Question

Reaction by which benzaldehyde can not be prepared is

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is D

Reaction by which Benzaldehyde cannot be prepared 

Benzaldehyde can be prepared by several reactions, such as:

• Partial oxidation of toluene
• Hydrolysis of benzal chloride (\(\text{C}_6\text{H}_5\text{CHCl}_2\))
• Gattermann–Koch reaction

However, Stephen’s reductiononly in the aliphatic series. It is not suitable for aromatic nitriles like benzonitrile because the reaction does not stop at the aldehyde stage—it tends to proceed further or fails due to ring stabilization.

Correct answer: Option (D) — the one involving Stephen’s reduction of benzonitrile.

Answer 2

The reaction by which benzaldehyde cannot be prepared is Option D.

Here's a breakdown of each reaction:

• Option A (Stephen Reduction): This is the Stephen reduction, where a nitrile (benzonitrile in this case) is reduced to an imine with SnCl₂ and HCl, followed by hydrolysis to form benzaldehyde. This is a valid method for preparing benzaldehyde.
• Option B (Rosenmund Reduction): This is the Rosenmund reduction, where an acyl chloride (benzoyl chloride in this case) is reduced to an aldehyde using H₂ gas over a palladium catalyst poisoned with barium sulfate (Pd/BaSO₄). This is a standard method to prepare benzaldehyde.
• Option C (Friedel-Crafts Acylation followed by reduction): This reaction sequence involves Friedel-Crafts acylation of benzene with an acyl chloride (acetyl chloride). This reaction will add an acetyl group to the benzene ring. While it's followed by Clemmensen reduction, that simply reduces the acetyl group to an ethyl group. So, you get ethylbenzene, **not** benzaldehyde. Therefore, benzaldehyde cannot be prepared by this reaction.
• Option D (Oxidation of Toluene): This is the Etard reaction using chromyl chloride. Toluene reacts with chromyl chloride (CrO₂Cl₂) to form a complex, which upon hydrolysis yields benzaldehyde.

Therefore, option D is the correct answer as it doesn't yield benzaldehyde as the product.

Correct Answer: Option D