Science of Synthesis

Wicha, J., Science of Synthesis, (2009) 48, 99.

Aldehydes and ketones are reduced to the corresponding methylene derivatives on treatment with zinc or amalgamated zinc in the presence of hydrochloric acid or another strong protic acid (the Clemmensen reduction) (‌Scheme 5‌).[‌7‌] Older literature on the Clemmensen reduction is reviewed in Houben–Weyl, Vol. 4/1c, p 721; Vol. 5/1a, p 244; and Vol. 7/2b, p 1998, as well as in several monographs and book chapters.[‌8‌–‌12‌] Zinc in the form of turnings, granulates, wool, dust (powder), or mossy zinc, or the respective amalgamated forms is used.[‌13‌,‌14‌] Amalgamated reagent is prepared immediately before use by activating the zinc with diluted hydrochloric acid (typically 2%) and then covering the bright metal with 5% aqueous mercury(II) chloride (a small amount of hydrochloric acid facilitates the process), shaking or stirring the suspension for an appropriate time (typically 2 hours), and decanting the liquid. The reaction is carried out (1) in aqueous hydrochloric acid (usually 20% or higher concentration) at reflux temperature (the classical conditions); (2) in aqueous hydrochloric acid in combination with water-miscible solvents, such as ethanol, acetic acid, or dioxane;[‌15‌] (3) in aqueous hydrochloric acid in combination with water-immiscible solvents, such as benzene or toluene (the Martin modification);[‌16‌] or (4) in dry diethyl ether, acetic anhydride, or other organic solvents saturated with gaseous hydrogen chloride at 0°C or lower (the Yamamura modification).[‌17‌,‌18‌] Some other modifications including those where the reaction is promoted with ultrasound irradiation[‌19‌] or with chlorotrimethylsilane[‌20‌] have also been developed.

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References

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• 8.Meeeee-Meee, (8888) M 88e8, 888.
• 8.Meeeee-Meee, (8888) M 8e, 888.
• 8.Meeeee-Meee, (8888) M 8e, 88.
• 8.Meeeee-Meee, (8888) M 8e8, 888.