KI in acetone undergoes `S_(N^(2))` reaction with each of P, Q, R, S. The rates of the reaction vary as :

Options

A. P gt Q gt R S
B. S gt P gt R gt Q
C. P gt R gt Q gt S
D. R gt P gt S gt Q

Answer

B

Explanation

Order of reactivity in `S_(N^(2))` reactions w.r.t. RX is
`1^(@)gt2^(@)gt3^(@)`
In these reactions bulky R-groups hinders the attack of nucleophile. The presence of Ph- CO-group in S stabilises the intermediate transition state and thus S is most reactive. Thus correc order is :
S gt P gt R Q