In general, SN1 substitution can take place when a stable carbocation can be formed. If not, the reaction follows the SN2 mechanism.
The SN1 with the removement of a hydroxide-ion out of the molecule, in this case tert-Butanol. By this, a positively charged carbocation with a planar geometry is formed. This step is also the rate-determing step because it is the slowest step in this reaction.
In the , the haloanion bound to the carbocation, and a neutral haloalkane is formed. With this step, the hydroxy-substituent is replaced by a halogen-substituent.
