question_answer

\[{{H}_{3}}C-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\mathop{CH}}\,-CH=C{{H}_{2}}+HBr\to A\] A (Predominantly) is

A) \[C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\mathop{CH}}\,-C{{H}_{2}}-C{{H}_{2}}Br\]

B) \[C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\overset{\begin{smallmatrix}  Br \\  | \end{smallmatrix}}{\mathop{C}}}\,-C{{H}_{2}}C{{H}_{3}}\]

C) \[C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  Br \end{smallmatrix}}{\mathop{CH}}\,-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\mathop{CH}}\,-C{{H}_{3}}\]

D) \[C{{H}_{3}}-\underset{\begin{smallmatrix}  | \\  C{{H}_{3}} \end{smallmatrix}}{\mathop{CH}}\,-\underset{\begin{smallmatrix}  | \\  Br \end{smallmatrix}}{\mathop{CH}}\,-C{{H}_{3}}\]

Correct Answer: B

Solution :

Key Idea: The mechanism of electrophilic addition reaction is consistent with the occurence of rearrangement leading to more stable carbocation. The order of stability of carbocation is as: \[T>S>P>C{{H}_{3}}\] \[{{H}_{3}}C-\underset{\begin{smallmatrix}  | \\  H \end{smallmatrix}}{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-CH=C{{H}_{2}}+HBr\xrightarrow[-Br]{{}}\] \[{{H}_{3}}C-\underset{\begin{smallmatrix}  {{3}^{o}}\,carbocation \\  (more\,stable) \end{smallmatrix}}{\mathop{\underset{\oplus }{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-}}\,C{{H}_{2}}-C{{H}_{3}}\xrightarrow[{}]{B{{r}^{-}}}\] \[{{H}_{3}}C-\underset{\begin{smallmatrix}  | \\  Br \end{smallmatrix}}{\overset{\begin{smallmatrix}  C{{H}_{3}} \\  | \end{smallmatrix}}{\mathop{C}}}\,-C{{H}_{2}}-C{{H}_{3}}\]