Effect of Alkali Metal Ions on Nucleophilic Substitution Reactions of 4-Nitrophenyl X-Substituted Benzoates with Alkali Metal Ethoxides in Anhydrous Ethanol

Pseudo-first-order rate constants (k(obsd)) have been measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl benzoate (5a), 4-nitrophenyl 4-methoxybenzoate (5b), and 4-nitrophenyl 4-hydroxybenzoate (5c) with alkali metal ethoxides, EtO-M+ (M+ = Li+, Na+ and K+) in anhydrous ethanol (EtOH) at 25.0 +/- 0.1 degrees C. The plots of k(obsd) vs. [EtO-M+] exhibit upward curvatures in all cases, indicating that M+ ions catalyze the reactions and ion-paired EtO-M+ species are more reactive than dissociated EtO-. Second-order rate constants for reactions with dissociated EtO- and ion-paired EtO-M+ (i.e., k(EtO)- and k(EtO)-(M+), respectively) have been calculated from ion-pair treatment for the reactions of 5a and 5b. However, such ion-pair treatment has failed to determine k(EtO)- and k(EtO)-(M+) values for the reactions of 5c. It has been concluded that reactions of 5a and 5b are catalyzed by one metal ion, which increases electrophilicity of the reaction center through coordination on the carbonyl oxygen. In contrast, reactions of 5c have been suggested to involve two metal ions, i.e., the one coordinated on the carbonyl oxygen increases the electrophilicity of the reaction center while the other one associated on the phenoxy oxygen decreases the charge repulsion between the anionic reagents (i.e., EtO- and deprotonated 5c). It has been found that the rate equation derived from the mechanism involving two metal ions fits nicely to the kinetic results obtained for the reactions of 5c.