Addition of HCN to carbonyl group results in formation of :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cyanohydrins
The C=O group of aldehydes and ketones undergoes nucleophilic addition. With HCN the reaction is very slow on its own, so a little base is added to generate the stronger nucleophile \( \ce{CN^{-}} \). The cyanide ion attacks the carbonyl carbon, and the alkoxide formed takes a proton from HCN:
\( \displaystyle \ce{R2C=O + HCN} \xrightarrow{\ce{OH^-}} \ce{R2C(OH)CN} \)
\( \displaystyle \ce{CH3CHO + HCN} \xrightarrow{\ce{OH^-}} \ce{CH3CH(OH)CN}\ \ (\text{ethanal cyanohydrin}) \)
The product carries an −OH and a −CN group on the same carbon, so it is a cyanohydrin, a useful synthetic intermediate (hydrolysis gives α-hydroxy acids).
Why the other options are incorrect
Option (1) Cyanides — Incorrect. Alkyl cyanides (nitriles, R–CN) are made by substituting a halide with KCN; they have no −OH group, while the addition product keeps the oxygen as −OH.
Option (2) Isocyanide — Incorrect. Isocyanides (R–NC) come from alkyl halides with AgCN or from the carbylamine reaction of primary amines, not from carbonyl compounds.
Option (4) Aldehydes — Incorrect. The aldehyde is the starting carbonyl compound; adding HCN changes it, it does not form one.
NCERT (2026-27 edition):
Addition of Hydrogen Cyanide (HCN) – “Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN−) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin. Cyanohydrins are useful synthetic intermediates.”