Clemmensen Reduction

Clemmensen Reduction is a chemical reaction that reduces aldehydes or ketones to alkanes. It uses hydrochloric acid and zinc amalgam for its reduction. Clemmensen Reduction Reaction has many uses in organic synthesis which is helpful to produce organic compounds.

In this article, we will discuss Clemmensen Reduction and its mechanism along with examples and applications.

What is Clemmensen Reduction?

Reduction means a chemical reaction that results in a decrease in the oxidation state of the compound. In a reduction reaction, an atom in the element gains an electron and becomes negatively charged. One such reduction is the Clemmensen Reduction. In Clemmensen reduction, a carbonyl group is deoxygenated to an alkyl group by the action of hydrochloric acid (HCL) and zinc amalgam (Zn-Hg) to produce hydrocarbon. It is also used to reduce carbonyl groups to methyl groups, and alkyl groups and to produce natural products and other complex organic compounds. This reaction is named after a Danish-American chemist, Erik Christian Clemmensen,

Clemmensen Reduction Reaction

The Clemmensen reduction reaction is a reaction in which the deoxygenation of carbonyl groups (mainly aldehydes or ketones) takes place. This reaction is beneficial for reducing aryl-alkyl ketones. These aryl-alkyl ketones are basically formed via Friedel Crafts acylation. This reaction is useful in the reduction of acyl benzene to alkyl benzene and in the presence of zinc metals.

Clemmensen Reduction Reaction

In this reaction, a carbonyl group reacts with zinc amalgam in the presence of concentrated hydrochloric acid to form an alkyl group.

Example of Clemmensen Reduction

A few examples of Clemmensen reduction reaction are as follows:

• Ketone cyclohexanone (C₆H₁₀O) is reduced to cyclohexane (C₆H₁₂) after the Clemmensen reduction reaction.

• When acetophenone (C₆H₅COCH₃) reacts with Zn-Hg and HCl, ethylbenzene (C₆H₅CH₂CH₃) is formed.

• If benzaldehyde (C₆H₅COCH) goes under the Clemmensen reduction reaction, methylbenzene also called toluene (C₆H₆CH₃) is formed.

• When aldehyde formaldehyde (HCHO) reacts with Zn-Hg and HCl, it is converted to methanol (CH₃OH) is formed.

• By Clemmensen reduction, ketone acetone (CH₃COCH₃) is reduced to propane (CH₃CH₂CH₃).

Mechanism of Clemmensen Reduction Reaction

According to the Clemmensen reduction reaction mechanism, a hydrocarbon is formed when the carbonyl group (like aldehydes or ketones) is reduced to alkanes using hydrochloric acid (HCL) and zinc amalgam (Zn-Hg). It can be explained in a better way by the following steps:

• First, the reactants (here carbonyl group) reacts with HCL and Zn-Hg.
• As a result of above reaction, the carbonyl oxygen is protonated forming a positively charged carbon (called carbocation) making it more vulnerable to be attacked.
• Next, the zinc from zinc amalgam donates its electrons to the formed carbocation. Here deoxygenation of carbonyl group takes place.
• Lastly, the proton is removed which results in the formation of the required alkyl group.

However, there is no other complete explanation of this reaction but the following two suggestions have been used for its understanding:

Carbanionic Mechanism

According to this mechanism, zinc attacks directly on the protonated carbon called carbocation.

Carbenoid Mechanism

In the Carbenoid mechanism, the reduction occurs on the zinc catalyst’s surface. This is a radical reaction in which alcohols are not postulated as intermediates and the substrate should be a stable strong acid.

Application of Clemmensen Reduction

The application of Clemmensen Reduction are listed below:

• Clemmensen Reduction is used in transformation of acyl benzene to alkyl benzene.
• It is used to reduce aryl-alkyl ketones which are formed via Friedel Crafts acylation.
• Clemmensen Reduction is used to convert the carbonyl group into a methyl group.
• It is useful in the production of unbranched side hydrocarbon chain aromatics and polycyclic aromatics.
• Clemmensen Reduction is also useful in the reduction of aliphatic and aliphatic-aromatic mixed carbonyl compounds.

Clemmensen Reduction – FAQs

1. What is Simillarity between Clemmensen Reduction and Wolf-Kishner Reduction?

Clemmensen Reduction and Wolf Kishner Reduction both reactions are effective way to reduce a carbonyl group to its respective alkanes.

2. What is Difference between Clemmensen Reduction and Wolf-Kishner Reduction?

Clemmensen Reduction and Wolf-Kishner Reduction both use different reagents and the reaction conditions are also different in both the cases.

3. What is an Example of Clemmensen’s Reduction Reaction?

When Benzaldehyde (C₆H₅COCH) goes under clemmensen reduction reaction, methylbenzene also called toluene (C₆H₆CH₃) is formed.

4. What is Name of Catalyst used in Clemmensen Reduction?

Zinc Amalgam (Zn-Hg) along with concentrated hydrochloric acid (HCL) is used as a catalyst in clemmensens reduction.

5. What are Uses of Clemmensen Reduction Reaction?

It is widely used to reduce aryl-alkyl ketones which are formed via Friedel Crafts acylation. It is also used in conversion of acyl benzene to alkyl benzene.

6. Does Clemmensen Reduction Reaction reduce Alcohol?

No, Clemmensen reaction does not reduce alcohol as there is no flow of electrons from zinc to carbonyl group.

7. What is End Product of Clemmensen Reduction Reaction?

Alkane is the product of Clemmensen reduction reaction.

8. Does Clemmensen reduce Ketone?

Yes, Clemmensen reaction is used to reduce Ketone.