Question

The product P in the reaction,

• A.
• B.
• C.
• D.

Hint:

When nitriles are reacted with Grignard reagents, they undergo a nucleophilic attack, resulting in the formation of ketones after hydrolysis. This is a common method for the formation of ketones from nitriles.

Answer 1

The Correct Option is D

Grignard Reaction with Nitriles: Ketone Formation

This transformation involves the nucleophilic addition of a Grignard reagent to a nitrile group, followed by acidic hydrolysis to yield a ketone.

1. Nucleophilic Addition of Grignard Reagent

R-C≡N + CH₃MgBr → R-C(-N=MgBr)(CH₃)

• The nucleophilic methyl group from CH₃MgBr attacks the electrophilic carbon of the nitrile triple bond
• The magnesium bromide coordinates with the nitrogen lone pair
• Forms an imine anion intermediate (ketimine salt)

2. Acidic Hydrolysis

R-C(-N=MgBr)(CH₃) + H₃O⁺ → R-CO-CH₃ + NH₄⁺ + MgBrOH

• Protonation of the nitrogen initiates hydrolysis
• Water adds to the imine carbon, forming an unstable amino alcohol
• Elimination of ammonia yields the final ketone product

Important Characteristics:

• Nitrile Reactivity: The C≡N triple bond is highly polarized, making the carbon electrophilic
• Stoichiometry: 1:1 ratio of Grignard reagent to nitrile is required
• Product Structure: The new ketone has one additional carbon (from the Grignard) compared to the starting nitrile
• Limitation: Cannot be used to prepare ketones with tertiary alkyl groups adjacent to carbonyl

Practical Considerations:

1. Reaction must be performed under anhydrous conditions initially
2. Hydrolysis is typically done with dilute acid at 0°C to room temperature
3. Common solvents: dry ether or THF

Final Product: The reaction converts the nitrile (R-C≡N) to a methyl ketone (R-CO-CH₃)

Therefore, the correct answer is D (a ketone with structure R-CO-CH₃).

Answer 2

Think of the nitrile as a "carbon delivery system":

1. The triple-bonded nitrogen acts like a handle that makes the carbon extremely electrophilic
2. The Grignard reagent (our nucleophile) latches onto this carbon
3. Acidic workup removes the nitrogen handle, leaving behind the new carbonyl group

Key Structural Features

1. The Nitrile Group

The -C≡N group is linear and highly polarized, with partial positive charge on carbon. This makes it:

• Perfect for nucleophilic attack
• More reactive than typical carbonyl groups

2. The Grignard Reagent

CH₃MgBr provides:

• A nucleophilic methyl group (CH₃^-)
• One new carbon atom to the final product

Our Specific Reaction

Starting with 3-methoxybenzonitrile:

3-Methoxybenzonitrile

+ CH₃MgBr →

3-Methoxyacetophenone

The methoxy group (-OCH₃) remains untouched because:

• It's not electrophilic enough for Grignard attack
• It's stable under both basic and acidic conditions used

Key Takeaway: Grignard + nitrile → ketone, every time.

Therefore, the correct product is D (3-methoxyacetophenone).