Clemmensen reduction is an organic reduction reaction shown by both aldehydes and ketones. This reaction is named after Erik Christian Clemmensen, a Danish chemist. This Clemmensen reduction reaction is carried by using Zinc amalgam (Zn-Hg) and Hydrochloric acid (HCl) which is often referred to as Clemmensen’s reducing agent.
It is found from experiments and research studies that the Clemmensen reduction is very effective in reducing aryl-alkyl ketones, such as those obtained in a Friedel-Crafts acylation reaction. Reduction with Zinc metal is much more effective especially for aliphatic or cyclic ketones.
Note: However, Clemmensen reduction reaction is not suitable for substances sensitive to acids, i.e., the substrate must be unreactive to the strongly acidic conditions of the Clemmensen reduction. Also, Carboxylic acid (-COOH) group can’t be reduced by this method (but the -COOH group can be reduced by treating it with soda lime [NaOH+ CaO] and then heating). For reducing acid sensitive substrates Wolff-Kishner reduction is suitable, since it utilizes basic condition.
The mechanism for the Clemmensen reduction is not yet fully understood but there are two popular theories that are proposed which aims in understanding this reaction mechanism from different approaches. These theories are “Carbanionic mechanism”, where the zinc attacks the protonated carbonyl directly, and the “Carbenoid mechanism”, which is a radical process and according to this, the Clemmensen reduction happens on the surface of the zinc metal. Carbenoid mechanism is widely accepted proposal for understanding the Clemmensen’s reduction reaction.
According to this Carbenoid mechanism of Clemmensen reduction, the reduction takes place at the surface of the zinc catalyst. In this reaction, alcohols are not postulated as intermediates. This mechanism employs the intermediacy of zinc carbenoids to rationalize the mechanism of the Clemmensen Reduction.
Note: Subjection of the corresponding alcohols to these same reaction conditions does not lead to the formation of alkanes.
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