Question

Which of the following reactions is an example of Clemmensen reduction?

• A. R-CO-Cl + H$_2 \xrightarrow{Pd/BaSO_4}$ R-CHO + HCl
• B. R-CHO $\xrightarrow{(i)NH_2NH_2, (ii)KOH/Glycol}$ R-CH$_3$
• C. R-CO-OC$_2$H$_5 \xrightarrow{1. DIBAL-H, 2. H_2O}$ R-CHO + C$_2$H$_5$OH
• D. R-CO-CH$_3 \xrightarrow{Zn-Hg/HCl}$ R-CH$_2$-CH$_3$

Hint:

There are two main named reactions to reduce a carbonyl group (aldehyde/ketone) completely to an alkane (-CH$_2$-): 1. Clemmensen Reduction: Uses Zn-Hg and conc. HCl. It is performed in acidic conditions and is not suitable for acid-sensitive compounds. 2. Wolff-Kishner Reduction: Uses NH$_2$NH$_2$ and a strong base (KOH). It is performed in basic conditions and is not suitable for base-sensitive compounds.

Answer 1

The Correct Option is D

Let's identify each named reaction.
The Clemmensen reduction is a chemical reaction used to reduce an aldehyde or a ketone to an alkane. The reaction uses zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl). The carbonyl group (C=O) is completely reduced to a methylene group (-CH$_2$-).
(A) This is the Rosenmund reduction. It is the catalytic hydrogenation of an acyl chloride over a palladium catalyst poisoned with barium sulfate. It reduces the acyl chloride to an aldehyde.
(B) This is the Wolff-Kishner reduction. It also reduces an aldehyde or a ketone to an alkane. The reagents are hydrazine (NH$_2$NH$_2$) followed by a strong base like KOH or potassium tert-butoxide in a high-boiling solvent like ethylene glycol. This reaction is performed under basic conditions.
(C) This is the reduction of an ester to an aldehyde using Diisobutylaluminium hydride (DIBAL-H) at low temperatures, followed by hydrolysis.
(D) This reaction shows a ketone (R-CO-CH$_3$) being reduced to an alkane (R-CH$_2$-CH$_3$) using zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl). This is the exact definition of the Clemmensen reduction.
Therefore, option (D) is the example of Clemmensen reduction.