Which solvent and substrate combination most strongly favors SN1 over SN2?

• A. A primary alkyl halide in polar aprotic solvent
• B. A tertiary alkyl halide in polar protic solvent
• C. A methyl halide in polar protic solvent
• D. A secondary alkyl halide in polar aprotic solvent

Answer: (B)

SN1 reactions rely on forming a carbocation in the rate-determining step, and they are promoted when that carbocation is highly stabilized and the solvent stabilizes charged species. A tertiary alkyl halide forms a very stable tertiary carbocation, and a polar protic solvent strongly solvates and stabilizes both the leaving group and the developing charges, lowering the barrier to ionization. At the same time, SN2 requires a backside attack on a hindered carbon; the tertiary center is too crowded for effective SN2, and the protic solvent further decreases nucleophile reactivity, making SN2 even less favorable. So this combination most strongly pushes the reaction toward SN1. By contrast, primary or methyl halides do not form stable carbocations, and polar aprotic solvents favor SN2 by keeping nucleophiles highly reactive, making those scenarios less compatible with SN1.