Question

Identify the product formed when benzoyl chloride is reduced by hydrogen using palladium catalyst poisoned with barium sulphate?

• A. Chlorobenzene
• B. Benzyl alcohol
• C. Benzene
• D. Benzaldehyde

Hint:

Whenever you see an acid chloride (R-COCl) reacting with $\text{H}_2$ alongside $\text{Pd/BaSO}_4$ (or sulfur/quinoline), instantly think of the Rosenmund Reduction!
The product of a Rosenmund reaction is always an aldehyde (R-CHO). Simply swap out the $-\text{Cl}$ atom for a $-\text{H}$ atom.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the organic product obtained when an acyl chloride, specifically benzoyl chloride, undergoes catalytic hydrogenation using a palladium catalyst that has been deliberately poisoned with barium sulphate ($\text{BaSO}_4$).

Step 2: Key Formula or Approach:
This specific reaction condition—hydrogen gas ($\text{H}_2$) in the presence of palladium on a barium sulphate support ($\text{Pd/BaSO}_4$)—is universally known as the Rosenmund Reduction.
The role of the $\text{BaSO}_4$ "poison" is to reduce the catalytic activity of the palladium, stopping the reduction at the aldehyde stage and preventing further reduction to a primary alcohol.

Step 3: Detailed Explanation:
Benzoyl chloride has the molecular structure $\text{C}_6\text{H}_5\text{COCl}$.
During the Rosenmund reduction, the acyl carbon-chlorine (C-Cl) bond undergoes hydrogenolysis. A hydrogen atom replaces the chlorine atom:
$$\text{C}_6\text{H}_5\text{COCl} + \text{H}_2 \xrightarrow{\text{Pd/BaSO}_4} \text{C}_6\text{H}_5\text{CHO} + \text{HCl}$$
The functional group transitions from an acid chloride directly into an aldehyde, yielding benzaldehyde as the primary organic product.
If pure palladium catalyst without $\text{BaSO}_4$ were used, the highly reactive benzaldehyde would immediately pick up more hydrogen to form benzyl alcohol ($\text{C}_6\text{H}_5\text{CH}_2\text{OH}$). Thus, the barium sulphate poison is critical to isolate the aldehyde.

Step 4: Final Answer:
The product formed is benzaldehyde, which perfectly corresponds to option (D).