question_answer 1)

                Write structures of various carbocations that can be obtained from 2-methylbutane. Arrange these carbocations in order of increasing stability.

Answer:

  Following four carbocations are possible from 2-methylbutane, they are given in increasing order of stability. \[C{{H}_{3}}\to -\underset{C{{H}_{3}}}{\mathop{\underset{\begin{smallmatrix}  | \\  \uparrow  \end{smallmatrix}}{\mathop{\overset{\beta }{\mathop{CH}}\,}}\,}}\,\to -\overset{\alpha }{\mathop{C{{H}_{\begin{smallmatrix}  2 \\  I({{1}^{{}^\circ }}) \end{smallmatrix}}}}}\,-\overset{+}{\mathop{C{{H}_{2}}}}\,<\] \[\overset{+}{\mathop{C{{H}_{2}}}}\,\to -\underset{C{{H}_{3}}}{\mathop{\underset{\begin{smallmatrix}  | \\  \uparrow  \end{smallmatrix}}{\mathop{CH}}\,}}\,\to -\overset{{}}{\mathop{C{{H}_{\begin{smallmatrix}  2 \\  II({{1}^{{}^\circ }}) \end{smallmatrix}}}}}\,-\overset{{}}{\mathop{C{{H}_{3}}}}\,<\] \[C{{H}_{3}}\to -\underset{\underset{\text{C}{{\text{H}}_{\text{3}}}}{\mathop{\begin{smallmatrix}  \text{ }\!\!|\!\!\text{ } \\  \uparrow  \end{smallmatrix}}}\,}{\mathop{CH}}\,-\leftarrow \overset{+}{\mathop{C{{H}_{\begin{smallmatrix}   \\  III({{2}^{{}^\circ }}) \end{smallmatrix}}}}}\,-\leftarrow C{{H}_{3}}<\] \[C{{H}_{3}}\to -\underset{\underset{\text{C}{{\text{H}}_{\text{3}}}}{\mathop{\begin{smallmatrix}  \text{ }\!\!|\!\!\text{ } \\  \uparrow  \end{smallmatrix}}}\,}{\mathop{\overset{+}{\mathop{C}}\,}}\,-\leftarrow C{{H}_{\begin{smallmatrix}  2 \\  IV({{3}^{{}^\circ }}) \end{smallmatrix}}}-\leftarrow C{{H}_{3}}\] The stability of carbocation decreases in the order\[3{}^\circ \text{ }>\text{ }2{}^\circ \text{ }>\text{ }1{}^\circ \]. Out of the two \[1{}^\circ \]carbocations, (II) carbocation has an electron donating \[C{{H}_{3}}\] group at \[\alpha \]-carbon while carbocation\[(I)\]has a \[C{{H}_{3}}\] group at \[\beta \]-carbon. Since, \[+I\] effect decreases with distance, therefore carbocation (II) is more stable than carbocation (I).