Question

Which compound undergoes Cannizzaro reaction?

• A. \( \mathrm{CH_3CHO} \)
• B. \( \mathrm{C_6H_5CHO} \)
• C. \( \mathrm{CH_3COCH_3} \)
• D. \( \mathrm{CH_3CH_2OH} \)

Hint:

Cannizzaro reaction occurs only in aldehydes \textbf{without \( \alpha \)-hydrogen}. Examples:

• Benzaldehyde \( (\mathrm{C_6H_5CHO}) \)
• Formaldehyde \( (\mathrm{HCHO}) \)

Aldehydes with \( \alpha \)-hydrogen undergo \textbf{Aldol reaction} instead.

Answer 1

The Correct Option is B

Concept: The Cannizzaro reaction is a disproportionation reaction shown by aldehydes that do not contain an \( \alpha \)-hydrogen atom in the presence of a strong base such as \( \mathrm{NaOH} \) or \( \mathrm{KOH} \). In this reaction, one molecule of aldehyde is oxidized to a carboxylate ion, while another molecule is reduced to an alcohol. General reaction: \[ 2RCHO + OH^- \rightarrow RCOO^- + RCH_2OH \] Thus, only aldehydes without \( \alpha \)-hydrogen undergo Cannizzaro reaction.
Step 1: {Check for presence of \( \alpha \)-hydrogen in each compound.}

• \( \mathrm{CH_3CHO} \) (Acetaldehyde) contains \( \alpha \)-hydrogen.
• \( \mathrm{C_6H_5CHO} \) (Benzaldehyde) has no \( \alpha \)-hydrogen.
• \( \mathrm{CH_3COCH_3} \) is a ketone, not an aldehyde.
• \( \mathrm{CH_3CH_2OH} \) is an alcohol.

Step 2: {Identify compound suitable for Cannizzaro reaction.} Among the given compounds, only benzaldehyde lacks \( \alpha \)-hydrogen. \[ 2\,\mathrm{C_6H_5CHO} + \mathrm{NaOH} \rightarrow \mathrm{C_6H_5COONa} + \mathrm{C_6H_5CH_2OH} \] Therefore, benzaldehyde undergoes the Cannizzaro reaction. Hence, the correct answer is (B) \( \mathrm{C_6H_5CHO} \).