Preparing Primary Aliphatic Amines

Primary aliphatic amines can be made by reacting a halogenoalkane with ammonia.Conditions: the halogenoalkane is heated with an excess of ammonia dissolved in ethanol, in a sealed tube (so the volatile ammonia cannot escape).The reaction is a nucleophilic substitution. The lone pair on the nitrogen of ammonia attacks the δ⁺ carbon of the C-X bond, and the halide ion leaves. This gives an alkylammonium ion, and a second ammonia molecule removes an H⁺ from it to release the amine. Two moles of ammonia are used overall:CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄BrThe products are ethylamine and ammonium bromide.The problem: further substitution. The primary amine product also has a lone pair, so it is a nucleophile too. It can attack more halogenoalkane, forming a secondary amine, then a tertiary amine and finally a quaternary ammonium salt. The result is a mixture of products that must be separated, which lowers the yield of the primary amine.Using a large excess of ammonia makes it much more likely that each halogenoalkane molecule meets ammonia rather than an amine, so the primary amine is the main product.Worked example: 1-chloropropane heated with excess ethanolic ammonia gives propylamine, CH₃CH₂CH₂NH₂, and ammonium chloride.Reducing a nitrile (next lesson) is an alternative route that gives only the primary amine.