SN2 mechanism is followed in case of primary and secondary alkyl halides i.e. SN2 reaction is favoured by small groups on the carbon atoms attached to halogen so,
CH3−X>R−CH2−X>R2CH−X>R3C−X
Primary is more reactive than secondary and tertiary alkyl halides. SN2 order :Methyl > Ethyl > Isopropyl > Tertiary butyl > Allyl > Benzyl