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Amine alkylation is a type of organic reaction between an alkyl halide and ammonia or an amine. The reaction is called nucleophilic aliphatic substitution , and the reaction product is a higher substituted amine. The method is widely used in the laboratory, but less so industrially, where alcohols are often preferred alkylating agents.
When the amine is a tertiary amine the reaction product is a quaternary ammonium salt in the Menshutkin reaction:
Amines and ammonia are generally sufficiently nucleophilic to undergo direct alkylation, often under mild conditions. The reactions are complicated by the tendency of the product to react with the alkylating agent. For example, reaction of 1-bromooctane with ammonia yields almost equal amounts of the primary amine and the secondary amine. Therefore, for laboratory purposes, N-alkylation is often limited to the synthesis of tertiary amines. An exception is the amination of alpha-halo carboxylic acids that do permit synthesis of primary amines with ammonia. Intramolecular reactions of haloamines X-n-NH2 give cyclic aziridines, azetidines and pyrrolidines.
N-alkylation is a general and useful route to quaternary ammonium salts from tertiary amines, because overalkylation is not possible.