A compound ‘A’ on reaction with \(NaCN/HCl\) gives compound B which on hydrolysis in acidic medium gives \(\alpha\) – Hydroxyphenyl acetic acid. Identify compound A
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → benzaldehyde
Aldehydes and ketones add hydrogen cyanide across the C=O group to give cyanohydrins. NaCN with HCl supplies HCN, and the cyanide ion, a strong nucleophile, attacks the carbonyl carbon. Acid hydrolysis then converts the –CN group into –COOH, which gives an α-hydroxy acid with one carbon atom more than the carbonyl compound.
The final product, α-hydroxyphenylacetic acid, is \( \ce{C6H5CH(OH)COOH} \). Working backwards, its –COOH came from –CN, so B is \( \ce{C6H5CH(OH)CN} \), and A is the aldehyde \( \ce{C6H5CHO} \):
\( \displaystyle \ce{C6H5CHO} \xrightarrow{\ce{NaCN/HCl}} \ce{C6H5CH(OH)CN}\ (\text{B}) \)
\( \displaystyle \ce{C6H5CH(OH)CN + 2H2O + H+ -> C6H5CH(OH)COOH + NH4+} \)
So compound A is benzaldehyde.
Why the other options are incorrect
Option (1) toluene — Incorrect. It has no carbonyl group, so it cannot add HCN.
Option (3) benzyl alcohol — Incorrect. It has no carbonyl group either; it is the reduction product of benzaldehyde.
Option (4) benzoic acid — Incorrect. The C=O of a carboxylic acid is stabilised by resonance with the –OH group and does not form a cyanohydrin.
NCERT (2026-27 edition):
Addition of Hydrogen Cyanide (HCN)
