Propanenitrile is treated with phenylmagnesium bromide in dry ether, and the product is then hydrolysed with dilute acid. Which of the following is the final organic product?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Propiophenone (1-phenylpropan-1-one)
Step 1 – Addition of the Grignard reagent: a Grignard reagent adds to the C≡N group of a nitrile. The phenyl group goes to the carbon and the MgBr part to the nitrogen, giving an imine salt:
\( \displaystyle \ce{CH3CH2C#N + C6H5MgBr} \xrightarrow{\text{dry ether}} \ce{CH3CH2C(=NMgBr)C6H5} \)
Step 2 – Hydrolysis: dilute acid then replaces =NMgBr by =O, and the nitrogen leaves as ammonia. The product is a ketone with the ethyl group of the nitrile and the phenyl group of the Grignard reagent on the carbonyl carbon:
\( \displaystyle \ce{CH3CH2C(=NMgBr)C6H5 + 2H2O -> CH3CH2COC6H5 + NH3 + Mg(OH)Br} \)
Why the other options are incorrect
Option (1) Propanal — Incorrect.
A nitrile gives an aldehyde when it is reduced to the imine, by stannous chloride and HCl (Stephen reaction) or by DIBAL-H, and then hydrolysed:
\( \displaystyle \ce{CH3CH2C#N} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{SnCl2 + HCl}} \ce{CH3CH2CHO} \)
A Grignard reagent adds a carbon group; it does not reduce the nitrile.
Option (2) Propanoic acid — Incorrect.
Propanoic acid is formed when the nitrile itself is hydrolysed with acid or alkali, through the amide:
\( \displaystyle \ce{CH3CH2C#N} \xrightarrow{\ce{H+}/\ce{H2O}} \ce{CH3CH2CONH2} \xrightarrow{\ce{H+}/\ce{H2O}} \ce{CH3CH2COOH} \)
Here the nitrile has already reacted with the Grignard reagent, and the dilute acid only hydrolyses the imine salt.
Option (3) 1-Phenylpropan-1-ol — Incorrect. This secondary alcohol is formed when phenylmagnesium bromide adds to an aldehyde, propanal. With the nitrile, the ketone appears only during the acid work-up, when no Grignard reagent is left to add to it.
NCERT (2026-27 edition):
8.2.3 Preparation of Ketones – “2. From nitriles
Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.”
\( \displaystyle \ce{CH3-CH2-C#N + C6H5MgBr} \xrightarrow{\text{ether}} \ce{CH3CH2-C(=NMgBr)C6H5} \xrightarrow{\ce{H3O+}} \ce{C2H5-CO-C6H5} \quad \text{Propiophenone (1-Phenylpropanone)} \)