Arrange the following in decreasing order of reactivity in S_N2 reaction:

Step-by-step solution

The reactivity of an alkyl halide in an S_ N 2 reaction is governed by steric hindrance at the reacting carbon and the stabilization of the transition state. Compound (S) is an -haloketone. The adjacent carbonyl group has a strong electron-withdrawing nature ( -I effect) which increases the electrophilic character of the reacting carbon. Furthermore, the adjacent system strongly stabilizes the S_ N 2 transition state through orbital overlap, making it the most reactive among the given compounds. Compound (P) is a b