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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 29, Number 1
BKCSDE 29(1)
January 20, 2008 

 
Title
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
Author
Ik-Hwan Um*, Seung-Eun Lee, Yeon-Ju Hong, Jee Eun Park
Keywords
Catalysis, Alkali metal ion, Ion-pair, Electrophilicity, Nucleofugality
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? (kEtO?) and EtO?M+ (kEtO?M+) have been calculated from ion-pairing treatments. The kEtO? and kEtO?M+ values decrease in the order kEtO?Na+ > kEtO?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.
Page
117 - 0
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