Question

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

Answer 1

Hint: In Gabriel’s phthalimide synthesis, sodium or potassium salt of phthalimide is N-alkylated with a primary alkyl halide. Alkyl halides are used because they can undergo nucleophilic substitution reaction when reacted with the phthalimide anion.

Complete step by step answer:
Gabriel’s phthalimide synthesis is the reaction that is used for the preparation of primary amines. In this reaction, the potassium salt of phthalimide is used which is prepared by the reaction between phthalimide and ethanolic potassium hydroxide. Phthalimide is the imide derivative of phthalic anhydride and can be prepared by reacting phthalic anhydride with alcoholic ammonia upon heating. In this reaction, the phthalimide salt reacts with the alkyl halide and forms corresponding primary amines.
But Gabriel’s phthalimide synthesis can only be used to prepare aliphatic primary amines. Aromatic primary amines cannot be prepared by this reaction because the reaction involves $S{N_2}$ a mechanism i.e. nucleophilic substitution reaction and for the preparation of aromatic primary amines, we will need aryl halide which does not undergo nucleophilic substitution with the phthalimide anion.
The aryl halides do not undergo $S{N_2}$ mechanism due to the steric hindrance caused by the benzene ring. Also, the c-x bond in the aryl halides shows a partial double bond character due to which the bond becomes stronger and difficult to break by $S{N_2}$ the mechanism.
Thus, aromatic primary amines cannot be prepared by Gabriel’s phthalimide synthesis.

Note: Gabriel’s phthalimide synthesis cannot be used to prepare secondary and tertiary amines also because secondary and tertiary amines do not show reaction with alkyl agent due to which over alkylation is not possible or in other words, there is no space for nitrogen to add upon rather than the terminal positions