Question

In the given $S_{N}2$ reaction rate is fastest, when (X) is:

• A. $-OH$
• B. $-N{H}_2$
• C.
• D.

Answer 1

The correct option is D

In both $S_{N}1$ and $S_{N}2$, the rate of the reaction depends on the ability of the leaving group.
Leaving group ability $\propto$ stability of anion.
Thus, more stable anion can leave easily.
In (d), the anion is resonance stabilised and have 3 equivalent resonating structures. Hence, it is the most stable anion

In (a), (b) and (c) there is no resonance stabilisation for the anion formed. Thus, they are poor leaving group.
Hence, the rate of reaction is fast when X is (d)