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Clemmensen Reduction

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The deoxygenation or reduction of carbonyl groups of aldehydes and ketones to methylene groups with a large excess of amalgamated zinc and hydrochloric acid is known as Clemmensen reduction.

Clemmensen Reduction

Clemmensen reaction is carried out by repluxing the carbonyl compounds with amalgamated zinc with concentrated hydrochloric acid. This reduction is especially useful for ketones having phenolic or carboxylic groups which remains unaffected. Ketones are reduced more often than aldehydes. Such reduction is also achieved by Wolff-Kishner reduction but Clemmensen reduction is easier to carry out. This reduction cannot be performed with acid sensitive and high molecular weight substrates. In the reduction of $\alpha$, $\beta$-unsaturated ketones, both the olefinic and carbonyl groups are reduced. However, the reduction has been reported to be highly specific for carbonyl groups of aldehydes and ketones having other functional and reducible groups.

Mechanism

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Although many mechanisms have been postulated for this reduction, yet they are contradictory and thus no specific mechanism could be postulated. A mechanism had been suggested which involved the intermediate formation of alcohol. However, this was rejected because the reagent fails to reduce most alcohols to hydrocarbons.

Clemmensen Reduction Mechanism

Applications 

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The applications of Clemmensen Reduction reaction are
  • To reduce aliphatic and mixed aliphatic-aromatic carbonyl compounds
CH3-(CH2)5-CHO $\xrightarrow[HCl]{\frac{Zn}{Hg}}$ CH3-(CH2)5-CH3


C6H5COC3H5 $\xrightarrow[HCl]{Zn-Hg}$ C6H5CH2C2H3

  • Reduction of keto acids, acids with a- and b-keto acids are generally not reduced by Clemmensen reduction.
C6H5COCH2CH2COOH $\xrightarrow[HCl]{Zn-Hg}$ C6H5CH2CH2CH2COOH