Assertion (A): Benzaldehyde and formaldehyde undergo the Cannizzaro reaction.
Reason (R): Benzaldehyde is more reactive than propanal in nucleophilic addition reactions.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
The Cannizzaro reaction is shown by aldehydes that have no α-hydrogen atom.
Assertion (A): Benzaldehyde and formaldehyde undergo the Cannizzaro reaction. — True. In \( \ce{HCHO} \) the carbonyl carbon carries only H atoms, and in \( \ce{C6H5CHO} \) it is joined to a ring carbon that has no hydrogen. Neither has an α-hydrogen, so both undergo the reaction:
\( \displaystyle \ce{2HCHO + NaOH} \xrightarrow[\Delta]{\text{conc.}} \ce{CH3OH + HCOONa} \)
\( \displaystyle \ce{2C6H5CHO + NaOH} \xrightarrow[\Delta]{\text{conc.}} \ce{C6H5CH2OH + C6H5COONa} \)
Reason (R): Benzaldehyde is more reactive than propanal in nucleophilic addition reactions. — False. In benzaldehyde the C=O group is in resonance with the benzene ring, which reduces the polarity of the carbonyl group. Its carbonyl carbon is less electrophilic than that of propanal, so benzaldehyde is less reactive than propanal.
NCERT (2026-27 edition):
8.4 Chemical Reactions: 5. Other reactions – “(i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”
8.4 Chemical Reactions: (ii) Reactivity – “The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance … and hence it is less reactive than propanal.”