Question

Which from following reactions converts carbonyl group of aldehydes and ketones to methylene group on treatment with zinc amalgam and concentrated HCl ?

• A. Wolf-Kishner reaction
• B. Stephen reduction
• C. Clemmensen reduction
• D. Rosenmund reduction

Hint:

To avoid confusing the two alkane-forming reductions:
Clemmensen uses Chloric acid (HCl) — Acidic medium.
Wolff-Kishner uses a Base (KOH) — Basic medium.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We must identify the specific named chemical reaction that completely reduces a carbonyl group (>C=O) to a methylene group ($\text{>CH}_2$) using the exact reagents: zinc amalgam (Zn-Hg) and concentrated hydrochloric acid (HCl).

Step 2: Key Formula or Approach:
There are two primary methods to strip the oxygen from an aldehyde or ketone and replace it with two hydrogens:
1. Using an acidic medium ($\text{Zn-Hg} / \text{conc. HCl}$)
2. Using a highly basic medium (Hydrazine $\text{NH}_2\text{NH}_2 / \text{KOH}$ in ethylene glycol)

Step 3: Detailed Explanation:
Let's analyze the named reactions in the options:
(A)

Wolff-Kishner reduction: This reaction achieves the exact same conversion (>C=O to $\text{>CH}_2$), but it uses hydrazine and a strong base (like KOH). It is ideal for acid-sensitive molecules.
(B)

Stephen reduction: This is the partial reduction of a nitrile ($\text{-C}\equiv\text{N}$) to an aldehyde using stannous chloride ($\text{SnCl}_2$) and HCl.
(C)

Clemmensen reduction: This is the specific reduction of aldehydes and ketones to alkanes using zinc amalgam and concentrated hydrochloric acid. It is ideal for base-sensitive molecules.
(D)

Rosenmund reduction: This is the partial reduction of an acid chloride (R-COCl) to an aldehyde using Hydrogen gas and a poisoned Palladium catalyst ($\text{Pd/BaSO}_4$).

Step 4: Final Answer:
The reaction utilizing zinc amalgam and conc. HCl is the Clemmensen reduction, which matches option (C).