Explain the mechanism of nucleophilic unimolecular substitution (SN¹). Reaction with one example.

In SN¹ reaction rate depends upon only the concentration of the alkyl halide and independent of nucleophile $\text{ Rate }\propto \left[\mathrm{RX}\right]$

Mechanism :- It takes place in two steps
Step-I: The alkyl halide Ionises to give carbonium ion and Br^–. This is slow step and is rate determining step.

rate ∝ [tertiary butyl halide]
Step -II: In the second step , the carbonium ion combines with nucleophile to form the final substituted product and it is fast step.