2002 AIPMT Aldehydes, Ketones and Carboxylic AcidsPreparation of Ketones from Nitriles Medium Question 85 of 91 in Aldehydes, Ketones and Carboxylic Acids

The reaction of m-methoxybenzonitrile (m-CH₃OC₆H₄CN) with CH₃MgBr followed by H₃O⁺ gives product P :

The reaction of m-methoxybenzonitrile ($m-CH_3OC_6H_4CN$) with $CH_3MgBr$ followed by $H_3O^+$ gives product P :
A -
B -
C -
D -

The answer is B.

Solution

A Grignard reagent adds once to the C$\equiv$N group of a nitrile, forming an imine magnesium salt:
$ArC\equiv N + CH_3MgBr \rightarrow Ar-C(CH_3)=N-MgBr$
Acid hydrolysis converts this imine salt into a ketone:
$Ar-C(CH_3)=NMgBr \xrightarrow{H_3O^+} Ar-CO-CH_3 + NH_3 + Mg(OH)Br$
Here Ar is the m-methoxyphenyl group, so P is m-methoxyacetophenone, $m-CH_3OC_6H_4COCH_3$.
(The ketone is not attacked further because it is formed only during the hydrolysis, after the Grignard reagent has been destroyed.)

Concept Grignard reagent with nitrile gives ketone

Aldehydes, Ketones and Carboxylic Acids in past papers

91 questions from this chapter have appeared across 27 exam years.