Which best describes the difference between SN1 and SN2 reactions?

The distinction between SN1 and SN2 reactions primarily lies in their mechanisms. SN1 reactions are characterized by a two-step process. The first step involves the formation of a carbocation intermediate after the leaving group departs, which can be a slower and rate-determining step. This intermediate can then be attacked by a nucleophile in the subsequent step. This mechanism allows the rates of SN1 reactions to depend only on the concentration of the substrate, which is why it is categorized as unimolecular nucleophilic substitution.

In contrast, the SN2 mechanism happens in a single concerted step where the nucleophile attacks the substrate at the same time as the leaving group departs, making it a bimolecular process. The transition state of the SN2 reaction involves both the substrate and the nucleophile, and the rate depends on both reactants.

Understanding this fundamental difference in their mechanisms is crucial for predicting the outcomes of these types of reactions, including factors like substrate structure and nucleophile strength.