Question

Benzoyl chloride on reduction with ${{\text{H}}_{\text{2}}}\text{/Pd-BaS}{{\text{O}}_{\text{4}}}$ produces _________________.
(A) Benzoic Acid
(B) Benzyl alcohol
(C) Benzoyl sulphate
(D) Benzaldehyde

Answer 1

Hint: The reaction given in this question is the Rosenmund Reduction. The Rosenmund reduction is a hydrogenation process in which an acyl chloride is reduced to an aldehyde in the presence of hydrogen on palladium deposited over Barium Sulphate.

Complete Stepwise Solution:
When Benzoyl chloride which is an acyl chloride is treated with hydrogen gas in presence of palladium coated barium sulphate, it leads to the formation of Benzaldehyde. Barium sulphate is used here because it has low surface area which reduces the however, there are certain acyl chlorides for which the reactivity must be further reduced by the addition of a catalyst poison and originally ‘thioquinanthrene’ was used for this purpose but now thiourea is used. Deactivation is required because the system should reduce the acyl chloride but not the subsequent aldehyde that is formed after the reaction. If the reaction continues to occur then the aldehyde will be reduced to primary alcohol that reacts with the remaining acyl chloride to form an ester.
Hence, the correct answer to this question is option D.

Note:
There are several other reducing agents that are used for the purpose of reduction in organic calcium carbonate or barium sulphate with an added catalyst poison and is called the Lindlar’s catalyst. This is used for the conversion of alkynes to alkenes without them being converted to alkanes due to complete hydrogenation. Another reducing agent, DIBAL or Diisobutylaluminium hydride is also used for the reduction of acyl chlorides to aldehydes.