Kinetic and Theoretical Studies on Alkaline Ethanolysis of 4-Nitrophenyl Salicylate: Effect of Alkali Metal Ions on Reactivity and Mechanism

摘要

Pseudo-first-order rate constants (k_obsd) for reactions of 4-nitrophenyl salicylate (7) with alkali metal ethoxides (EtOM, M=K, Na, and Li) in anhydrous ethanol have been measured spectrophotometrically. Interestingly, the k_obsd value decreases significantly as the concentration of EtOM increases. Because the phenolic moiety of substrate 7 would be deprotonated and exist as an anionic form (i.e., 7−) under kinetic conditions, the ground-state stabilization of 7− through formation of a six-membered cyclic complex with M+ (i.e., 8) is proposed to be responsible for the decreasing k_obsd trend. The k_obsd value at a given concentration of EtOK increases steeply upon addition of [18]crown-6 ether (18C6) up to [18C6]/[EtOK]=1 in the reaction mixture and then remains relatively constant thereafter. In contrast, k_obsd decreases upon addition of salts (e.g., LiClO₄ or KSCN) to the reaction mixture, which indicates that M+ ions inhibit the reaction. However, in the presence of 18C6, the k_obsd value is independent of the concentration of EtOK but remains constant, which indicates that the reaction proceeds through a unimolecular mechanism in the presence of the complexing agent. Although two conceivable unimolecular pathways (formation of ketene 9 and lactone 10) can account for the kinetic results, the reaction has been concluded to proceed via formation of ketene 9 as the reactive intermediate on the basis of theoretical calculations.