Amine synthesis
Gabriel synthesis of a primary amine
How the Gabriel synthesis of a primary amine works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Potassium phthalimide; R–X; then N₂H₄. Conditions: DMF, 60–90 °C; then EtOH reflux. Bonds formed: C–N σ bond; primary amine R–NH₂; bonds broken: C–X; two C(O)–N bonds. Stereochemistry: Inversion at a chiral primary/secondary carbon (SN2). Rate and selectivity: SN2: methyl > 1° ≫ 2°; no 3° or aryl halides.
At a glance
Step 1: Phthalimide anion attacks R–Br (SN2)
Potassium phthalimide's nitrogen — freed of its acidic proton by base — acts as a nucleophile in a standard SN2, displacing halide from an alkyl halide.
Step 2: N-alkylphthalimide: no over-alkylation
This gives an N-alkylphthalimide. Because the nitrogen is now doubly flanked by carbonyls, it's not nucleophilic or basic enough to over-alkylate — unlike a direct amine alkylation, which is hard to stop at one substitution.
Step 3: Hydrazinolysis frees the amine
Hydrazine attacks both phthalimide carbonyls in sequence, releasing the alkyl group as a clean, unpolluted primary amine plus a cyclic hydrazide byproduct.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- X
- Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
- DMF
- N,N-Dimethylformamide. Polar aprotic solvent – dissolves ionic nucleophiles but does not H-bond to them, speeding SN2 and SNAr.
- EtOH
- Ethanol. Polar protic solvent.
- K2CO3
- Potassium carbonate. Mild inorganic base.
- KOH
- Potassium hydroxide. Strong aqueous/alcoholic base.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
- Δ
- Heat. Reaction is run at elevated temperature.
- pKa
- Acid-dissociation constant (–log). Lower pKa = stronger acid; used to judge which proton a base removes.
How it compares with related reactions
| Gabriel synthesis of a primary amine | Delépine reaction | Aza-Wittig reaction | |
|---|---|---|---|
| Reagents | Potassium phthalimide; R–X; then N₂H₄ | Hexamethylenetetramine (HMTA), then HCl/EtOH (or H₃O⁺) | R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone |
| Conditions | DMF, 60–90 °C; then EtOH reflux | CHCl₃ or EtOH, reflux; then ethanolic HCl | THF or toluene, 25–110 °C, anhydrous |
| Stereochemistry | Inversion at a chiral primary/secondary carbon (SN2) | Inversion at a stereogenic primary/benzylic carbon (SN2) | Imine E/Z set at ring collapse; no stereocentre needed |
| Rate / selectivity | SN2: methyl > 1° ≫ 2°; no 3° or aryl halides | SN2 step rate-limiting; works for benzylic, allylic, α-halo carbonyls | Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Gabriel synthesis of a primary amine?
Potassium phthalimide; R–X; then N₂H₄. Conditions: DMF, 60–90 °C; then EtOH reflux.
Which bonds are formed and which are broken?
Formed: C–N σ bond; primary amine R–NH₂. Broken: C–X; two C(O)–N bonds.
What is the stereochemical outcome?
Inversion at a chiral primary/secondary carbon (SN2)
What controls the rate or the selectivity?
SN2: methyl > 1° ≫ 2°; no 3° or aryl halides
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go