Question

The order of reactivities of the following alklyl halides for a $S{N}^2$ reaction is

• A. RF > RCl > RBr > RI
• B. RF > RBr > RCl > RI
• C. RCl > RBr > RF > RI
• D. RI > RBr > RCl > RF

Answer 1

Answer: (D)

The reactivity of the alkyl halide is decided by the ease with which the halide leaves the substrate. As with the decrease in size of halogen, C - X bond energy increases and so reactivity of alkyl halide decreases.
It means the leaving ability, order is $I^{-}>B{r}^{-}>C{l}^{-}>F^{-}$ and the order of $S_{N}2$ ​ reaction is RI > RBr > RCl > RF.