Reactivity-selectivity relationships. Part 11. Effect of leaving group on selectivity in S_N2 reactions. a frontier orbital analysis

A perturbation molecular orbital treatment based on frontier orbitals is utilized to explain the selectivity order of a series of octyl, 1 -methylheptyl, and benzyl derivatives toward m-chloroaniline and ethanol in a nucleophilic substitution reaction. The observed selectivity increases in the order ROSO ₂Ar < RCl < RBr < Rl. The model suggests that it is the differential HOMO-LUMO gap for a given substrate and the two reacting nucleophiles that governs substrate selectivity. Implications of this result to the reactivity-selectivity principle are discussed.