Clemmensen Reduction

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What is Clemmensen Reduction?

The Clemmensen reaction was first reported by Clemmensen of Park Davis in 1913. The aldehydes and ketones reacted with zinc amalgam (Zn/Hg alloy) in concentrated hydrochloric acid (HCL) in Clemmensen Reduction, resulting in aldehyde or ketone hydrocarbon formation. The Clemmensen reduction uses zinc and mercury in presence of strong acid.

The general reaction is:

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The reaction mentioned above is particularly effective in aryl-alkyl ketones reduction formed in Friedel-Crafts acylation. The reduction of Clemmensen is most widely used to transform acyl benzene (from acylation by Friedel-Crafts) to alkylbenzene but it also works well with other acid-insensitive ketones or aldehydes. The two-step sequence of Friedel-Crafts acylation is followed by Clemmensen reduction. It constitutes a classical strategy for the primary alkylation of arenes.

Clemmensen Reduction Mechanism

It allows the deoxygenation of aldehydes or ketones to form the corresponding hydrocarbon. The stratum must be stable to strong acid. The Clemmensen Reduction works well with other acid-insensitive ketones or aldehydes. With an excess of amalgamated zinc (mercury treated zinc, Zn (Hg) and concentrated hydrochloric acid (HCl), the carbonyl compound is heated. On the surface of the zinc, the reduction happens. There are two proposals for the Clemmensen Reduction Mechanism:

  • Carbanionic Mechanism: In the Carbanionic mechanism, zinc attacks directly to the protonated carbon.

  • Carbenoid Mechanism: It is a radical process and reduces the happenings on the metal surface of zinc. The reaction of the carbenoid mechanism takes place at the surface of the zinc catalyst.

The equation below follows the intermediacy of zinc carbenoids to justify the Clemmensen Reduction mechanism. The Clemmensen Reduction enables aldehydes or ketones to be deoxygenated to obtain the corresponding hydrocarbon. The substrate must be a strong acid that is stable. The reduction of the Clemmensen is complementary to the reduction of the Wolff-Kishner, which is operated under very simple conditions. To explain the Clemmensen Reduction process, the following equation employs the intermediacy of zinc carbenoids.

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Wolff-Kishner reduction reaction is similar to Clemmensen Reduction Reaction. In the Wolff-Kishner reduction reaction, carbonyl compounds are heated with hydrazine and potassium hydroxide in presence of boiling solvents like ethylene glycol or diethylene glycol to form alkanes. In this, carbonyl compounds react with hydrazine to form hydrazone. On heating under the normal conditions, these form alkanes with the evolution of Nitrogen gas (N₂). 

The Clemmensen reduction reaction and Wolff Kishner reduction reaction differ in a few conditions. In Clemmensen Reaction, the conversion of ketones or aldehydes into alkanes takes place, whereas, in the case of Wolff-Kishner Reaction, the conversion of carbonyl groups into methylene groups takes place. These conversions are processed by reducing functional groups. Both the reactions require specific reaction conditions and the catalyst for the successful progression of the reaction.

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Applications of Clemmensen Reduction

  1. Alkane from alkenyl chloride (halide) can be prepared from any organic compound which can be transformed into alkenyl halide.

  2. The reaction is widely used to convert the carbonyl group into a methyl group.

  3. Preparation of polycyclic aromatics and aromatics containing unbranched side hydrocarbon chain.

  4. The reaction helps to reduce the aliphatic and mixed aliphatic-aromatic carbonyl compounds.

  5. The reduction of Clemmensen is most widely used to transform acyl benzene (from acylation by Friedel-Crafts) to alkylbenzene.

Toluene can be Formed with the Help of Clemmensen Reduction as Explained Below:

In this, Benzaldehyde undergoes Clemmensen's Reduction in the presence of zinc amalgam and concentrated hydrochloric acid to form Toluene. 

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FAQ (Frequently Asked Questions)

Q1. Which Catalyst is Used in Chemical Reduction Reaction and Why is it Used?

The catalyst used in the chemical reduction reaction is the Zinc catalyst. In these reactions, the alcohols are not postulated as intermediates, because subjection of the alcohols to the same reaction conditions does not lead to the formation of alkanes. The reduction of the Clemmensen reaction takes place at the Zinc surface. Clemmensen Reduction is a chemical reaction that is explained by the use of zinc amalgam and concentrated hydrochloric acid (HCL) as a reduction of ketones or aldehydes in alkanes.

The zinc amalgam is used in Clemmensen Reduction as the reaction of zinc with hydrochloric acid releases hydrogen gas (H₂). In this way, zinc amalgam acts to trap the active H₂ gas as it is formed, and it also allows to attack the carbonyl compound rather than it released H₂ gas.

Q2. Does the Clemmensen Reduction Reaction Reduce Alcohol?

In general, the reduction of carbonyl groups (in aldehydes and ketones) to methylene groups with zinc amalgam and hydrochloric acid is known as the reduction reaction of Clemmensen. It has been concluded that alcohol is not the intermediate of the Clemmensen reduction and carbonium is involved in this Clemmensen reduction. Benzylic and allylic alcohols can be reduced under the Clemmensen reduction conditions. In particular, the Clemmensen reduction conditions are reactive to the reduction of aryl-alkyl ketones, such as those produced in the Friedel-Crafts acylation reaction, and these alcohols are firstly converted into the respective benzyl and allylic chlorides reacted by the HCL and then react with the zinc amalgam.

Q3. Does Clemmensen Reduction Reduce Double Bonds?

The double bond is referred to as a chemical bond in which the two pairs of electrons are shared between the two atoms to form a bond among the elements. The double bond is formed when two atoms share two pairs of electrons. The Clemmensen Reduction includes the addition of Zn(Hg) which is dissolved in heated hydrochloric acid (HCl) to reducible form. Note that this process can accidentally chlorinate a double bond which is also present on the reactant side of the Clemmensen reduction reaction.