Question

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

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

Answer 1

The correct option is C $C_6{H}_{5}C\left(C{H}_3\right)\left(C_6{H}_5\right)Br$

$S_{N}1$ reaction proceed $via$ the formation of a carbocation intermediate.
More stable is the carbocation more reactive is the alkyl/aryl halide towards $S_{N}1$.
In $C_6{H}_5{C}^{+}\left(C{H}_3\right)C_6{H}_5)$ carbocation, the two phenyl rings by theirs $-R$ effect and $-C{H}_3$ by its $+I$ effect deminish the postive charge and make it stable.