Clemmensen还原
利用锌汞齐和氯化氢将醛酮的羰基还原成为亚甲基的反应。
浓盐酸回流下，用锌汞齐将醛或酮中的羰基还原为亚甲基，与芳香环共轭的酮的反应活性最高。在非水溶液的条件下，用氯化氢的醚溶液，甚至能还原非共轭的羰基。这种情况只需要用金属锌就可以，不需要用到毒性高的汞合金。
本反应在酸性条件下进行，而与之相对应的有碱性环境下的沃尔夫-凯西纳还原反应，以及中性条件下的雷尼镍脱硫还原反应。
反应机理
具体的反应机理不明，通常认为反应发生在锌催化剂表面，由于将相应的醇用于此反应条件并不能得到亚甲基化合物，因此该反应中间态可能没有醇产生。可能是生成了一种锌-卡宾中间体，进而在酸性条件下得到产物。
反应实例
参考文献
1. Clemmensen, E. Ber. 1913, 46, 1837-1843. Erik C. Clemmensen (1876-1941) was born in Odense, Denmark. He received the M.S. degree from the Royal Polytechnic Institute in Copenhagen. In 1900, Clemmensen immigrated to the United States, and worked at Parke, Davis and Company in Detroit (coincidently, this author’s first employer!) as a research chemist for 14 years, where he discovered the reduction of carbonyl compounds with amalgamated zinc. Clemmensen later founded a few chemical companies and was the president of one of them, the Clemmensen Chemical Corporation in Newark, New Jersey.
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Erik C. Clemmensen (1876-1941) 出生于丹麦的欧登塞，获得哥本哈根皇家理工学院的理科硕士学位。1900年移民到美国，在Parke, Davis&公司（现在是辉瑞公司的一部分）作为化学研究院工作了14年，在那里他发现了利用锌汞齐还原羰基化合物的方法。后来Clemmensen自己创建了一些小的化工企业，其中有一个在纽约的纽华克，叫Clemmensen Chemical Corporation，本人担任董事长。
编译自：J.J. Li, Name Reactions: A Collection of Detailed Mechanisms and Synthetic Applications, Clemmensen reduction，page 153-154.
相关操作
Amalgamated zinc is prepared by shaking for five minutes a mixture of 120 g. of mossy zinc, 12 g of mercuric chloride, 200 mL of water, and 5–6 mL of concentrated hydrochloric acid contained in a 1-L round-bottomed flask. The solution is decanted and the following reagents are added, in the order named, to the zinc: 75 mL of water, 175 mL of concentrated hydrochloric acid, 100 mL of toluene, and 50 g (0.28 mol) of β-benzoylpropionic acid. The flask is fitted with a vertical condenser connected to a gas absorption trap, and the reaction mixture is boiled vigorously for twenty-five to thirty hours. Three 50 mL portions of concentrated hydrochloric acid are added at approximately six-hour intervals during the refluxing period. After cooling to room temperature the layers are separated. The aqueous layer is diluted with 200 mL of water and extracted with three 75 mL portions of ether. The toluene layer and the ether extracts are combined, washed with water, and dried over calcium chloride. The solvents are removed by distillation under reduced pressure on the steam bath, after which the χ-phenylbutyric acid is distilled at 178–181°/19 mm. (148–154°/8–10 mm.,
125–130°/3 mm.). The yield of acid, which melts at 46–48°, is 38–41 g. (82–89%).
Organic Syntheses, Coll. Vol. 2, p.499; Vol. 15, p.64