In SN1 reactions, which intermediate forms during the rate-determining step?

• A. Carbocation
• B. Carbanion
• C. Radical
• D. Carbene

Answer: (A)

In SN1 reactions the slow, rate-determining step is the ionization of the substrate to form a carbocation. When the leaving group departs heterolytically, the electrons stay with the leaving group and a positively charged carbon—carbocation—remains. This carbocation is then attacked by the nucleophile in a fast follow-up step to give the product. The rate of the overall reaction depends only on the concentration of the substrate, since only this first ionization step governs the speed.

This is why the carbocation is the correct intermediate. A carbanion would require the carbon to carry a negative charge, which isn’t what results from the leaving-group departure in SN1. A radical would imply homolytic cleavage and a radical pathway, not the typical SN1 mechanism. A carbene is a highly reactive neutral species with two nonbonded electrons on carbon and is not formed as the common intermediate in SN1.