Preparation of Amines

Specification Reference Organic chemistry, Amines 3.3.11.1

Quick Notes

Primary amines can be prepared by:
- Reaction of ammonia with halogenoalkanes (nucleophilic substitution).
- Reduction of nitriles (using LiAlH₄ or catalytic hydrogenation).
Aromatic amines are prepared by the reduction of nitro compounds and are used in dye manufacture.

AQA A-Level Chemistry diagram showing reduction of a nitroarene to an aromatic amine

Full Notes

Preparation of Aliphatic Amines

Note - aliphatic means an organic molecule with no aromatic ring. At this level, that really just means any organic molecule that doesn’t have a benzene ring in it!

There are two main ways to produce (primary) aliphatic amines - reaction of halogenoalkanes with ammonia or the reduction of nitriles.

Reaction of Ammonia with Halogenoalkanes

Reagents: Excess ammonia (NH₃) and haloalkane (R–X)

AQA A-Level Chemistry overview of halogenoalkane reacting with ammonia to give a primary amine and ammonium halide

Conditions: Ethanol as a solvent, sealed tube (to prevent escape of NH₃).

Mechanism – Nucleophilic substitution:

AQA A-Level Chemistry curly-arrow mechanism for nucleophilic substitution of a halogenoalkane by ammonia to form a primary amine

Limitations:
If NH₃ isn’t in excess then further substitution may occur, leading to secondary and tertiary amines. This is because the primary amine formed at the start can act as a nucleophile and react with any left over halogenoalkane.

Reduction of Nitriles

Nitriles (–CN) can be reduced to primary amines (–CH₂NH₂).

AQA A-Level Chemistry route showing nitrile reduced to primary amine via LiAlH4 in dry ether or H2/Ni catalytic hydrogenation

Methods of Reduction:

Using LiAlH₄ in dry ether (laboratory method): R–CN + 4[H] → R–CH₂NH₂
Catalytic hydrogenation (industrial method):
Reagent: H₂ gas + Nickel catalyst
Conditions: High pressure and temperature
Advantage: Produces only primary amines (unlike halogenoalkane method).

Preparation of Aromatic Amines

Aromatic amines (e.g., phenylamine) are prepared by reducing nitro compounds.

Uses:

Aromatic amines are key intermediates in dye synthesis
Used in making pharmaceuticals and polymers

Example: Reduction of Nitrobenzene to Phenylamine

Reagents: Tin (Sn) + Concentrated HCl

AQA A-Level Chemistry reduction of nitrobenzene to phenylamine using tin and concentrated hydrochloric acid, then neutralisation

Conditions: Heat under reflux

Equation:
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Final Step: Excess HCl is neutralised with NaOH to obtain free amine.

Summary Table: Preparation of Amines

Type of Amine	Preparation Method	Reagents	Conditions
Primary Aliphatic Amine	From halogenoalkane	NH₃ (excess)	Ethanol solvent
Primary Aliphatic Amine	Reduction of nitrile	LiAlH₄ or H₂/Ni	Dry ether (LiAlH₄) or high pressure (H₂/Ni)
Aromatic Amine	Reduction of nitro compound	Sn + HCl	Reflux, then neutralisation with NaOH