$C_{6} H_{5} C H\left(C_{6} H_{5}\right) B r \rightarrow$$\left(C_{6} H_{5}\right)_{2} \stackrel{+}{C} H+B r^{-} C_{6} H_{5} C H\left(C H_{3}\right) B r \rightarrow$$ C_{6} H_{5} \stackrel{+}{C} H\left(C H_{3}\right)+B r^{-}$
$C_{6} H_{5} C\left(C H_{3}\right)\left(C_{6} H_{5}\right) B r \rightarrow$$\left(C_{6} H_{5}\right)_{2}\stackrel{+}{C}\left(C H_{3}\right)+B r^{-} C_{6} H_{5} C H_{2} B r \rightarrow$$ C_{6} H_{5}\stackrel{+}{C} H_{2}+B r^{-}$
The order of stability of these carbocations is as
$\left(C_{6} H_{5}\right)_{2} \stackrel{+}{C}\left(C H_{3}\right)>\left(C_{6} H_{5}\right)_{2} \stackrel{+}{C} H$ $>C_{6} H_{5} \stackrel{+}{C} H\left(C H_{3}\right)>C_{6} H_{5} \stackrel{+}{C} H_{2}$
Thus, $C_{6} H_{5} C\left(C H_{3}\right)\left(C_{6} H_{5}\right) B r$ is most reactive towards ${S_{{N^1}}}$ reaction.
$(I)\,C{H_3}C{H_2}C{H_2}Cl$ $(II)\,C{H_2} = CH - CH(Cl)C{H_3}$
$(III)\,C{H_3}C{H_2}CH(Cl)C{H_3}$
$R - Br + C{l^ - }\xrightarrow{{DMF}}R - Cl + B{r^ - }$ ,
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