Question

Which of the following will give the fastest nucleophilic substitution reaction by $S_{N}2$ pathway ?

• A.
• B.
• C.
• D.

Answer 1

The correct option is A

$S_{N}2$ reaction is a concerted reaction which goes through transition state. So the rate of reaction is governed by stability of transition state. Greater is the stability faster will be the rate. When a carbonyl is present adjacent to halogen it stabilise the transition state at a greater extent. Therefore, $S_{N}2$ reaction will be faster in case of option (a).
Another thing is that addition of nucleophile at carbonyl carbon can also be possible but for that, halide should not be a leaving group. Here halide is $B{r}^{-}$ which is a good leaving group so substitution takes place instead of addition.