The least reactive aldehyde towards HCN would be

The correct answer is Option (4) → 2,2-Dimethylpropanal

HCN adds to aldehydes by nucleophilic addition: the \( \ce{CN^{-}} \) ion attacks the carbonyl carbon, and the cyanohydrin is formed.

    \( \ce{RCHO + HCN} \xrightarrow{\ce{OH^-}} \ce{RCH(OH)CN} \)

The reactivity falls as the group R becomes larger, for two reasons:

• Steric: a bulky group hinders the approach of the nucleophile to the carbonyl carbon.

• Electronic: alkyl groups release electrons (+I effect) and reduce the positive charge (electrophilicity) of the carbonyl carbon.

For the given aldehydes, R = H, \( \ce{CH3} \), \( \ce{CH2CH3} \) and \( \ce{C(CH3)3} \):

    HCHO > \( \ce{CH3CHO} \) > \( \ce{CH3CH2CHO} \) > \( \ce{(CH3)3CCHO} \)

The tert-butyl group of 2,2-dimethylpropanal is both the bulkiest and the most electron-releasing, so 2,2-dimethylpropanal is the least reactive.

Why the other options are incorrect

• Option (1) Methanal — Incorrect. It has no alkyl group at all (two H atoms on the carbonyl carbon), so it is the most reactive aldehyde.

• Option (2) Ethanal — Incorrect. One small methyl group; it is more reactive than propanal and 2,2-dimethylpropanal.

• Option (3) Propanal — Incorrect. The ethyl group is larger than methyl but far smaller than tert-butyl, so propanal still reacts faster than 2,2-dimethylpropanal.

NCERT (2026-27 edition):
Reactivity – “Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.”