a
Order of basic strength of aliphatic amine in aqueous solution is as follows (order of \(K_{b}\) )

\(\left(\mathrm{CH}_{3}\right)_{2} \bar {N}H>\mathrm{CH}_{3} \bar {N}H_{2}>\)\(\left(CH_{3}\right)_{3} \bar {N}>\mathrm{C}_{6}\ mathrm{H}_{5} \bar {N} \mathrm{H}_{2}\)

As we know \(\mathrm{pk}_{\mathrm{b}}=-\log \mathrm{K}_{\mathrm{b}}\)

So \( (CH_3) _{2}\)  \(NH\) will have smallest \(pK_{b}\) value. In the case of phenylamine, \(N\) is attached to \(s p^{2}\) hybridised carbon, hence it has highest \(p K_{b}\) and least basic strength.