Effect of Alkali Metal Ions on Alkaline Ethanolysis of 2-Pyridyl and 4-Pyridyl Benzoates in Anhydrous Ethanol

Abstract

Pseudo-first-order rate constants (<TEX>$k_{obsd}$</TEX>) have been measured for nucleophilic substitution reactions of 2-pyridyl benzoate 5 with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol. The plots of <TEX>$k_{obsd}$</TEX> vs. <TEX>$[EtOM]_o$</TEX> are curved upwardly but linear in the excess presence of 18-crown-6-ether (18C6) with significant decreased <TEX>$k_{obsd}$</TEX> values in the reaction with EtOK. The <TEX>$k_{obsd}$</TEX> value for the reaction of 5 with a given EtONa concentration decreases steeply upon addition of 15-crown-5-ether (15C5) to the reaction medium up to ca. [15C5]/<TEX>$[EtONa]_o$</TEX> = 1, and remains nearly constant thereafter, indicating that <TEX>$M^+$</TEX> ions catalyze the reaction in the absence of the complexing agents. Dissection <TEX>$k_{obsd}$</TEX> into <TEX>$k_{EtO^-}$</TEX>- and <TEX>$k_{EtOM}$</TEX>, i.e., the second-order rate constants for the reaction with the dissociated <TEX>$EtO^-$</TEX> and the ion-paired EtOM, respectively has revealed that ion-paired EtOM is 3.2 - 4.6 times more reactive than dissociated <TEX>$EtO^-$</TEX>. It has been concluded that <TEX>$M^+$</TEX> ions increase the electrophilicity of the reaction center through a 6-membered cyclic transition state. This idea has been examined from the corresponding reactions of 4-pyridyl benzoate 6, which cannot form such a 6-membered cyclic transition state.