Experimental and theoretical study on the kinetics and mechanism of the amine-catalyzed reaction of oxiranes with carboxylic acids

Abstract

In this work, a systematic study on the kinetics and mechanism of ring-opening reaction of oxirane by carboxylic acid initiated by a tertiary amine is presented. Kinetic parameters of β-hydroxypropyl ester formation including reaction orders, rate constants, and activation energies were established at the temperature range 323–353 K. The experimental values of Δ^≠H° and Δ^≠S° are characteristic for the S_N2-like processes. In the initial reaction system, the acid, oxirane and amine exist mainly as hydrogen-bonded complex acid-oxirane, free oxirane and free base. H-bonding was analyzed using IR-spectroscopy. The reaction pathways were examined by the density functional theory (DFT) method at the B3LYP/6-31+G** level. Optimized equilibrium configurations of transition states and corresponding activation parameters were established. In accordance to both experimental and theoretical approaches, it is reasonable to suggest that amine-catalyzed ring-opening reaction of oxirane by carboxylic acid is a series of parallel consecutive stages: (1) quaternization of tertiary amine by activated oxirane; (2) carboxylate anion participation in ring-opening of both nonactivated and activated oxirane. The kinetic model, which adequately describing all observations, is proposed.