Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Benzoates with Potassium Ethoxide in Anhydrous Ethanol: Reaction Mechanism and Role of K size='1'>+ Ion

摘要

Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for the reactions of Ysubstituted- phenyl benzoates (5a-j) with potassium ethoxide (EtOK) in anhydrous ethanol at 25.0 ± 0.1 oC. The plots of kobsd vs. [EtOK] curve upward regardless of the electronic nature of the substituent Y in the leaving group. Dissection of kobsd into the second-order rate constants for the reactions with the dissociated EtO– and ion-paired EtOK (i.e., kEtO and kEtOK, respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated EtO–. The Brønsted-type plots for the reactions with the dissociated EtO– and ion-paired EtOK exhibit highly scattered points with lg = –0.5 ± 0.1. The Hammett plots correlated with o constants result in excellent linear correlations, indicating that no negative charge develops on the O atom of the leaving Ysubstituted- 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.