Which statement best distinguishes SN1 and SN2 mechanisms?

• A. SN2 is unimolecular; SN1 is bimolecular.
• B. SN2 is concerted and bimolecular; SN1 proceeds via a carbocation and is unimolecular.
• C. SN2 occurs only with weak nucleophiles.
• D. SN1 rate depends on [nucleophile].

Answer: (B)

SN1 and SN2 differ in how the reaction proceeds and how the rate is determined. SN2 is a single, concerted step where the nucleophile attacks as the leaving group leaves, so the reaction rate depends on both the substrate and the nucleophile concentrations—bimolecular. SN1 proceeds in two steps: the leaving group leaves first to form a carbocation, then the nucleophile attacks the carbocation; the rate-determining step is the unimolecular formation of the carbocation, so the rate depends only on the substrate concentration.

That’s why the statement describing SN2 as concerted and bimolecular and SN1 as proceeding via a carbocation and being unimolecular is the best choice. The other statements misstate the kinetics or the mechanism: SN2 is not unimolecular; SN2 is not limited to weak nucleophiles; and SN1 rate does not depend on nucleophile concentration. Context: SN2 usually favors strong nucleophiles and less hindered substrates, while SN1 is favored by weak nucleophiles and more stable carbocation formation.