Question

Benzyl alcohol is obtained from benzaldehyde by the following reaction.

• A. Aldol condensation
• B. Perkin reaction
• C. Cross aldol condensation
• D. Cannizzaro reaction

Hint:

Benzaldehyde has no \( \alpha \)-hydrogen, so with strong base it disproportionates (Cannizzaro) to benzyl alcohol plus benzoate.

Answer 1

The Correct Option is D

Step 1: Identify the substrate. Benzaldehyde, \( C_6H_5CHO \), has no \( \alpha \)-hydrogen atom (the carbon next to the carbonyl is part of the benzene ring and carries no H that can be removed as in aldol chemistry).
Step 2: Aldehydes without \( \alpha \)-hydrogen, when treated with concentrated alkali (NaOH/KOH), undergo the Cannizzaro reaction, a self oxidation-reduction (disproportionation): one molecule is reduced to an alcohol and the other is oxidised to the salt of a carboxylic acid.
Step 3: For benzaldehyde:
\[ 2\,C_6H_5CHO + NaOH \rightarrow C_6H_5CH_2OH + C_6H_5COONa \]
Benzyl alcohol \( (C_6H_5CH_2OH) \) is the reduction product and sodium benzoate is the oxidation product.
Step 4: Aldol and cross aldol condensation both need an \( \alpha \)-hydrogen, which benzaldehyde lacks, and the Perkin reaction gives cinnamic acid with anhydride, not benzyl alcohol. Hence option (iv) Cannizzaro reaction is correct.
\[\boxed{\text{Cannizzaro reaction}}\]