Arrange the following in the increasing order of their pKb values: C6H5NH2, C2H5NH2, C6H5NHCH3

The increasing order for pKb values is C2H5NH2 (3.29) < C6H5NHCH3 (9.30) < C6H5NH2 (9.38). The numbers in bracket indicate pKb. Higher is the pKb value, lower is the basic strength of amine. Thus, the increasing order for pKb values indicates decreasing order of the basic strength of amine. Aniline is least basic and ethanamine is most basic. Aniline is least basic as the unshared pair of electrons on the N atom is involved in resonance with the benzene ring and cannot be easily donated. When an alkyl group such as methyl or ethyl is present, the electron-releasing inductive effect (+I effect) of the alkyl group increases the electron density on the N atom and the unshared pair of electrons on the N atom can be easily donated.