What is the first step of a unimolecular nucleophilic substitution (SN1) reaction?

In a unimolecular nucleophilic substitution (SN1) reaction, the first step involves the departure of the leaving group, leading to the formation of a carbocation. This step is crucial because it sets the stage for the subsequent nucleophilic attack. The leaving group must depart first to create an unstable carbocation intermediate, which is pivotal for the reaction mechanism.

Once the leaving group is gone, the carbocation is formed. This positively charged species is highly reactive and can then be attacked by a nucleophile in a later step, ultimately leading to the formation of the final product. The stability of the carbocation formed can significantly influence the reaction rate and is an important characteristic of SN1 reactions, as more stable carbocations (such as tertiary versus primary) will lead to faster reactions.

This sequential progression emphasizes why the departure of the leaving group is the hallmark of the first step in SN1 reactions, distinguishing it from bimolecular mechanisms where the nucleophile attacks simultaneously with the departure of the leaving group. This understanding incorporates the role that carbocation formation plays in dictating the pathway and kinetics of the reaction.