Clemmensen Reduction: Mechanism and Applications

The Clemmensen reduction is a reduction of aldehyde and ketones into the saturated hydrocarbon. This reaction takes place in the presence of zinc amalgam and concentrated hydrochloric acid. Hence, the substrate must be stable to strong acid. This is named after Danish chemist Erik Christian Clemmensen. It was first reported in 1913. It is compatible with a wolf-kishner reduction which uses basic conditions.

Introduction of Clemmensen reduction

“The reduction of carbonyl groups (aldehyde or ketone) into the methylene group by using zing amalgam and hydrochloric acid is known as Clemmensen reduction.”

An image for clemmenson reaction - general

This reduction is particularly effective in reducing aryl-alkyl ketones. It is mostly used for the conversion of acyl benzene into the alkyl benzene. However, it can also reduce other aldehyde and ketones that are not sensitive to acid.

Mechanism of Clemmensen reduction

The mechanism of the Clemmensen reduction is not fully understood. This is because some complex reactions may happen on the surface of zinc catalysts. This reduction is not involved in the formation of alcohol.

Carbanoinic MECHANISM

This mechanism involves the protonation of the carbonyl oxygen. Hence, Zn attacks the partially positive charged carbonyl carbon.

Carbenoid mechanism

It is a radical process that involves the reduction on the surface of Zn metal.

Examples of Clemmensen reduction

Example 1

Example 2

Example 3

Application of Clemmensen reduction

  • This reduction is widely used in the conversion of the carbonyl group into the methylene group.
  • This reaction is widely used in the preparation of polycyclic aromatics.
  • It produces aromatic compounds containing unbranched side hydrocarbon chains.
  • It involves the reduction of aliphatic, as well as mixed aliphatic-aromatic carbonyl compounds.

Concepts Berg

Why Would You Prefer The Wolff-Kishner Over the Clemmensen, Or Vice Versa?

The Clemmensen reduction is made under acidic conditions. Whereas the wolf-kishner reduction uses basic conditions. Therefore, the preference to choose between Clemmensen or Wolf-kishner reduction depends on the nature of the reactant.

What is Clemmensen reduction with an example?

The reduction of aldehyde and ketone into hydrocarbons is known as Clemmensen reduction. For example, the conversion of acetaldehyde into ethane.

Which reagent is used in Clemmensen reduction?

The amalgamated Zn with hydrochloric acid is used as reagents.

Does Zn Hg HCl reduce alcohol?

The amalgamated zinc and hydrochloric acid cannot be used to reduce alcohol. This is because in the presence of HCl alcohols are dehydrated.

In Clemmensen reduction, can zinc be replaced by sodium?

The zinc cannot be replaced with sodium metal. This is because sodium reacts with HCl violently. It results in the formation of hydrogen gas which causes an explosion.

Does rearrangement occur in Clemmensen reduction?

Leonard and Ruyle studied that alpha-aminoketones undergo rearrangement during Clemmensen reduction.

Why doesn’t carboxylic acid undergo Clemmensen reduction?

The carboxylic acid doesn’t undergo Clemmensen reduction. This is because the carbonyl carbon of carboxylic acid does not contain a partial positive charge. In this way, Zn electrons cannot transfer to the carbonyl carbon.

Reference Books

  • Textbook of Organic Chemistry by Sana Ullah

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