Give reason: aromatic primary amines cannot be prepared by Gabriel’s phthalimide synthesis.

The Gabriel's phthalimide synthesis is a method used to prepare primary amines from alkyl halides (alkyl halides have a leaving group attached to a carbon atom). While this method is effective for the synthesis of aliphatic primary amines, it cannot be used to prepare aromatic primary amines. The reason for this limitation lies in the mechanism of the reaction and the stability of intermediates involved.

In the Gabriel's phthalimide synthesis, the first step involves the reaction of an alkyl halide with potassium phthalimide, forming an N-alkyl phthalimide intermediate. The second step involves the removal of the alkyl group from the N-alkyl phthalimide to obtain the primary amine.

The crucial step in this reaction is the cleavage of the N-alkyl phthalimide to yield the primary amine. For aliphatic primary amines, this step proceeds smoothly because the intermediate formed is relatively stable. However, in the case of aromatic primary amines, the intermediate formed after cleavage is an aryl amine, which is highly unstable under the reaction conditions. This instability leads to side reactions, making the reaction impractical for the synthesis of aromatic primary amines.

One of the significant challenges with aryl amines is their tendency to undergo undesirable side reactions, such as aromatic nucleophilic substitution, leading to unwanted products. Additionally, the aryl amines formed may undergo further reactions, such as oxidation, that hinder the formation of the desired aromatic primary amine.

Due to these challenges and limitations in the reaction mechanism, Gabriel's phthalimide synthesis is not suitable for the preparation of aromatic primary amines, and alternative methods must be employed for their synthesis.