Alkali Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Benzoate with Alkali Metal Ethoxides in Anhydrous Ethanol: Unusually High Na₊ Ion Selectivity

Alkali Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Benzoate with Alkali Metal Ethoxides in Anhydrous Ethanol: Unusually High Na₊ Ion Selectivity

Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for nucleophilic substitution reactions of 5-nitro-8-quinolyl benzoate (5) with alkali metal ethoxides, EtO?M+ (M+ = Li+, Na+ and K+) in anhydrous ethanol (EtOH) at 25.0 0.1 C. The plots of kobsd vs. [EtO?M+] exhibit upward curvatures, while the corresponding plots for the reactions of 5 with EtO?Na+ and EtO?K+ in the presence of complexing agents, 15-crown-5-ether and 18-crown-6-ether are linear with rate retardation. The reactions of 5 with EtO?Na+ and EtO?Li+ result in significant rate enhancements on additions of Na+ClO4, indicating that the M+ ions behave as a catalyst. The dissociated EtO and ion-paired EtOM+ have been proposed to react with 5. The second-order rate constants for the reactions with EtO (kEtO) and EtOM+ (kEtOM+) have been calculated from ion-pairing treatments. The kEtO and kEtOM+ values decrease in the order kEtONa+ > kEtOK+ > kEtOLi+ > kEtO, indicating that ion-paired EtOM+ species are more reactive than the dissociated EtO ion, and Na+ ion exhibits the largest catalytic effect. The M+ ions in this study form stronger complex with the transition state than with the ground state. Coordination of the M+ ions with the O and N atoms in the leaving group of 5 has been suggested to be responsible for the catalytic effect shown by the alkali metal ions in this study.