The main step in the Gabriel phthalimide synthesis is a reaction in which the nucleophile, phthalimide anion displaces the halide ion from alkyl halides to form N-alkyl phthalimide which by subsequent acid or alkaline hydrolysis gives the corresponding aliphatic primary amine.
However, in aryl halides, nucleophilic substitution reaction does not occur easily due to the resonance in Ar—X. That is why, aromatic primary amines cannot be prepared by Gabriel phthalimide reaction.