question_answer

Predict, giving reasons, the order of basicity of the following compounds in (i) gaseous phase and (ii) in aqueous solution. \[{{(C{{H}_{3}})}_{3}}N,\,\,{{(C{{H}_{3}})}_{2}}NH,\,\,C{{H}_{3}}N{{H}_{2}},\,N{{H}_{3}}\]

Answer:

                Due to \[+I\]-effect of the alkyl groups, the electron density on the N-atom of \[1{}^\circ ,\,2{}^\circ \] and \[3{}^\circ \] amines is higher than that on the N-atom in \[N{{H}_{3}}\]. Therefore, all amines are more basic than \[N{{H}_{3}}\]. (i) In gaseous phase, solvation effects are absent and hence the relative basicity of amines depends only on the \[+I\]-effect of the alkyl groups. Now since the \[+I\]-effect increases in going from \[1{}^\circ \] to \[2{}^\circ \] to \[3{}^\circ \]amine, therefore, the relative basicity of amines decreases in the order : \[3{}^\circ \] amine > \[2{}^\circ \] amine > \[1{}^\circ \]amine. In other words, in the gaseous phase, the basicity of methylamines relative to \[N{{H}_{3}}\] decreases in the order : \[{{(C{{H}_{3}})}_{3}}N>{{(C{{H}_{3}})}_{2}}NH>C{{H}_{3}}N{{H}_{2}}>N{{H}_{3}}\] (ii) In aqueous solution, the basicity of methylammes depends upon two factors : (a) \[+I\]-effect of the \[C{{H}_{3}}\] groups and (b) Solvation effect, i.e., stabilization of the conjugate acid (formed by addition of a proton to amine) by H-bonding. As explained above, on the basis of \[+I\]-effect alone, the relative basicity of amines should decrease in the order :\[{{(C{{H}_{3}})}_{3}}N>{{(C{{H}_{3}})}_{2}}NH>C{{H}_{3}}N{{H}_{2}}>N{{H}_{3}}\] On the basis of stabilization of the conjugate acids by H- bonding alone (as explained below), the basicity should decrease in the order: \[C{{H}_{3}}N{{H}_{2}}>{{(C{{H}_{3}})}_{2}}NH>{{(C{{H}_{3}})}_{3}}N\]     The combined effect of these two opposing factors is that \[{{(C{{H}_{3}})}_{2}}NH\] is the strongest base. In case of \[C{{H}_{3}}N{{H}_{2}}\] and \[{{(C{{H}_{3}})}_{3}}N,\] the stability due to H-bonding predominates over stability due to \[+I\]-effect of the\[C{{H}_{3}}\] groups thereby making \[C{{H}_{3}}N{{H}_{2}}\] stronger than \[{{(C{{H}_{3}})}_{3}}N\]. Thus, the overall relative basicity of methylamines w.r.t. ammonia in aqueous solution decreases in the order : \[{{(C{{H}_{3}})}_{2}}NH>C{{H}_{3}}N{{H}_{2}}>{{(C{{H}_{3}})}_{3}}N>N{{H}_{3}}\]