Increasing order of reactivity of the following compounds for S N 1 substitution is:

${\text{SN}}_1$ reactivity order $\propto$ stability of carbocation

SN1 is unimolecular because the slow step of the reaction involves only the substrate, the reaction is unimolecular. Because only the substrate is present in the transition state, the rate of the reaction depends only on its concentration and not on the concentration of the nucleophile.