Question

Which of the following can undergo disproportion reaction on heating with conc. alkali?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Aldehydes without \(\alpha\)-hydrogen undergo Cannizzaro reaction with concentrated alkali. One molecule is oxidized to carboxylate ion and another molecule is reduced to alcohol.

Answer 1

The Correct Option is A

Step 1: Identify the reaction mentioned in the question.
Disproportionation of aldehydes in the presence of concentrated alkali is known as the Cannizzaro reaction.
This reaction is shown by aldehydes that do not contain an \(\alpha\)-hydrogen atom.
\[ 2RCHO \xrightarrow{\text{conc. alkali}} RCOO^- + RCH_2OH \]

Step 2: Check benzaldehyde.
Benzaldehyde has the structure \[ C_6H_5CHO \] In benzaldehyde, the carbonyl carbon is directly attached to the benzene ring.
There is no \(\alpha\)-hydrogen present.
Therefore, benzaldehyde undergoes Cannizzaro reaction with concentrated alkali.
\[ 2C_6H_5CHO + KOH \rightarrow C_6H_5COOK + C_6H_5CH_2OH \]

Step 3: Check the other options.
Benzoic acid is already a carboxylic acid and does not undergo Cannizzaro disproportionation.
Benzyl alcohol is an alcohol, not an aldehyde. Hence, it does not undergo Cannizzaro reaction.
Phenylacetaldehyde has the structure \[ C_6H_5CH_2CHO \] It contains \(\alpha\)-hydrogen atoms and therefore does not undergo Cannizzaro reaction. Instead, aldehydes with \(\alpha\)-hydrogen generally undergo aldol type reactions.

Step 4: Final conclusion.
Hence, the compound that undergoes disproportionation on heating with concentrated alkali is \[ \boxed{\text{Benzaldehyde}} \] Therefore, the correct option is \[ \boxed{(1)} \]