Primary amines

· Primary amines contain the amine functional group, –NH₂, attached to an alkyl group: R–NH₂.
· General structure: R–NH₂, where R = alkyl group.
· Example: ethylamine, CH₃CH₂NH₂, from bromoethane, CH₃CH₂Br.
· In CIE AS Level, classification of amines is not tested, but you must recognise primary amines as products.
· Key idea: halogen in a halogenoalkane is replaced by –NH₂.

This diagram helps identify the primary amine functional group as nitrogen bonded to one carbon group and two hydrogen atoms. Source

Production from halogenoalkanes

· Primary amines are produced by reacting a halogenoalkane with ammonia, NH₃.
· Required conditions: NH₃ in ethanol, heat, under pressure / sealed tube.
· General equation: R–X + 2NH₃ → R–NH₂ + NH₄X.
· X = halogen: Cl, Br or I.
· Example: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br.
· Product formed: primary amine + ammonium halide salt.

This page shows the key CIE reaction: a halogenoalkane reacts with ammonia to form a primary amine. It also highlights why the reaction is done in a sealed tube with ethanolic ammonia. Source

Reaction type and exam wording

· Reaction type: nucleophilic substitution.
· Ammonia acts as a nucleophile because nitrogen has a lone pair of electrons.
· The C–X bond in a halogenoalkane is polar: Cδ+–Xδ−.
· NH₃ attacks the δ+ carbon, and the halide ion leaves.
· A second NH₃ molecule removes H⁺, forming the neutral primary amine.
· Exam phrase: “reaction of a halogenoalkane with NH₃ in ethanol heated under pressure”.

The diagram shows why this reaction is a nucleophilic substitution: NH₃ donates a lone pair to the carbon bonded to halogen. The halogen leaves as a halide ion, allowing the C–N bond to form. Source

Why excess ammonia is used

· The primary amine product still has a lone pair on nitrogen.
· Therefore, the primary amine can react further with more halogenoalkane.
· This may form secondary amines, tertiary amines or quaternary ammonium salts.
· To favour the primary amine, use excess ammonia.
· For CIE 19.1, focus on the required product: primary amine, R–NH₂.

Common exam mistakes

· Do not write the product as R–NH₃; the neutral primary amine is R–NH₂.
· Do not forget the by-product: NH₄X.
· Do not write aqueous NH₃ if the syllabus asks for NH₃ in ethanol.
· Do not omit heat under pressure / sealed tube.
· Do not confuse this with forming nitriles using KCN in ethanol and heat.