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

Abstract

Pseudo-first-order rate constants (<TEX>$k_{obsd}$</TEX>) have been measured spectrophotometrically for the reactions of Y-substituted-phenyl benzoates (5a-j) with potassium ethoxide (EtOK) in anhydrous ethanol at <TEX>$25.0{\pm}0.1^{\circ}C$</TEX>. The plots of <TEX>$k_{obsd}$</TEX> vs. [EtOK] curve upward regardless of the electronic nature of the substituent Y in the leaving group. Dissection of <TEX>$k_{obsd}$</TEX> into the second-order rate constants for the reactions with the dissociated <TEX>$EtO^-$</TEX> and ion-paired EtOK (i.e., <TEX>$k_{EtO^-}$</TEX> and <TEX>$k_{EtOK}$</TEX>, respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated <TEX>$EtO^-$</TEX>. The Br<TEX>${\phi}$</TEX>nsted-type plots for the reactions with the dissociated <TEX>$EtO^-$</TEX> and ion-paired EtOK exhibit highly scattered points with <TEX>${\beta}_{lg}$</TEX> = -<TEX>$0.5{\pm}0.1$</TEX>. The Hammett plots correlated with <TEX>${\sigma}^o$</TEX> 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 <TEX>$EtO^-$</TEX> and ion-paired EtOK proceed through a stepwise mechanism, in which departure of the leaving group occurs after the RDS, and that <TEX>$K^+$</TEX> ion catalyzes the reactions by increasing the electrophilicity of the reaction center through a four-membered cyclic TS structure.