Arrange the following in increasing order of their basic strength :
`C_2 H_5 NH_2 ,(C_2H_5)_2 NH, (C_2 H_5)_3 N, C_5 H_5 NH_2`

Explanation

Due to +I effect of `C_2 H_5` group, the electron density on nitrogen atom increases. Hence, the basic character of `C_2 H_5 NH_2`,`(C_2 H_5 ) NH and (C_2 H_5) _3 N ` is expected to follow the order : ` C_2 H_5 NH_2 lt (C_2 H_5)_2 NH lt (C_2 H_5 )_3 NH` In `C_6 H_5 NH_2`, the electron density on the N atom decreases due to the delocalization of the lone pair of electrons over the benzene ring. Hence, `C_6 H_5 NH_2` is a weaker base than all the three ethylamines, though `(C_2 H_5 )_3N` is expected to be more basic than `(C_2 H_5 )_2 NH`, it is less basic because the stability of alkyl substituted ammonium ions depends upon the resultant of +I effect, steric hinderance and solvation of ions due to the increase in the number of alkyl groups which result into much crowding and makes the attack of protons difficult and stabilization of alkyl substituted ammonium ion by solvation. Larger the size of the ion, lesser will be the solvation. Hence, larger size of the ion reduces the stability and decreases basic strength. In case of primary and secondary amines, inductive effect dominates and `(C_2 H_5 )_2NH` is more basic than `C_2 H_5 NH_2`. Hence, `(C_2 H_5 )_3N` is less basic than `(C_2 H_5 )_2NH` but is more basic than `C_2 H_5 NH_2`. Therefore, the correct order of basic strength is ` C_6 H_5 NH_2 lt C_2 H_5 NH_2 lt (C_2 H_5 )_3 N lt (C_2 H_5) _2 NH`