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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 6
BKCSDE 35(6)
June 20, 2014 

K+ Ion Catalysis in Nucleophilic Displacement Reaction of Y-Substituted-Phenyl Picolinates with Potassium Ethoxide: Effect of Substituent Y on Reactivity and Transition State Structure
Hyun-Ju Im, Jieun Lee, Mi-Yeon Kim, Ik-Hwan Um*
Aryl picolinates, Metal-ion catalysis, Hammett plot, Electrophilicity, Nucleofugality
Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for the nucleophilic substitution reaction of Y-substituted-phenyl picolinates (7a-f) with potassium ethoxide (EtOK) in anhydrous ethanol at 25.0 ± 0.1 oC. The plot of kobsd vs. [EtOK] curves upward while the plot of kobsd/[EtO–]eq vs. [EtO–]eq is linear with a positive intercept in all cases. Dissection of kobsd into kEtO− and kEtOK (i.e., the second-order rate constants for the reactions with the dissociated EtO– ion and ion-paired EtOK, respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated EtO–. The σo constants result in a much better Hammett correlation than σ– constants, indicating that the reaction proceeds through a stepwise mechanism in which departure of the leaving group occurs after the rate-determining step (RDS). K+ ion catalyzes the reaction by increasing the electrophilicity of the reaction center through formation of a cyclic transition state (TS). The catalytic effect decreases as the substituent Y becomes a stronger electron-withdrawing group (EWG). Development of a positive charge on the N atom of the picolinyl moiety through resonance interactions is responsible for the decreasing K+ ion catalysis.
1749 - 1753
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