Gabriel synthesis: practice and mechanism
The phthalimide anion is alkylated only once (SN2) because the N has no H left afterwards. Hydrazine releases the primary amine. This gives clean R–NH₂ without overalkylation.
Example
The phthalimide anion is alkylated via SN2. Because the nitrogen then carries no H, a second alkylation is impossible. Hydrazine releases the primary amine. This gives clean primary amines without over-alkylation.
Mechanism step by step
- 1
Potassium phthalimide: the N–H between two C=O groups is acidic (pKa ≈ 8) and the anion is a good nucleophile.
- 2
The phthalimide anion attacks the primary haloalkane via SN2: N-alkylphthalimide. There is no H left on N, so a second alkylation is impossible.
- 3
Hydrazine attacks both C=O groups, forming phthalhydrazide, and the primary amine is released.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: SN2 with ammonia · SN2 with azide
Frequently asked questions
Why not just NH₃ and a haloalkane?
The primary amine is more nucleophilic than NH₃ and reacts on to secondary and tertiary amines. With Gabriel this is impossible.
Does it work with secondary haloalkanes?
Poorly. The phthalimide anion is bulky; with secondary and tertiary substrates it becomes slow or elimination takes over.