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

Abstract

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 EtO-M+ have been proposed to react with 5. The second-order rate constants for the reactions with EtO- (k EtO-) and EtO-M+ (kEtO-M+) have been calculated from ion-pairing treatments. The kEtO- and k EtO-M+ values decrease in the order kEtO-Na+>k EtO-K+>kEtO-Li+>kEtO-, indicating that ion-paired EtO-M+ 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.