A kinetic study on ethylaminolysis of phenyl y-substituted-phenyl carbonates: Effect of leaving-group substituents on reactivity and reaction mechanism

Abstract

A kinetic study on nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5j) with ethylamine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC is reported. The plots of kobsd vs. [amine] are linear for the reactions of substrates possessing a strong electron-withdrawing group (EWG) but curve upward for those of substrates bearing a weak EWG, indicating that the electronic nature of the substituent Y in the leaving group governs the reaction mechanism. The reactions have been concluded to proceed through a stepwise mechanism with one or two intermediates (a zwitterionic tetrahedral intermediate T± and its deprotonated form T-) depending on the nature of the substituent Y. Analysis of Brønsted-type plots and dissection of kobsd into microscopic rate constants have revealed that the reactions of substrates possessing a strong EWG (e.g., 5a-5f) proceed through T± with its formation being the rate-determining step, while those of substrates bearing a weak EWG (e.g., 5g-5j) proceed through T± and T-.