Step 1: Purpose of the reaction.
Rosenmund's reduction converts an acyl chloride (acid chloride) selectively to an aldehyde by partial hydrogenation. Direct reduction of a carboxylic acid to an aldehyde is not possible, so this indirect route is used.
Step 2: Reagents and conditions.
The acyl chloride is treated with hydrogen gas ($H_2$) over a palladium catalyst deposited on barium sulfate ($Pd/BaSO_4$) under controlled conditions.
Step 3: Role of $BaSO_4$.
Barium sulfate acts as a catalyst poison that reduces the activity of palladium. Without it, the aldehyde formed would be further reduced to a primary alcohol. The poisoned catalyst stops reduction at the aldehyde stage.
Step 4: Reaction.
\[ R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl \] Example: benzoyl chloride ($C_6H_5COCl$) gives benzaldehyde ($C_6H_5CHO$).