What is formed when acetophenone is treated with HCN?

The correct answer is Option (2) → Acetophenone cyanohydrin

Acetophenone, \( \ce{C6H5COCH3} \), is a ketone. HCN adds across its C=O bond: the cyanide ion (a strong nucleophile, generated by a base catalyst) attacks the carbonyl carbon and the oxygen takes up a proton. The product carries an –OH and a –CN group on the same carbon, a cyanohydrin:

    \( \displaystyle \ce{C6H5COCH3 + HCN} \xrightarrow{\ce{OH^-}} \ce{C6H5C(OH)(CN)CH3} \)

Why the other options are incorrect

Option (1) Acetophenone phenylhydrazone — Incorrect. A phenylhydrazone forms with phenylhydrazine, \( \ce{C6H5NHNH2} \), which adds to C=O and then loses water; HCN gives no such product.

Option (3) Methyl phenyl carbinol — Incorrect. Methyl phenyl carbinol, \( \ce{C6H5CH(OH)CH3} \), is the secondary alcohol formed by reduction of acetophenone with \( \ce{NaBH4} \) or \( \ce{LiAlH4} \), not by addition of HCN.

NCERT (2026-27 edition):
8.4 Chemical Reactions – “(a) 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.”