question_answer

Rearrange the following in an increasing order of their basic strengths : \[{{C}_{6}}{{H}_{5}}N{{H}_{2}},\,{{C}_{6}}{{H}_{5}}N{{(C{{H}_{3}})}_{2}},\,{{({{C}_{6}}{{H}_{5}})}_{2}}NH\]and \[C{{H}_{3}}N{{H}_{2}}\].

Answer:

                (i) Due to delocalization of lone pair of electrons on the N-atom into the benzene ring, all aromatic amines are less basic than alkylamines, i.e., \[C{{H}_{3}}N{{H}_{2}}\].                 (ii) Presence of electron-donating groups (i.e., \[C{{H}_{3}}\]) on the N-atom increases the basicity of substituted aniline w.r.t. \[{{C}_{6}}{{H}_{5}}N{{H}_{2}}.\] (iii) In \[{{({{C}_{6}}{{H}_{5}})}_{2}}NH,\] the lone pair of electrons on the N-atom is delocalized over two benzene rings instead of one in \[{{C}_{6}}{{H}_{5}}N{{H}_{2}},\] therefore, \[{{({{C}_{6}}{{H}_{5}})}_{2}}NH\]is much  less basic than \[{{C}_{6}}{{H}_{5}}N{{H}_{2}}\]. Combining all the three trends together, the basicity of the four amines increases in the order : \[{{({{C}_{6}}{{H}_{5}})}_{2}}NH<{{C}_{6}}{{H}_{5}}N{{H}_{2}}<{{C}_{6}}{{H}_{5}}N{{(C{{H}_{3}})}_{2}}\]\[<C{{H}_{3}}N{{H}_{2}}\].