Question

The reagent used in Clemmensen’s reduction is:
(A) Conc. \[{H_2}S{O_4}\]
(B) \[Zn - Hg/conc.HCl\]
(C) \[aq.KOH\]
(D) \[\;\;\;alc.KOH\]

Answer 1

Hint: In Clemmensen reduction, reduction of ketones or aldehydes to alkanes take place. A metal-amalgam is used along with an acid to provide a surface area for reaction to occur.

Complete step-by-step answer:
The Clemmensen reduction got its name after a Danish chemist, Erik Christian Clemmensen.The Clemmensen reduction is a reaction that is performed to reduce carbonyl compounds such as aldehydes or ketones to hydrocarbons that is alkanes using concentrated hydrochloric acid and zinc amalgam.
The mercury in the zinc-amalgam is alloyed with the zinc metal and does not take part in the reaction, rather it serves only to provide a clean active metal surface. The reaction occurs with zinc but not with other metals of comparable reduction potential.
This method works best for aromatic ketones, but not so effective for non-aromatic ones. Aliphatic alcohols are not the intermediates of this reduction; however, allylic and benzylic alcohols easily undergo this reduction. Currently, there are two mechanisms proposed but they are contradictory.
In one of the mechanisms, the rate determining step involves the attack of zinc and chloride ion on the carbonyl group and the intermediates are carbanions, whereas in other heterogeneous process, the formation of a radical intermediate occurs and a zinc carbenoid species is formed.
So, the reagent used in Clemmensen’s reduction is \[Zn - Hg/conc.HCl\].

Hence, the correct option is (B).

Note: Clemmensen reduction is complementary to another type of reduction, Wolff-Kishner reduction, which also converts carbonyl compounds such as aldehydes and ketones to hydrocarbons but in basic conditions, while the former takes place in acidic condition.