Question

$KI$ in acetone undergoes $S_{N}2$ reaction with each of P, Q, R, S. The rate of reaction follows the order:

• A. P > Q > R > S
• B. S > P > R > Q
• C. P > R > Q > S
• D. S > R > Q > P

Answer 1

The correct option is D S > R > Q > P

Rate of this reaction is governed by steric factor and $\delta +$ charge created at carbon atom bearing leaqving group.
Rate of $S_{N}2$ follows the order: $3^o>2^o>1^o>\text{methyl}$
S will be most reactive as electron withdrawing $>CO$ group increase the $\delta +$ charge at electrophilic carbon centre.
P is the least reactive as it has tertiary carbon.
So the order becomes S > R > Q > P