Nucleophile Effects on the E2/S N 2 Competition for the X - + CH 3 CH 2 Cl Reactions: A Theoretical Study.

The effect of nucleophiles on the gas-phase E2/S N 2 competition is still not completely elucidated, despite its importance in chemistry. In the current work, the electronic structure calculations of prototypical reactions X - + CH 3 CH 2 Cl (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 N 2 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 N 2 and E2 reactions become more exothermic, with the reaction energy in the ranges from -51.9 to 10.8 kcal mol -1 (S N 2) 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 N 2 < syn-E2 < anti-E2 ∼ inv-S N 2. 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 N 2. 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 N 2. 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 N 2 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.