Lesson 3 of 12 · 6 min
Gabriel and Hoffmann methods
NCERT §9.4
A customer wants pure propan-1-amine, with no secondary or tertiary amine at all. Ravi knows ammonolysis will not do. The chemist hands him two older recipes.
The lesson in notes
In short
Gabriel phthalimide synthesis gives primary amines only. Phthalimide with ethanolic KOH forms potassium phthalimide; heating this with an alkyl halide gives an N-alkylphthalimide, and alkaline hydrolysis releases the primary amine.
Gabriel synthesis cannot give aniline or any aromatic primary amine: an aryl halide will not be attacked by the phthalimide anion in a substitution.
Hoffmann bromamide degradation (NCERT's spelling): an amide is treated with bromine in aqueous or ethanolic NaOH. An alkyl or aryl group migrates from the carbonyl carbon to nitrogen, and the primary amine formed has one carbon fewer than the amide.
Equation: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O.
Worked: propanamine (three carbons) comes from butanamide (four); benzamide (seven carbons) gives aniline (six).
Carbon count at a glance: nitrile reduction adds one carbon to the alkyl halide, Hoffmann degradation removes one from the amide, and Gabriel synthesis and ammonolysis keep the halide's carbon count.