SN1 reaction is two steps reaction in which carbocation is formed as an intermediate in step I (rate determining step). Greater the stability of carbocation, greater will be its ease of formation from alkylhalide and faster will be the rate of reaction. Stahility of carbocation follows the order: tertiary > secondary > primary
due to decreasing +I-effect. Thus, the order of reactivity of haloalkanes towards SN1 reaction is tertiary halide > secondary halide > primary halide.