KEAM 2014 Physics and Chemistry Paper

With increase in the size of halogen atom in R - X, (where X=halogen) bond length increases or R - X bond strength decreases . Thus rate of reaction in SN1 as well as SN2 reaction increases . The order of size of halogen atom is
I > Br > Cl > F
Thus, the order of reactivity is
R ─ I > R ─ Br > R ─ Cl > R ─ F