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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 1
BKCSDE 35(1)
January 20, 2014 

Kinetic Study on Nucleophilic Substitution Reactions of 4-Nitrophenyl X-Substituted- Benzoates with Potassium Ethoxide: Reaction Mechanism and Role of K+ Ion
Song-I Kim, Min-Young Kim, Ik-Hwan Um*
Metal-ion catalysis, Electrophilicity, Nucleofugality, Stepwise mechanism, Hammett plot
A kinetic study on nucleophilic substitution reactions of 4-nitrophenyl X-substituted-benzoates (7a-i) with EtOK in anhydrous ethanol at 25.0 ± 0.1 oC is reported. The plots of pseudo-first-order rate constants (kobsd) vs. [EtOK] curve upward. Dissection of kobsd into the second-order rate constants for the reactions with the dissociated EtO– and ion-paired EtOK (i.e., kEtO and kEtOK, respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated EtO–. Hammett plots for the reactions of 7a-i with the dissociated EtO– and ion-paired EtOK exhibit excellent linear correlations with X = 3.00 and 2.47, respectively. The reactions have been suggested to proceed through a stepwise mechanism in which departure of the leaving-group occurs after the RDS. The correlation of the kEtOK/kEtO ratio with the X constants exhibits excellent linearity with a slope of –0.53. It is concluded that the ion-paired EtOK catalyzes the reaction by increasing the electrophilicity of the reaction center rather than by enhancing the nucleofugality of the leaving group.
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