Start: Phthalimide (cyclic imide from phthalic anhydride + $NH_{3}$)

• Structure: Bicyclic with –CO–NH–CO– unit; mildly acidic N–H (pKa ≈ 8.3)

Step 1 — Deprotonation: Phthalimide + KOH → Potassium phthalimide (K-salt)

• The N–H is acidic enough to be deprotonated; $K^{+}$ ion pair stabilises negative charge on nitrogen

Step 2 — N-Alkylation (SN2): Potassium phthalimide + RX (primary alkyl halide) → N-Alkylphthalimide + KX

• SN2 requires a primary (1°) alkyl halide; secondary and tertiary halides give elimination instead
• ArX does NOT react because C–X in arenes has partial π character; SN2 is blocked

Step 3 — Hydrolysis (Hydrazinolysis): N-Alkylphthalimide + $N_{2}H_{4}$ (hydrazine) → $RNH_{2}$ + Phthalhydrazide (phthalic hydrazide)

• Alternatively, dilute acid/base hydrolysis can be used (but gives phthalic acid + $RNH_{2}$)

Product: Primary aliphatic amine ($RNH_{2}$) — pure and uncontaminated by 2° or 3° amines

Key Constraint: Only works with primary alkyl halides; aromatic halides, secondary/tertiary halides are incompatible

Why it's valued: Gabriel synthesis is the ONLY selective method for primary aliphatic amines without contamination by higher substitution amines; unlike ammonolysis (which gives mixtures)