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

Pseudo‐first‐order rate constants (kobsd) 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 kobsd 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 kobsd trend. The kobsd 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, kobsd 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 kobsd 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. Choose the right path: The reaction of 1− with EtOM is strongly inhibited by the formation of stable complex 2, and proceeds unimolecularly via ketene 3 as the reactive intermediate and also through a bimolecular reaction of complex 2 with dissociated EtO−.