The following reaction is known as

The correct answer is Option (2) → Rosenmund reduction reaction

In the Rosenmund reduction an acyl chloride is hydrogenated over palladium on barium sulphate. The barium sulphate (often with a little sulphur or quinoline) partly poisons the palladium, so the reduction stops at the aldehyde instead of going on to the alcohol. Here benzoyl chloride gives benzaldehyde:

    \( \displaystyle \ce{C6H5COCl + H2} \xrightarrow{\ce{Pd-BaSO4}} \ce{C6H5CHO + HCl} \)

So the reaction is the Rosenmund reduction.

Why the other options are incorrect

• Option (1) Wolff-Kishner reduction — Incorrect. It reduces the C=O group of an aldehyde or ketone to \( \ce{CH2} \) through the hydrazone, heated with KOH in ethylene glycol: \( \ce{R2C=O} \xrightarrow[\ce{KOH}/\text{ethylene glycol, heat}]{\ce{NH2NH2}} \ce{R2CH2 + N2} \). It starts from a carbonyl compound, not an acyl chloride.

• Option (3) Birch reduction reaction — Incorrect. It reduces an aromatic ring with sodium in liquid ammonia and an alcohol, giving a cyclohexa-1,4-diene; the ring, not an acyl chloride, is reduced:

      \( \displaystyle \ce{C6H6} \xrightarrow{\ce{Na},\ \text{liq. }\ce{NH3},\ \ce{ROH}} \text{cyclohexa-1,4-diene}\ (\ce{C6H8}) \)

• Option (4) Hell-Volhard-Zelinsky reaction — Incorrect. It is the α-halogenation of a carboxylic acid with chlorine or bromine and red phosphorus: \( \ce{RCH2COOH} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{X2}/\text{red P}} \ce{RCH(X)COOH} \). It is not a reduction.

NCERT (2026-27 edition):
From Acyl Chloride (Acid Chloride)

Note: The same/similar question has been asked in different years by NTA as per the details given below: