Question

(c) (ii) Why ammonolysis of alkyl halides is not a good method to prepare pure amines?

Hint:

Ammonolysis of alkyl halides often leads to a mixture of primary, secondary, and tertiary amines. To avoid this, alternative synthetic methods, like Gabriel synthesis, can be used to obtain pure primary amines.

Answer 1

Step 1: The Ammonolysis Process Ammonolysis of alkyl halides involves reacting an alkyl halide with ammonia (NH₃), where the nucleophilic nitrogen from ammonia attacks the electrophilic carbon in the alkyl halide, displacing the halide ion and forming a primary amine. However, this reaction suffers from a major drawback: the product amine can further react with excess alkyl halide, leading to the formation of secondary and tertiary amines. \bigskip Step 2: Multiple Reactions The alkyl groups attached to the nitrogen of the amines formed during ammonolysis can make the reaction progress further to form secondary and tertiary amines. This leads to a mixture of amines, rather than a pure primary amine. For example, if methylamine (CH₃NH₂) is formed in the first step, it can then react with another molecule of alkyl halide to form dimethylamine (CH₃NHCH₃), and further to trimethylamine (CH₃N(CH₃)₂). \bigskip Step 3: Alternative Methods To obtain pure primary amines, other methods, such as reductive amination or Gabriel synthesis, are preferred as they avoid the formation of secondary and tertiary amines.