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19.1 Primary amines

Syllabus
9701–2028–2029
Topic
19.1
Level
AS

Prepare a primary amine with ammonia in ethanol under pressure

Heat a halogenoalkane with excess NH₃ in ethanol under pressure. Ammonia acts as the nucleophile and the carbon skeleton of the halogenoalkane is retained while X is replaced by NH₂.

The attacking NH₃ first forms an alkylammonium species, RNH₃⁺. A second NH₃ molecule removes a proton, giving the amine RNH₂ and NH₄⁺; the leaving halide X⁻ therefore appears in the ammonium salt.

RX+2 NHX3→ethanol,  heat,  pressureRNHX2+NHX4X\ce{RX + 2NH3 ->[ethanol,\ heat,\ pressure] RNH2 + NH4X}

CHX3CHX2Br+2 NHX3→CHX3CHX2NHX2+NHX4Br\ce{CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br}

Use excess ammonia to favour the primary amine because the amine product can otherwise undergo further alkylation. Detailed classification of amines is outside the stated AS assessment scope.