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Alkali metal ion catalysis and inhibition in alkaline ethanolysis of O-Y-substituted-phenyl O-phenyl thionocarbonates: contrasting M+ ion effects upon changing electrophilic centre from C=O to C=S
- Alkali metal ion catalysis and inhibition in alkaline ethanolysis of O-Y-substituted-phenyl O-phenyl thionocarbonates: contrasting M+ ion effects upon changing electrophilic centre from C=O to C=S
- Um, Ik-Hwan; Kang, Ji-Sun; Dust, Julian M.
- Ewha Authors
- SCOPUS Author ID
- Issue Date
- Journal Title
- CANADIAN JOURNAL OF CHEMISTRY
- CANADIAN JOURNAL OF CHEMISTRY vol. 95, no. 1, pp. 45 - 50
- alkali metal ion catalysis and inhibition; alkaline ethanolysis; electrophilic centre; reaction mechanism; Yukawa-Tsuno equation
- CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS
- SCIE; SCOPUS
- Document Type
- Pseudo-first-order rate constants (k(obsd)) were measured for nucleophilic substitution reactions of O-Y-substituted-phenyl O-phenyl thionocarbonates (4a-4h) with alkali metal ethoxides (EtOM, M = Li, Na, or K) in anhydrous ethanol at 25.0 +/- 0.1 degrees C. Plots of k(obsd) vs. [EtOM] exhibited upward curvature for the reaction of O-4-nitrophenyl O-phenyl thionocarbonate (4a) with EtOK in the presence of 18-crown-6-ether (18C6), but showed downward curvature for the reaction with EtOLi, indicating that the reaction is catalyzed by the 18C6-crowned K+ ion, but is inhibited by Li+ ion. The k(obsd) values were dissected into k(EtO)- and k(EtOM), the second-order rate constant for the reaction with dissociated EtO- and ion-paired EtOM, respectively. The reactivity of EtOM toward 4a increases in the order EtOLi < EtONa < EtO- < EtOK < EtOK/18C6, which is in contrast to that reported previously for the corresponding reaction of 4-nitrophenyl phenyl carbonate (a C=O analogue of 4a), e.g., EtO- approximate to EtOK/18C6 < EtOLi < EtONa < EtOK. The reaction mechanism, including the transition-state model and the origin of the contrasting reactivity patterns found for the reactions of the C=O and C=S compounds, are discussed.
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