Question

Which one is most reactive towards $S_{N}1$ reaction?

• A. $C_6{H}_{5}C{H}_{2}Br$
• B. $C_6{H}_{5}CH(C_6{H}_5)Br$
• C. $C_6{H}_{5}CH(C{H}_3)Br$
• D. $C_6{H}_{5}C(C{H}_3)(C_6{H}_5)Br$

Answer 1

Answer: (D)

$S_{N}1$ reaction involves the formation of carbonation intermediate. More the stability of carbonation, more is the reactivity of alkyl/aryl halides towards $S_{N}1$ reaction. The intermediate carbonations formed by given halides are as:
$C_6{H}_{5}CH\left(C_6{H}_5\right)Br\to {\left(C_6{H}_5\right)}_2\stackrel{+}{C}H+B{r}^{-}{C}_6{H}_{5}CH\left(C{H}_3\right)Br\to C_6{H}_5\stackrel{+}{C}H\left(C{H}_3\right)+B{r}^{-}\text{(c) }{C}_6{H}_{5}C\left(C{H}_3\right)\left(C_6{H}_5\right)Br\to {\left(C_6{H}_5\right)}_2\stackrel{+}{C}\left(C{H}_3\right)+B{r}^{-}{C}_6{H}_{5}C{H}_{2}Br\to C_6{H}_5\stackrel{+}{C}{H}_2+B{r}^{-}$
The order of stability of these carbocations is as
${\left(C_6{H}_5\right)}_2\stackrel{+}{C}\left(C{H}_3\right)>{\left(C_6{H}_5\right)}_2\stackrel{+}{C}H>C_6{H}_5\stackrel{+}{C}H\left(C{H}_3\right)>C_6{H}_5{\stackrel{+}{C}}_2$
Thus, $C_6{H}_{5}C\left(C{H}_3\right)\left(C_6{H}_5\right)Br$ is most reactive towards $S_{N}1$ reaction.