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 the reaction of an alkyl halide with ammonia (\( \text{NH}_3 \)), where the nucleophilic nitrogen from ammonia attacks the electrophilic carbon in the alkyl halide. This displaces the halide ion and forms a primary amine. However, this reaction has a significant limitation: the product amine can further react with excess alkyl halide, resulting in 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 cause the reaction to proceed further, leading to secondary and tertiary amines. This results in a mixture of amines, rather than a pure primary amine. For example, if methylamine (\( \text{CH}_3\text{NH}_2 \)) is formed in the first step, it can react with another molecule of alkyl halide to form dimethylamine (\( \text{CH}_3\text{NH}\text{CH}_3 \)), and further to trimethylamine (\( \text{CH}_3\text{N}(\text{CH}_3)_2 \)). \bigskip Step 3: Alternative Methods To obtain pure primary amines, alternative methods such as reductive amination or Gabriel synthesis are preferred. These methods avoid the formation of secondary and tertiary amines.