Question
Explain the Gabriel phthalimide synthesis for the preparation of primary amines. Why can’t this method be used to prepare aromatic primary amines?
(JEE Main 2022, similar pattern)
Solution — Step by Step
Phthalimide (a cyclic imide derived from phthalic acid) is treated with KOH to form potassium phthalimide:
The N-H of phthalimide is acidic enough (due to two flanking C=O groups) to be deprotonated by KOH, giving the potassium salt with a nucleophilic nitrogen.
The potassium phthalimide reacts with a primary alkyl halide (R-X) via an S2 mechanism:
The nucleophilic N attacks the carbon bearing the halide, forming an N-C bond.
The N-alkylphthalimide is hydrolysed with aqueous NaOH (or acidic hydrolysis with HCl, or hydrazinolysis with NHNH):
The primary amine is released cleanly. No secondary or tertiary amines are formed — this is the key advantage over direct ammonolysis of alkyl halides.
Aryl halides (like chlorobenzene) cannot undergo S2 reactions because:
- The C-Cl bond in aryl halides has partial double bond character (resonance with the ring)
- The carbon is sp-hybridised, making backside attack impossible
Since step 2 requires S2 displacement, aryl halides do not react with potassium phthalimide. Aromatic amines like aniline must be prepared by other methods (e.g., reduction of nitrobenzene).
Why This Works
The Gabriel synthesis is elegant because it uses the phthalimide as a “protected” form of NH. Only one N-C bond forms (the nitrogen is locked between two C=O groups and can only make one more bond). This prevents over-alkylation, which is the major problem with direct ammonolysis (NH + R-X gives a mixture of primary, secondary, tertiary amines, and quaternary salts).
Alternative Method — Hofmann Bromamide Degradation
For preparing primary amines with one fewer carbon than the starting material, use the Hofmann bromamide degradation: RCONH + Br + 4NaOH → RNH + NaCO. This works for both aliphatic and aromatic primary amines.
The Gabriel synthesis gives pure primary amines only. If the question asks “How to prepare a pure primary amine without contamination from secondary/tertiary amines?” — Gabriel synthesis is the answer. This is tested frequently in both JEE and CBSE boards.
Common Mistake
Students try to use Gabriel synthesis with secondary or tertiary alkyl halides. The S2 mechanism in step 2 works best with primary alkyl halides. Secondary halides give poor yields (competing elimination), and tertiary halides give almost entirely elimination products. Always specify “primary alkyl halide” in your answer.