Question

Which of the following orders is correct regarding the basicity of aliphatic amines?

• A. ${CH}_3{NH}_2>({CH}_3{)}_{2}NH>({CH}_3{)}_{3}N$
• B. ${CH}_3{NH}_2<({CH}_3{)}_{2}NH<({CH}_3{)}_{3}N$
• C. ${CH}_3{NH}_2>({CH}_3{)}_{2}NH<({CH}_3{)}_{3}N$
• D. ${CH}_3{NH}_2<({CH}_3{)}_{2}NH>({CH}_3{)}_{3}N$

Answer 1

The correct option is D ${CH}_3{NH}_2<({CH}_3{)}_{2}NH>({CH}_3{)}_{3}N$

Except the amines containing tertiary butyl group, all lower aliphatic amines are stronger bases than ammonia because of +I (inductive) effect. The alkyl groups, which are electron releasing groups, increase the electron density around the nitrogen thereby, increasing the availability of the lone pair of electrons to proton or Lewis acids and making the amine more basic.

The observed order in the case of lower members is found to be as secondary > primary > tertiary. This anomalous behavior of tertiary amines is due to steric factors i.e. crowding of alkyl groups cover nitrogen atom from all sides and thus makes it unable for protonation. Thus the relative strength is in order CH3NH2<(CH3)2NH>(CH3)3N

The correct option is D.