Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Benzoates with Potassium Ethoxide in Anhydrous Ethanol: Reaction Mechanism and Role of K~+ Ion

摘要

Pseudo-first-order rate constants (k_(obsd)) have been measured spectrophotometrically for the reactions of Y-substituted-phenyl benzoates (5a-j) with potassium ethoxide (EtOK) in anhydrous ethanol at 25.0 ± 0.1 °C. The plots of k_(obsd) vs. [EtOK] curve upward regardless of the electronic nature of the substituent Y in the leaving group. Dissection of k_(obsd) into the second-order rate constants for the reactions with the dissociated EtO~- and ion-paired EtOK (i.e., k_(EtO)- and k_(EtOK), respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated EtO~-. The Bronsted-type plots for the reactions with the dissociated EtO~- and ion-paired EtOK exhibit highly scattered points with β_(lg) = -0.5 ± 0.1. The Hammett plots correlated with σ° constants result in excellent linear correlations, indicating that no negative charge develops on the O atom of the leaving Y-substituted-phenoxide ion in transition state. Thus, it has been concluded that the reactions with the dissociated EtO~- and ion-paired EtOK proceed through a stepwise mechanism, in which departure of the leaving group occurs after the RDS, and that K~+ ion catalyzes the reactions by increasing the electrophilicity of the reaction center through a four-membered cyclic TS structure.