question_answer 19)

Explain why \[{{(C{{H}_{3}})}_{3}}{{C}^{+}}\] is more stable than \[C{{H}_{3}}\overset{+}{\mathop{C{{H}_{2}}}}\,\] and \[\overset{+}{\mathop{C{{H}_{3}}}}\,\] is the least stable cation.

Answer:

Hyper conjugation interaction in \[{{(C{{H}_{3}})}_{3}}{{C}^{+}}\] is greater than in \[C{{H}_{3}}\overset{+}{\mathop{C{{H}_{2}}}}\,\] as the \[{{(C{{H}_{3}})}_{3}}\overset{+}{\mathop{C}}\,\] has nine C-H bonds while\[C{{H}_{3}}\overset{+}{\mathop{C{{H}_{2}}}}\,\] has only three C-H bonds. As a result, \[{{(C{{H}_{3}})}_{3}}{{C}^{+}}\] is more stable than\[C{{H}_{3}}\overset{+}{\mathop{C{{H}_{2}}}}\,\]. In\[C{{H}_{3}}\], the vacant p-orbital is perpendicular to the plane in which C-H bonds lie and hence cannot overlap with it. Thus, \[\overset{+}{\mathop{C{{H}_{3}}}}\,\] is not stabilized by hyper conjugation and hence is least stable of the three cations.