What is the rate-determining step in SN1 reactions?

• A. Formation of the carbocation by loss of the leaving group
• B. Attack by the nucleophile
• C. Proton transfer step
• D. Rearrangement step

Answer: (A)

SN1 reactions are governed by a slow, unimolecular ionization step that creates a carbocation. The rate of the overall reaction is determined by this initial departure of the leaving group to form the carbocation, because that step has the highest energy barrier. Once the carbocation exists, the nucleophile can attack rapidly, delivering the product. The carbocation can rearrange to a more stable cation, but rearrangements happen quickly after the carbocation forms and do not set the pace of the reaction. The proton transfer step, when it occurs, can help the leaving group depart but is not the slow, rate-determining step. Thus, the rate-determining step is the formation of the carbocation by loss of the leaving group.