Primary and secondary amines

· Amines are organic derivatives of NH₃ where one or more H atoms are replaced by alkyl groups.
· Primary amine: RNH₂; nitrogen bonded to one alkyl group and two H atoms.
· Secondary amine: R₂NH; nitrogen bonded to two alkyl groups and one H atom.
· Key exam idea: amines contain a lone pair on nitrogen, so they can act as bases and nucleophiles.

This diagram shows how amines are classified by counting the number of carbon-containing groups bonded directly to nitrogen. For this topic, focus on primary amines, RNH₂, and secondary amines, R₂NH. Source

Producing amines from halogenoalkanes

· Primary amines can be made by heating a halogenoalkane with NH₃ in ethanol in a sealed tube / under pressure.
· General reaction: R–X + NH₃ → RNH₂ + HX.
· This is a nucleophilic substitution reaction: NH₃ acts as the nucleophile using its lone pair.
· A large excess of NH₃ favours formation of the primary amine and reduces further substitution.
· Secondary amines can be made by reacting a halogenoalkane with a primary amine in ethanol, heated in a sealed tube / under pressure.
· General reaction: R–X + R′NH₂ → R–NHR′ + HX.
· Exam trap: the amine product is still nucleophilic, so further substitution can produce mixtures unless conditions are controlled.

The diagram shows NH₃ attacking the δ⁺ carbon bonded to the halogen. This replaces the halogen and leads to a primary amine after deprotonation. Source

Producing amines by reduction

· Amides can be reduced using LiAlH₄ to form amines.
· Key transformation: amide → amine; the C=O group is reduced.
· Nitriles can be reduced to primary amines using LiAlH₄ or H₂/Ni.
· Key transformation: R–C≡N → R–CH₂NH₂.
· Exam shorthand: reduction may be shown using [H], e.g. R–C≡N + 4[H] → R–CH₂NH₂.
· Remember: nitrile reduction increases the carbon chain retained from the nitrile carbon, giving –CH₂NH₂.

Condensation with acyl chlorides

· Ammonia reacts with an acyl chloride at room temperature to form a primary amide.
· General reaction: RCOCl + 2NH₃ → RCONH₂ + NH₄Cl.
· A primary amine reacts with an acyl chloride at room temperature to form a secondary amide.
· General reaction: RCOCl + 2R′NH₂ → RCONHR′ + R′NH₃Cl.
· A secondary amine reacts with an acyl chloride at room temperature to form a tertiary amide.
· General reaction: RCOCl + R′₂NH → RCONR′₂ + HCl, with base/amine removing HCl.
· This is a condensation reaction because a small molecule, usually HCl, is eliminated.
· Exam wording: use “amide”, not “amine”, for the product of reaction with an acyl chloride.

The diagram summarises how acyl chlorides form different amides depending on whether the nitrogen reagent is NH₃, a primary amine or a secondary amine. This is a high-yield reaction route for recognising amide formation. Source

Basicity of aqueous amines

· Amines are weak bases in water because the lone pair on nitrogen accepts H⁺.
· General equation: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻.
· For secondary amines: R₂NH + H₂O ⇌ R₂NH₂⁺ + OH⁻.
· Aqueous amine solutions are alkaline because they produce OH⁻ ions.
· Alkyl groups have a positive inductive effect, pushing electron density towards nitrogen.
· More electron density on nitrogen makes the lone pair more available to accept H⁺, increasing basicity.
· Exam phrase: amines are basic because the nitrogen lone pair accepts a proton to form an alkylammonium ion.

The image highlights that the nitrogen lone pair is the key feature responsible for amine basicity. In exam explanations, link basicity to proton acceptance and formation of an ammonium ion. Source

Exam reaction summary

· Halogenoalkane + NH₃ / ethanol / heat under pressure → primary amine.
· Halogenoalkane + primary amine / ethanol / sealed tube or pressure → secondary amine.
· Amide + LiAlH₄ → amine.
· Nitrile + LiAlH₄ → primary amine.
· Nitrile + H₂/Ni → primary amine.
· NH₃ or amine + acyl chloride / room temperature → amide + HCl or ammonium salt.
· Amine + water ⇌ substituted ammonium ion + OH⁻ explains alkaline aqueous solutions.