Overall reaction
Phthalimide → (KOH) potassium phthalimide → (R–X) N-alkylphthalimide → (N₂H₄ or H₃O⁺) R–NH₂
Phthalimide as a protected NH₂: primary amines without over-alkylation.
Name reaction · mechanism
Gabriel Phthalimide Synthesis
The Gabriel synthesis makes pure primary amines. Phthalimide is deprotonated (its N–H, between two carbonyl groups, has pKa about 8.3), the anion is alkylated by a primary alkyl halide in an SN2 reaction, and hydrazine (or hydrolysis) then releases the amine. Because the N-alkylphthalimide cannot be alkylated again, no secondary or tertiary amines form.
Mechanism step by step
- Deprotonation. The N–H between two C=O is acidic (pKa 8.3)
- N-Alkylation (SN2). The imide anion displaces bromide
- Hydrazine attacks. First of two addition–elimination steps (Ing–Manske)
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- The N-alkyl phthalimide is no longer nucleophilic: only one alkyl group adds.
- Hydrazine attacks both C=O groups in turn, releasing the amine and phthalhydrazide.
- Acid or base hydrolysis also works but needs harsher conditions.
- Only for primary alkyl halides (SN2); aryl halides do not react.
For JEE and NEET
- Gives only primary amines — no over-alkylation, unlike ammonia + alkyl halide.
- Aryl halides (e.g. chlorobenzene) do not work: they do not undergo SN2.
- Aromatic primary amines (aniline) cannot be made this way.
- Hydrazinolysis by-product is phthalhydrazide.
FAQ
Why can aniline not be prepared by Gabriel synthesis?
The alkylation step is SN2, and aryl halides do not undergo nucleophilic substitution by the phthalimide anion.
Why is phthalimide acidic?
The N–H sits between two carbonyl groups, so the anion is delocalised onto both oxygens (pKa ≈ 8.3).
Why is the Gabriel synthesis preferred over NH₃ + R–X?
Ammonia gives mixtures of primary, secondary and tertiary amines and quaternary salts; the N-alkylphthalimide stops at one alkyl group.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.