Computational Comparison of S(N)()2 Substitution Reactions of CHX(*-)() and CH(2)()X(-)() with CH(3)()X (X = Cl, Br). Do Open-Shell and Closed-Shell Anions React Differently?
The S(N)2 displacement reaction of the radical anions (CHCl(*-) and CHBr(*-)) and the closed-shell anions (CH(2)Cl(-) and CH(2)Br(-)) with CH(3)Cl and CH(3)Br were studied with density functional theory. It was determined that the anions CH(2)Cl(-) and CH(2)Br(-) were more reactive than the radical anions CHCl(*-) and CHBr(*-), in agreement with experiment. The degree of charge transfer in the reactant complex is the best indicator of the reactivity trend. It was found that two reactions (CH(2)Cl(-) with CH(3)Cl and CH(3)Br) proceeded without an activation barrier at the B3LYP/6-31+G(d) level. The backside transition states (leading to inversion of configuration at the methyl group) were 20-30 kcal/mol lower in energy than the frontside transition states (leading to retention of configuration). Interestingly, it was found that the mechanism of backside and frontside attack by radical anions was different. In backside attack, the radical anion leads with the lone pair directed toward the methyl group to form an incipient two-center, two-electron bond. In frontside attack, the radical anion leads with the unpaired electron directed toward the methyl group to form an incipient two-center, one-electron bond.
McKee
Michael L.
ML
Department of Chemistry, Auburn University, Auburn, Alabama 36849.