The SN1 reaction is a type of nucleophilic substitution reaction where the rate-determining step involves the formation of a carbocation intermediate. This process occurs in two main steps: first, the leaving group departs, creating a positively charged carbocation, and then a nucleophile attacks this carbocation to form the final product. The SN1 mechanism is favored in polar protic solvents, which stabilize the carbocation and the leaving group. Typically, tertiary substrates are more reactive in SN1 reactions due to their ability to stabilize the carbocation through hyperconjugation and inductive effects.