Question

Which one of the following is the strongest base in aqueous solution ?(a) Methylamine (b) Trimethylamine(c) Aniline (d) Dimethylamine.

Answer 1

The increasing order of basicity of the given compounds is (CH3)2NH > CH3NH2 > (CH3)3N > C6H5NH2.

Due to the +I effect of alkyl groups, the electron density on nitrogen increases and thus the availability of the lone pair of electrons to proton increases and hence the basicity of amines also increases.

So aliphatic amines are more basic than aniline. In case of tertiary anine (CH3)3N, the covering of alkyl groups over nitrogen atom from all sides makes the approach and bonding by a proton relatively difficult (steric hinderance), hence the basicity decreases. Electron withdrawing (C6H5–) groups decreases electron density on nitrogen atom and thereby decreasing basicity.

Hope it helps :)