Single solvent molecule effect over S_N2 and E2 competition in the hydroperoxide anion reaction with ethyl-iodide

The influence of individual solvent molecules on the dynamics of competing reactions remains largely unexplored for many important chemical systems. Herein, direct dynamics simulations revealed that a single water molecule has multifaceted effects on the reaction between the hydroperoxide anion HOO⁻ and C₂H₅I. The introduction of one water reduced the overall reaction rate and shifted the preference from elimination (E2) to substitution (S_N2) reactions because of the differential solvation effect. Increasing the collision energy lowered the overall reactivity but did not change the S_N2-to-E2 pathway ratio. Notably, the additional water molecules also induced new competing pathways that used HO⁻ as an attacking nucleophile via proton transfer within the nucleophile HOO⁻(H₂O); here, both the HO⁻-E2 and HO⁻-S_N2 trajectories were observed at small percentages. The occurrence of the HO⁻ paths was driven by the extensive proton transfer within the pre-reaction complex well, but was suppressed by the entropy effect and increased barriers. In addition, water molecules complicated the reaction mechanisms, increased the percentage of indirect mechanisms, and affected the dynamic features of proton transfer. As in the solvent-free system, protons were frequently exchanged between the nucleophiles and substrates, whereas in the singly solvated system, proton exchange mainly occurred within the nucleophiles. This work highlights the dynamic role of solvent molecules and may have profound impacts on reaction dynamics, with relevance to organic synthesis and chemistry in biosystems, microdroplets, and aerosols.