In an SN1 reaction, what is the rate-limiting step?

In an SN1 reaction, the rate-limiting step is the formation of the carbocation. This is because the reaction mechanism begins with the departure of the leaving group, which results in the formation of a positively charged carbocation intermediate. The stability of this carbocation plays a critical role in determining the rate of the overall reaction.

The formation of the carbocation is the slowest step in the mechanism, which is why it is considered rate-limiting. Once the carbocation is formed, the subsequent attack by the nucleophile occurs rapidly, as nucleophiles typically react quickly with positively charged species. Thus, the rate of the reaction depends mainly on how easily the leaving group can depart to create the carbocation.

The transition state formation and deprotonation process, while necessary components of the reaction, do not serve as the rate-limiting step in an SN1 mechanism. The key factor driving the reaction rate is the initial step of carbocation formation, making it the most critical part of determining the overall reaction kinetics.