Arrange the following in increasing order of their basic strength :
(i) `C_(2)H_(5)NH_(2), C_(6)H_(5)NH_(2), NH_(3), C_(6)H_(5)CH_(2)NH_(2)` and `(C_(2)H_(5))_(2)NH` (ii) `C_(2)H_(5)NH_(2), (C_(2)H_(5))_(2)NH, (C_(2)H_(5))_(3)N, C_(6)H_(5)NH_(2)`
(iii) `CH_(3)NH_(2), (CH_(3))_(2)NH, (CH_(3))_(3)N, C_(6)H_(5)NH_(2), C_(6)H_(5)CH_(2)NH_(2)`

Explanation

(i) `(C2)H_(5))_(2)NH gt C_(2)H_(5)NH_(2) gt C_(6)H_(5)CH_(2)NH_(2) gt C_(6)H_(5)NH_(2) gt NH_(3)`
Due to +I effect of the two `C_(2)H_(5)` -groups in `(C_(2)H_(5))_(2)NH` as compared to one in `C_(2)H_(5)NH_(2)` the lone pair on N is more available in `(C_(2)H_(5))_(2)NH` than `C_(2)H_(5)NH_(2)` and therefore, `(C_(2)H_(5))_(2)NH` is more basic than `C_(2)H_(5)NH_(2)`. Now aromatic amine, `C_(6)H_(5)NH_(2)` is less basic than both `(C_(2)H_(5))_(2)NH` and `C_(2)H_(5)NH_(2)` due to -I effect of `C_(6)H_(5)` -group. Due to the presence of `C_(6)H_(5)` -group, the electron density on the N atom becomes lower and hence less basic. Comparing `C_(6)H_(5)NH_(2)` and `C_(6)H_(5)CH_(2)NH_(3)`, N is directly bonded to the benzene ring and hence the lone pair of electrons on the N atom is delocalised over the benzene ring. In contrast, N in `C_(6)H_(5)CH_(2)NH_(2)` is not directly bonded to the benzene ring and hence its lone pair is not delocal ised over the benzene ring. Therefore, the lone pair of electrons on N atom in `C_(6)H_(5)CH_(2)NH_(2)` is more easily available for protonation than that on the N atom in `C_(6)H_(5)NH_(2)`. Hence `C_(6)H_(5)CH_(2)NH_(2)` is more basic than `C_(6)H_(5)NH_(2)`. Hence, the correct order of basic character.
(ii) Due to increase in +I inductive effect of `C_(2)H_(5)` -group, the electron density on N-atom increases and therefore, the basic character is expected to increase as `C_(2)H_(5)NH_(2) gt (C_(2)H_(5))_(2)NH gt (C_(2)H_(5))_(3)N`. In `C_(6)H_(5)NH_(2)`, the electron density on the N atom decreases due to the delocalisation of the lone pair of electrons over the benzene ring. Therefore, all the three ethyl amines are more basic than `C_(6)H_(5)NH_(2)`. Though `(C_(2)H_(5))_(3)` N is expected to be more basic than `(C_(2)H_(5))_(2)NH`, it is less basic because of steric hindrance to H-bonding for solvation of conjugate acid derived from `(C_(2)H_(5))_(3)` N. Therefore, `(C_(2)H_(5))_(3)N` is less basic than `(C_(2)H_(5))_(2)NH` but more basic than `C_(2)H_(5)NH_(2)`. So, the correct order of basic strength is :
`(C_(2)H_(5))_(2)NH gt (C_(2)H_(5))_(3)N gt C_(2)H_(5)NH_(2) gt C_(6)H_(5)NH_(2)`.
(iii) As explained in answer (i), `C_(6)H_(5)NH_(2)` is less basic than `C_(6)H_(5)CH_(2)NH_(2)` and all methyl amines are more basic than these amines due to +I effect of `-CH_(3)` group sanswer (ii)]. However, `(CH_(3))_(3)` N is less basic than `(CH_(3))_(2)NH` and `CH_(3)NH_(2)` due to steric hindrance and less stabilization by H-bonding. Therefore, the correct order is :
`(CH_(3))_(2)NH gt CH_(3)NH_(2) gt (CH_(3))_(3)N gt C_(6)H_(5)CH_(2)NH_(2) gt C_(6)H_(5)NH_(2)`