Primary and Secondary Amines

Specification Reference Organic Chemistry, Nitrogen compounds 34.1

Quick Notes

Amine preparation methods:
- From halogenoalkanes:
  - NH₃ in ethanol, heated under pressure (forms a primary amine)
  - Primary amine in ethanol, heated in sealed tube (forms a secondary amine)
- Reduction of nitriles using LiAlH₄ or H₂/Ni (forms primary amines)
- Reduction of amides using LiAlH₄
Amines react with acyl chlorides to form amides at room temperature in condensation reactions.
Amines can act as weak bases due to the lone pair on the nitrogen, which can accept a proton (H⁺).

There are several methods of amine preparation that you need to know.

Halogenoalkanes and Ammonia

Reagent: NH₃ in ethanol
Conditions: Heated under pressure
Reaction: R–X + 2NH₃ → RNH₂ + NH₄X
- Produces primary amines via nucleophilic substitution.
- Excess ammonia is used to favour formation of primary amines.

From Halogenoalkanes and Primary Amines

Reagent: Primary amine (e.g. CH₃NH₂)
Conditions: Heated in a sealed tube or under pressure
Reaction: R–X + R′NH₂ → R–NHR′ + HX
- Produces secondary amines.
- Further reaction can lead to tertiary amines or quaternary ammonium salts.

Reduction of Amides

Reagent: LiAlH₄ (in dry ether)
Reaction: RCONH₂ + 4[H] → RCH₂NH₂ + H₂O
- Converts amides into primary amines.

Reduction of Nitriles

Reagents:
- LiAlH₄ in dry ether
- OR catalytic hydrogenation (H₂ with Ni catalyst)
Reaction: R–C≡N + 4[H] → RCH₂NH₂
Both methods give primary amines.

Reaction type: Nucleophilic addition–elimination (condensation reaction)
Reagents: Ammonia or amine + acyl chloride
Conditions: Room temperature
Products:
- With ammonia: RCOCl + 2NH₃ → RCONH₂ + NH₄Cl (forms primary amide)
- With primary amine: RCOCl + 2R′NH₂ → RCONHR′ + R′NH₃Cl (forms secondary amide)

Ammonium halide salts are formed because HCl by-product reacts with excess amine or ammonia to form a salt.

Amines can act as Bronsted–Lowry bases. They accept protons via the lone pair on the nitrogen atom.

Electron-donating groups (e.g. alkyl groups) increase electron density on N → stronger base.

Aromatic amines (e.g. phenylamine) are weaker bases because the lone pair is delocalised into the benzene ring, making it less available for bonding with H⁺.
Solubility in water also affects how readily amines act as bases in aqueous solution.
- Example Reaction in Water: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻

This equilibrium shows why aqueous solutions of amines are slightly alkaline.

Amines can be prepared from halogenoalkanes, nitriles, or amides.
Primary amines are produced with ammonia, secondary amines with primary amines.
Amines and ammonia react with acyl chlorides to form amides via nucleophilic addition–elimination.
Amines act as weak bases; basicity depends on electron-donating/withdrawing groups and delocalisation effects.