Explain the reactivity of `SN_1` reaction ?

Explanation

The order ofreactivity depends upon the stability of carbocation in the first step, Greater the stability of the carbocation greatenwill be its ease of formation form alkyl halide and hence faster will be the rate of reaction. As we know `3^@` carbocation is most stable therefore the tert-alkyl halide will undergo `SN^(1)` reaction very fast. The order of reactivity will be
Tertiary alkyl halide > Secondary alkyl halide > primary alkyl halide > Methyl halide