The order of reactivity of haloalkanes towards SN l reaction is tertiary halide > secondary halide > primary halide because the order of reactivity of SNl reactions depends on the stability of carbocation intermediate formed during nucleophilic substitution reaction, hence, compounds I, IV and VI are more reactive.
Whereas in II, III, V structures, 1∘ carbocation is form which is least reactive for SN2 reactive.