What is the final product when phenylacetonitrile (C6H5CH2CN) is heated with dilute sulphuric acid?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Phenylacetic acid
On heating with dilute acid, a nitrile is hydrolysed in two stages: water adds to the C≡N group to give an amide, and the amide is then hydrolysed to the carboxylic acid and an ammonium salt. The carbon of the –CN group becomes the carbon of –COOH, so no carbon is lost or gained.
For phenylacetonitrile, the amide stage gives phenylacetamide:
\( \displaystyle \ce{C6H5CH2CN + H2O} \xrightarrow{\ce{H^+}} \ce{C6H5CH2CONH2} \)
Further heating with dilute sulphuric acid hydrolyses the amide:
\( \displaystyle \ce{2C6H5CH2CONH2 + 2H2O + H2SO4} \xrightarrow{\text{heat}} \ce{2C6H5CH2COOH + (NH4)2SO4} \)
So the final product is phenylacetic acid (2-phenylethanoic acid).
Why the other options are incorrect
Option (1) Phenyl acetate — Incorrect. \( \ce{CH3COOC6H5} \) is an ester of phenol, made by acetylating phenol. Hydrolysis of a nitrile gives an acid, not an ester.
Option (3) Methyl phenyl ether — Incorrect. Anisole, \( \ce{C6H5OCH3} \), is an ether, made by the Williamson synthesis from sodium phenoxide and a methyl halide. Hydrolysis does not form a C–O–C link; it turns –CN into –COOH.
Option (4) Benzoic acid — Incorrect. \( \ce{C6H5COOH} \) has one carbon fewer than \( \ce{C6H5CH2COOH} \). The \( \ce{-CH2-} \) of phenylacetonitrile stays in the product.
NCERT (2026-27 edition):
8.7 Methods of Preparation of Carboxylic Acids
