What NEET Asks
- Direct questions on reactants and products of Gabriel synthesis.
- Mechanism-based questions involving intermediates like N-alkylphthalimide.
- Questions testing the limitations, specifically why aromatic primary amines cannot be prepared.
- Often appears in assertion-reason or match-the-column types, carrying 4 marks each.
Key Points
- Gabriel synthesis is exclusively used for the preparation of pure primary aliphatic amines.
- It starts with phthalimide, which is treated with alcoholic KOH to form potassium phthalimide.
- Potassium phthalimide is a strong nucleophile. It reacts with alkyl halides (R-X) via an SN2 mechanism.
- The resulting N-alkylphthalimide undergoes hydrolysis (acidic or alkaline) to yield the primary amine (R-NH2) and phthalic acid/phthalate.
- Aryl halides do not undergo SN2 reaction with potassium phthalimide; hence aromatic primary amines cannot be prepared.
- The method prevents the formation of secondary or tertiary amines due to the protecting effect of the phthalimide group.
Must-Know Formula / Reaction
Step 1: Phthalimide + KOH (alcoholic) → Potassium Phthalimide Step 2: Potassium Phthalimide + R-X → N-Alkylphthalimide Step 3: N-Alkylphthalimide + H₂O/OH⁻ (hydrolysis) → R-NH₂ + Phthalic acid/phthalate
- Phthalimide: Starting material, provides the nitrogen.
- KOH (alc.): Strong base, deprotonates phthalimide to form an anion.
- R-X: Alkyl halide, source of the 'R' group for the amine, must be primary or secondary.
- H₂O/OH⁻: Hydrolyzing agent (acidic or basic).
- R-NH₂: The desired primary amine.
Common Mistakes
- Students often try to synthesize aromatic primary amines using this method. Remember aryl halides are unreactive towards SN2.
- Don't confuse the role of alcoholic KOH (deprotonation) and the final hydrolysis step (amine release).
- Incorrectly identifying secondary or tertiary amines as products – it exclusively makes primary amines.
Rapid Revision
Gabriel synthesis: Phthalimide → Potassium Phthalimide → N-Alkylphthalimide → Primary Amine. Only for aliphatic primary amines. Aryl amines not possible. Hydrolysis releases the amine and phthalic acid/salt. Highly specific and prevents polyalkylation.