Nucleophile Effects on the E2/S_N2 Competition for the X^- + CH₃CH₂Cl Reactions: A Theoretical Study

The effect of nucleophiles on the gas-phase E2/S_N2 competition is still not completely elucidated, despite its importance in chemistry. In the current work, the electronic structure calculations of prototypical reactions X^- + CH₃CH₂Cl (X = OH, F, Cl, Br, and I) are performed at the MP2 level with aug-cc-pVDZ or ECP/d. The effects of nucleophiles on the competing E2 and S_N2 reactions in terms of the correlation between the barrier height and reaction energy, electronegativity of X, bond length, charge distribution, and proton affinity of anionic nucleophile X^- are explored and emphasized. As the nucleophile changes from OH^- to I^-, both S_N2 and E2 reactions become more exothermic, with the reaction energy in the ranges from -51.9 to 10.8 kcal mol^-1 (S_N2) and -36.8 to 38.0 kcal mol^-1(E2). For X^- = F^- and OH^-, the sequence of reactivity for the four pathways is ret-S_N2 < syn-E2 < anti-E2 ∼ inv-S_N2. However, for X^- = Cl^-, Br^-, and I^-, the anti-E2 barrier is much higher in energy (17.1-29.4 kcal mol^-1) than that of inv-S_N2. Energy decomposition analysis illustrates that the anti-E2 pathway possesses the highly destabilizing characteristic distortion, resulting in a larger reaction barrier and hence becoming a more unfavorable pathway than inv-S_N2. More interestingly, only ion-dipole complex exists in the entrance channel for reactions involving OH^-, Cl^-, Br^-, and I^-, and in contrast, a significant hydrogen-bonded complex formation is also revealed for X^- = F^-, which can further affect E2/S_N2 competition and atomic-level mechanisms, especially, for the isoelectronic nucleophile F^- and OH^-. It has been revealed here that electronegativity of central atoms in X and ionic radii of nucleophiles are the important factors affecting the entrance channel complex.