3.4.7 Amines - Preparation
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Preparation of amines
1. From haloalkanes
Ammonia reacts with haloalkanes by nucleophilic substitution to form amines.
However, the reaction has a complication in that the product also reacts with ammonia making secondary and then tertiary amines.
To get around this problem the reactants are mixed so that there is a stoichiometric amount of ammonia equivalent to a 1:1 reaction with the haloalkane. This does not eliminate the side products, but it goes some way towards improving the yield of the primary amine.
2. From nitriles
Nitriles can be reduced using hydrogen gas with a catalyst to form amines directly.
This reaction affords a means of extending the carbon chain by one carbon atom when starting with a haloalkane. The haloalkane undergoes nucleophilic substitution with cyanide ions and then the corresponding nitrile can be reduced.
The conditions required are hydrogen under pressure in the presence of a nickel catalyst.
Preparation of aromatic amines
Phenylamine can be prepared by reduction of nitrobenzene. The conditions required are tin and hydrochloric acid, followed by dilute base.
The tin reacts with the hydrochloric acid forming hydrogen gas and the product of the reaction of this hydrogen gas with the nitro group complexes with the tin chloride pulling the reaction to the right hand side. The tin complex formed is broken down by mild basic hydrolysis, warming with dilute sodium hydroxide solution, to yield the final product.
