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.
Gabriel phthalimide synthesis is used for the preparation of which type of amines?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Primary aliphatic amines
The passage states that Gabriel synthesis is used for the preparation of primary amines, and that aromatic primary amines cannot be prepared by it. So it gives primary aliphatic amines only.
The alkyl group comes from the alkyl halide: the phthalimide anion replaces the halogen, and alkaline hydrolysis of the N-alkylphthalimide sets the amine R–NH2 free (R = alkyl group):
\( \displaystyle \ce{C6H4(CO)2N^-K^+ + R-X} \xrightarrow{\Delta} \ce{C6H4(CO)2N-R + KX} \)
\( \displaystyle \ce{C6H4(CO)2N-R + 2NaOH -> C6H4(COONa)2 + R-NH2} \)
Why the other options are incorrect
Option (1) Primary aromatic amines — Incorrect. Aryl halides do not undergo nucleophilic substitution with the phthalimide anion, so no aryl group can be put on the nitrogen.
Option (2) Secondary amines — Incorrect. Phthalimide has only one N–H, so only one alkyl group reaches the nitrogen; the amine set free carries one alkyl group.
Option (4) Tertiary amines — Incorrect. For the same reason, three alkyl groups can never be put on the nitrogen.