Pk_b value is the negative logarithm of the basicity constant (K_b) .i.e., pK_b = -logK_b

Evidently, smaller the value of pK_b , stronger is the base (strong tendency to donate electrons)

⇒ The structure of methyl amine is CH₃NH₂

⇒ The structure of aniline is:

Aniline (C₆H₅NH₂) shows resonance:

As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalized over benzene ring. As a result, electron density on the nitrogen decreases and thus is less easily available to donate electrons making it less basic. In contrast, in methyl amine (CH₃NH₂), delocalization of the lone pair of electrons on the nitrogen atom by resonance is not possible. Furthermore, CH₃ being an electron- releasing group, due to which +I effect of CH₃ increases the electron density on the N- atom and thus is more easily available to donate electrons making it more basic.

Therefore, aniline is weaker base than methylamine and hence its pK_b value is higher than that of methylamine.