Question

Which of the following compounds will show retention in configuration on nucleophile substitution by OH⁻ ion?

Options

• \[\begin{array}{cc}
  \ce{CH3 - CH - CH2Br}\\
  |\phantom{.....}\\
  \ce{C2H5}\phantom{..}
  \end{array}\]

• \[\begin{array}{cc}
  \ce{CH3 - CH - Br}\\
  |\\
  \phantom{...}\ce{CH3}
  \end{array}\]

• \[\begin{array}{cc}
  \phantom{....}\ce{Br}\\
  \phantom{...}|\\
  \ce{CH3 - C - H}\\
  \phantom{...}|\\
  \phantom{.......}\ce{C6H13}
  \end{array}\]

• \[\begin{array}{cc}
  \ce{CH3 - CH - Br}\\
  |\\
  \phantom{...}\ce{C6H5}
  \end{array}\]

Answer 1

\[\begin{array}{cc}
\ce{CH3 - CH - Br}\\
|\\
\phantom{...}\ce{C6H5}
\end{array}\]

Explanation:

Due to the presence of the +R effect −C₆H₅ group, carbocation formed by \[\begin{array}{cc}\ce{CH3 - CH - Br}\\|\\\phantom{...}\ce{C6H5}\end{array}\] is most stable and the reaction will undergo by S_N1 mechanism. Hence, the intended product is obtained.