Question

Clemmensen reduction of a ketone is carried out in the presence of which of the following?

• A. $Zn-Hg$ with $HCl$
• B. $LiAlH_4$
• C. $H_2$ and $Pt$ as catalyst
• D. Glycol with $KOH$

Answer 1

The Correct Option is A

The Clemmensen reduction is a chemical reaction where a ketone is reduced to a hydrocarbon. It is a useful method for the deoxygenation of ketones to alkanes and is widely used in organic synthesis. This reaction specifically involves the reduction of carbonyl groups (\(C=O\)) in ketones and aldehydes to methylene groups (\(CH_2\)).

The Clemmensen reduction is carried out in the presence of zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl). Here is why each of the given options is considered or ruled out:

• Zn-Hg with HCl: This is the correct choice. In Clemmensen reduction, zinc amalgam acts as a reducing agent and hydrochloric acid provides an acidic medium necessary for the reaction.
• LiAlH_4: Lithium aluminium hydride is a strong reducing agent, commonly used for the reduction of carbonyl compounds. However, it does not participate in the Clemmensen reduction, as it performs reductions under different conditions (in non-acidic environments).
• H_2 and Pt as catalyst: This represents catalytic hydrogenation, another method for reducing ketones, which is not utilized in the Clemmensen reduction. It is more commonly used in the presence of a hydrogen gas and a metal catalyst.
• Glycol with KOH: This indicates a different reduction process known as the Wolff–Kishner reduction, which also converts carbonyl groups to methylene groups but under basic conditions.

Thus, the correct answer is Zn-Hg with HCl as it is the reaction conditions required for Clemmensen reduction.

Important Note: Clemmensen reduction is particularly useful for substrates that are not compatible with strong bases, which is why it is preferable over the Wolff–Kishner reduction in such cases.