Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine. Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.
Which of the following cannot be prepared by Gabriel phthalimide synthesis?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Aniline
The passage states that Gabriel synthesis gives primary amines, and that aromatic primary amines cannot be prepared by it because aryl halides do not undergo nucleophilic substitution with the phthalimide anion. The alkyl group of the amine always comes from the halide R–X:
\( \displaystyle \ce{C6H4(CO)2N^-K^+ + R-X} \xrightarrow{\Delta} \ce{C6H4(CO)2N-R + KX} \)
\( \displaystyle \ce{C6H4(CO)2N-R + 2NaOH -> C6H4(COONa)2 + R-NH2} \)
Option (1) Methylamine — Can be prepared by Gabriel synthesis. A primary aliphatic amine, from methyl halide.
Option (2) Ethylamine — Can be prepared by Gabriel synthesis. A primary aliphatic amine, from ethyl halide.
Option (3) Propylamine — Can be prepared by Gabriel synthesis. A primary aliphatic amine, from propyl halide.
Option (4) Aniline — Cannot be prepared by Gabriel synthesis. An aromatic primary amine: it would need an aryl halide such as chlorobenzene, which does not react with the phthalimide anion.