Pkb value is the negative logarithm of the basicity constant (Kb) .i.e., pKb = -logKb
Evidently, smaller the value of pKb , 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 pKb value is higher than that of methylamine.
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