Read the following passage and answer the next five questions based on it.
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two large groups in ketones hinders the attack of nucleophile to carbonyl carbon than in aldehydes. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in the former.
Which among the following compound is formed, when aldehyde reacts with HCN in presence of base ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cyanohydrin
Aldehydes and ketones undergo nucleophilic addition across the C=O group. With HCN, the base generates the strong nucleophile \( \ce{CN-} \), which attacks the carbonyl carbon; the alkoxide formed then takes a proton. The product, carrying –OH and –CN on the same carbon, is a cyanohydrin:
\( \displaystyle \ce{HCN + OH^- <=> CN^- + H2O} \)
\( \displaystyle \ce{R-CHO + HCN} \xrightarrow{\ce{OH^-}} \ce{R-CH(OH)CN} \)
Why the other options are incorrect
Option (1) Cyanide — Incorrect. The –CN group is added, but the product also contains –OH; it is not a simple cyanide.
Option (2) Isocyanide — Incorrect. Isocyanides (R–NC) are formed in the carbylamine reaction of primary amines, not from aldehydes.
NCERT (2026-27 edition):
Addition of Hydrogen Cyanide (HCN)
