Preparation of Aliphatic Amines

There are two main ways to make aliphatic amines.1. Halogenoalkane and ammonia. A halogenoalkane is heated with excess ammonia dissolved in ethanol in a sealed tube (ammonia is a gas, so it would escape from an open flask). Ammonia acts as a nucleophile and substitutes for the halogen:CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄BrThe second ammonia molecule removes H⁺ from the intermediate ion. The problem is that the amine product also has a lone pair, so it is itself a nucleophile. It can react with more halogenoalkane in further substitution, giving secondary and tertiary amines and finally a quaternary ammonium salt such as (CH₃CH₂)₄N⁺Br⁻. Using a large excess of ammonia makes it more likely that the halogenoalkane meets an ammonia molecule, so the primary amine is the main product, but a mixture still forms.2. Reduction of nitriles. A nitrile is reduced by LiAlH₄ in dry ether:CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂This gives only the primary amine, so it is a cleaner route.Worked example: make propylamine from bromoethane. Bromoethane has two carbons and propylamine has three, so a carbon must be added. Heat bromoethane under reflux with potassium cyanide in ethanol to give propanenitrile, then reduce it with LiAlH₄ to give propylamine.