Question Details

The major product of the following reaction is

Options

Show Answer

Correct Answer :

Option B

Option 2

Solution :

Correct Answer: Option 2 (corresponding to the structure 5-hydroxy-5-phenylhexan-2-one)

To determine the major product of the reaction, let us analyze the functional groups present in the starting material and their reactivity towards the Grignard reagent under the given conditions:

1. Reactants and Functional Groups:
The starting material is 4-oxo-4-phenylbutanenitrile, which contains:
• A ketone carbonyl group ( $C_{6} H_{5} - CO -$ ) conjugated with a phenyl ring.
• A nitrile group ( $- C \equiv N$ ).
The reagents used are excess methylmagnesium bromide ( $C H_{3} MgBr$ ) followed by acidic workup ( $H_{3} O^{+}$ ).

2. Reactivity towards Grignard Reagent:
• Ketones are more electrophilic and react significantly faster with Grignard reagents than nitriles.
• Since $C H_{3} MgBr$ is used in excess, both reactive functional groups (the ketone and the nitrile) will undergo nucleophilic addition.

3. Step-by-Step Reaction Mechanism:
Step 1: Nucleophilic attack on the ketone:
The first equivalent of $C H_{3} MgBr$ attacks the carbonyl carbon of the ketone, forming a tertiary magnesium alkoxide intermediate:

$C_{6} H_{5} - CO - C H_{2} C H_{2} CN + C H_{3} MgBr \to C_{6} H_{5} - C (OMgBr) (C H_{3}) - C H_{2} C H_{2} CN$

Step 2: Nucleophilic attack on the nitrile:
Another equivalent of

C H_{3} MgBr

attacks the electrophilic carbon of the nitrile group, producing a magnesium imine salt intermediate:

$C_{6} H_{5} - C (OMgBr) (C H_{3}) - C H_{2} C H_{2} - C (C H_{3}) = NMgBr$

Note that the imine salt intermediate is stable under the reaction conditions and does not undergo further nucleophilic addition by the Grignard reagent.

Step 3: Acidic Hydrolysis:
Upon treatment with aqueous acid ( $H_{3} O^{+}$ ):
• The tertiary alkoxide is protonated to yield a tertiary alcohol:

$- C (OMgBr) (C H_{3}) \to - C (OH) (C H_{3})$