Question

Why aniline has high pkb value?

Answer 1

Answer:

Aniline undergoes resonance and as a result, the electrons on the N-atom are delocalized over the benzene ring. Therefore, the electrons on the N-atom are less available to donate.On the other hand, in case of methylamine (due to the +I effect of methyl group), the electron density on the N-atom is increased. As a result, aniline is less basic than methylamine. Thus, pKb of aniline is more than that of methylamine.

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Answer 2

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In aniline, the lone pair of electrons on N atom is delocalized over the benzene ring. As a result, electron density on the nitrogen decreases.

On the other hand, in $CH_3NH_2$, + I effect of $CH_3$ group increases the electron density on N atom.

Therefore, aniline is less basic than methylamine and hence pK$_b$ of aniline is higher than that of methylamine and aniline is less basic than methylamine.