Question

Account for the following:

pK_b of aniline is more than that of methylamine.

Answer 1

The one which has less basicity has more pK_b. The pK_b of aniline is more than that of methylamine because aniline has five resonance structures, due to which the unshared electron pair of the nitrogen atom present in it gets delocalized on the benzene ring. This reduces the electron density and aniline does not donate electrons; i.e., the basicity of aniline is less.

On the other hand, the presence of a methyl group in methylamine increases the +I effect, due to which the electron density on the nitrogen atom increases and it readily donates electrons, i.e., methylamine is more basic. Therefore, the pK_b of aniline is more than that of methylamine.

Answer 2

In aniline, the lone pair of electrons on the N atom is delocalized over the benzene ring. As a result, the electron density of nitrogen decreases. In contrast, in CH₃NH₂ (+I) effect of CH₃ increases the electron density on the N atom. Therefore, aniline is a weaker base than methylamine, and hence its pK_b value is higher than that of methylamine.