Considering the basic strength of amines in aqueous solution, which one has

the smallest pKb value?

(1) (CH3)3N

(2) C6H5NH2

(3) (CH3)2NH

(4) CH3NH2

Answer ::

Because of resonance, aryalamines are less basic than alkyl amines. In aryalamines, the lone pair of electrons on N is partly shared with the ring and is less available for sharing with a proton. In alkylamines, the electron releasing alkyl group increases the electron density on nitrogen atom. Hence increases the protonation ability of amines. So, more the number of alkyl groups, higher will be basicity of amine. Because of steric effect, a slight discrepancy occurs in case of trimethyl amine. So (CH3)2NH has the smallest pKb value.