Kinetic Study on Nucleophilic Substitution Reaction of 5-Nitro-8-quinolyl Benzoate, Picolinate, Nicotinate and Isonicotinate with Alkali Metal Ethoxide: Effect of Nonleaving Group on Reactivity and Transition State Structure

摘要

Pseudo-first-order rate constants (k_(obsd)) have been measured spectrophotometrically for the reactions of 5-nitro-8-quinolyl nicotinate (4) and 5-nitro-8-quinolyl isonicotinate (S) with alkali metal ethoxides (EtOM; M = K, Na and Li) in anhydrous ethanol at 25.0 ±0.1 °C. The plots of k_(obsd) vs. [EtOM] curve slightly upward for the reactions with EtOK and EtONa but are linear for the reactions with EtOLi and for those with EtOK in the presence of 18-crown-6-ether. Dissection of k_(obsd) into k_(EtO~-) and k_(EtOM) (i.e., the second-order rate constants for the reactions with the dissociated EtO~- and ion-paired EtOM, respectively) has revealed that the reactivity increases in the order EtO~- = EtOLi < EtOK < EtONa for the reactions of 4 and EtOLi < EtO~- < EtOK < EtONa for the reactions of 5. Comparison of the kinetic results for the reactions of 4 and 5 with those reported previously for the corresponding reactions of 5-nitro-8-quinolyl benzoate (2) and picolinate (3) has revealed that the esters possessing a pyridine ring (i.e., 3-5) are significantly more reactive than the benzoate ester 2 due to the presence of the electronegative N atom (e.g., 2 3 < 4 < 5). It has been concluded that M~+ ion catalyzes the reactions of 3-5 by increasing the electrophilicity of the reaction center through a five-membered cyclic transition state (TS) for the reaction of 3 and via a four-membered cyclic TS for the reactions of 4 and 5.