Question

Gabriel phthalimide synthesis of amines produces

• A. tertiary amines.
• B. secondary amines.
• C. primary amines.
• D. aromatic secondary amines.

Hint:

Gabriel synthesis highlights:
- Exclusively produces: Pure primary aliphatic amines (\(1^\circ\)).
- Cannot prepare: Aniline or aromatic amines (aryl halides do not undergo \(\text{S}_\text{N}2\)).

Answer 1

The Correct Option is C

Concept:
The Gabriel phthalimide synthesis is a selective laboratory method used for preparing pure primary aliphatic amines without contamination by secondary or tertiary amines.

Step 1: Generation of the Phthalimide Anion:
Phthalimide is treated with ethanolic potassium hydroxide (\(\text{KOH}\)).
The acidic imide hydrogen between the two carbonyl groups is deprotonated, yielding potassium phthalimide, which serves as a nucleophile:
\[ \text{C}_6\text{H}_4(\text{CO})_2\text{NH} + \text{KOH} \rightarrow \text{C}_6\text{H}_4(\text{CO})_2\text{N}^-\text{K}^+ + \text{H}_2\text{O} \]

Step 2: Nucleophilic Alkylation via \(\text{S}_\text{N}2\) Pathway:
Potassium phthalimide is heated with a primary alkyl halide (\(\text{R}-\text{X}\)).
The phthalimide anion attacks the alkyl halide via an \(\text{S}_\text{N}2\) mechanism, displacing the halide ion to produce N-alkylphthalimide:
\[ \text{C}_6\text{H}_4(\text{CO})_2\text{N}^-\text{K}^+ + \text{R}-\text{X} \rightarrow \text{C}_6\text{H}_4(\text{CO})_2\text{N}-\text{R} + \text{KX} \] Because N-alkylphthalimide lacks acidic protons on the nitrogen, further alkylation cannot take place, preventing the formation of secondary or tertiary amines.

Step 3: Hydrolysis or Hydrazinolysis to Free Primary Amine:
Alkaline hydrolysis of N-alkylphthalimide with aqueous sodium hydroxide yields phthalate salt and liberates the pure primary aliphatic amine:
\[ \text{C}_6\text{H}_4(\text{CO})_2\text{N}-\text{R} + 2\text{NaOH} \rightarrow \text{C}_6\text{H}_4(\text{COONa})_2 + \text{R}-\text{NH}_2 \] Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the phthalimide anion under standard conditions.
Final Answer:
Gabriel phthalimide synthesis selectively produces primary amines, corresponding to option (C).