On what factors does the rate (reactivity) of the $S_{N}1$ reaction depend? Explain with suitable examples.

(N/A) The rate of an $S_{N}1$ reaction depends only on the concentration of the substrate. The rate-determining step is the formation of a carbocation. Therefore,the more stable the carbocation formed in the first step,the easier it is for the alkyl halide to form the carbocation,and the higher the rate of the $S_{N}1$ reaction.
$(a)$ Reactivity of $1^{\circ}, 2^{\circ}, 3^{\circ}$ alkyl halides: Tertiary $(3^{\circ})$ halides react fastest in $S_{N}1$ reactions because the $3^{\circ}$ carbocation is the most stable. The order of reactivity is: $3^{\circ} \text{ halide} > 2^{\circ} \text{ halide} > 1^{\circ} \text{ halide}$.
$(b)$ Allylic halides: Allylic halides show high reactivity towards $S_{N}1$ reactions because the allylic carbocation formed in the first step is resonance-stabilized. For example: $CH_{2}=CH-CH_{2}Cl \rightarrow CH_{2}=CH-CH_{2}^{+} + Cl^{-}$.
$(c)$ Benzylic halides: Benzylic halides are highly reactive towards $S_{N}1$ reactions because the benzylic carbocation formed after the departure of $Cl^{-}$ is resonance-stabilized by the benzene ring.