Since SN1 reactions involve the formation of carbocation as intermediate in the rate determining step, more is the stability of carbocation higher will be reactivity of alkyl halides towards SN1 route. Now we know that stability of carbocations follows the order : 3∘>2∘>1∘,SN1 reactivity should also follow the same order. 3∘>2∘>1> Methyl (SN1. reactivity)