓盐酸，加热回流（常加甲苯作共溶剂）
Zinc amalgam and concentrated HCl at reflux (often with toluene as co-solvent)
羰基在锌表面接受电子并质子化。
The carbonyl accepts electrons and protons at the zinc surface.
经有机锌中间体脱去氧。
Oxygen is removed via organozinc intermediates.
最终得到亚甲基。
The methylene group results.
芳基酮还原为烷基苯。
An aryl ketone reduced to an alkylbenzene.
年表Timeline
- 1913
克莱门森报道该反应
Clemmensen reports the reaction