Question

Give plausible explanation for each of the following:
Aromatic primary amines cannot be prepared by Gabriel Phthalimide synthesis.

Hint:

Aromatic rings are less reactive in nucleophilic substitution reactions due to the resonance stabilization of the ring.

Answer 1

To solve the problem, we need to explain why aromatic primary amines cannot be prepared by the Gabriel Phthalimide synthesis.

1. Understanding the Gabriel Phthalimide Synthesis:
The Gabriel Phthalimide synthesis is a method used to prepare primary amines by alkylating phthalimide (a derivative of phthalic acid) with an alkyl halide in the presence of a strong base like potassium hydroxide (KOH). This results in the formation of an alkylated phthalimide, which, upon hydrolysis, yields the corresponding primary amine.

2. The Mechanism:
The Gabriel synthesis works well with alkyl halides, where the alkyl group can be easily transferred to the phthalimide anion. The reaction is typically carried out in a basic medium, which deprotonates the phthalimide, making it a good nucleophile. The alkyl halide then reacts with the phthalimide anion to form an imide intermediate, which, after hydrolysis, leads to the primary amine.

3. Issue with Aromatic Compounds:
The problem arises when trying to apply this reaction to aromatic amines (amines attached to a benzene ring). Aromatic amines are not easily alkylated using the Gabriel synthesis due to the instability of the intermediate that would form when phthalimide reacts with an aromatic alkyl halide.

4. The Challenge with Aromatic Rings:
Aromatic rings, such as benzene, have a stable π-electron cloud that resists nucleophilic attack. In addition, when trying to alkylate the phthalimide with an aromatic alkyl group (like phenyl group), the reaction is not efficient. This is due to the instability of the intermediate carbocation that would be formed in an aromatic system during the alkylation step.

5. Final Answer:
Aromatic primary amines cannot be prepared by the Gabriel Phthalimide synthesis because aromatic rings resist nucleophilic substitution reactions due to their stable electron structure, and the alkylation step does not proceed efficiently.