Kinetic Study on Nucleophilic Substitution Reactions of 4-Nitrophenyl X-Substituted-Benzoates with Potassium Ethoxide: Reaction Mechanism and Role of K⁺Ion

Abstract

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 o C is reported. The plots of pseudo-first-order rate constants (k obsd ) vs.[EtOK] curve upward. Dissection of k obsd into the second-order rate constants for the reactions with the dissociated EtO -and ion-paired EtOK (i.e., k EtO  and k EtOK , 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 k EtOK /k EtO  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.