The order ofreactivities of the following alkyl halides for a ${\mathrm{S}}_{{\mathrm{N}}^2}$ reaction is

The reactivity increases with increasing size of halogen. More good the leaving group more will be reactivity of alkyl halide in ${\mathrm{SN}}_2$ since  order of leaving group is $\mathrm{I}>\mathrm{Br}>\mathrm{Cl}>\mathrm{F}$ So,order of reactivity of alkyl halide in ${\mathrm{Sn}}_2$ reaction is $\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}$.