Abstract: Nucleophilic substitution (S_N2) reactions constitute a core component of fundamental organic chemistry education. However, conventional teaching approaches often inadequately address allylic S_N2' reactions, resulting in students’ prevalent misconception that the S_N2 pathway represents the exclusive reaction mechanism. This study employs the CH₃S^-/CH₃O^- + CH₂=CHCH₂Br system as a pedagogical model, utilizing M06-2X methodology with SMD solvation models to quantitatively examine the competition between S_N2 and S_N2' pathways. Computational results reveal that the S_N2 pathway maintains significantly lower activation barriers than S_N2' in both gas phase and aqueous solution, with aqueous solvation substantially increasing the energy barriers for both pathways while preserving their relative selectivity. Further analysis demonstrates that the elevated barriers in solution arise from increased geometric distortion during the reaction process. Notably, aqueous conditions enhance the nucleophilicity of CH₃S^- relative to CH₃O^-, which correlates with sulfur’s smaller solvation-induced geometric distortion. Through visualization of transition state evolution and potential energy profiles, this approach facilitates students’ mechanistic understanding of competitive reaction pathways. The study establishes an effective “quantitative calculation-structural analysis-discussion” pedagogical framework, providing a successful case study for integrating computational chemistry into undergraduate chemistry education.

Key words: Teaching ideas, S_N2 reaction, Quantum chemical calculations, Potential energy profiles, Solvent effect