Origin of autocatalysis in the biphasic alkaline hydrolysis of C-4 to C-8 ethyl alkanoates

The alkaline hydrolysis of C-4 to C-8 (butanoate to octanoate) ethyl esters shows autocatalytic kinetics when performed under two-phase conditions without any mixing solvent. Alkanoate anions and ethanol are the products of the reaction. A dynamic model is proposed that describes quantitatively this kinetic behavior. The model includes the main processes occurring in the biphasic medium and the corresponding thermodynamic calculations of the average size and stoichiometry of the molecular aggregates. Modeling indicates that salting-in and solvent effects caused by the alkanoate anions and ethanol determine the autocatalytic kinetics in the hydrolysis of C-4 ethyl ester where no aggregation occurs. In the C-5 to C-8 experiments, ester-containing micelles (ECM) are mainly formed by cooperative aggregation of alkanoate anions with ester molecules. ECM is formed only after a threshold concentration of the alkanoate anion has been reached. In a phase-transfer-like process, ECM carries ester molecules into the aqueous phase, where hydrolysis takes place yielding alkanoate anions. Additionally, in C-6 and C-7 ethyl ester hydrolysis, autocatalysis appears to be delayed, since acceleration only starts after the extent of hydrolysis has reached a certain level. A transient storage of alkanoate anions in a reservoir has been assumed to explain this delay. Collective adsorption of alkanoate anions at the oil-water interface, which occurs without any threshold concentration, could play the role of such a transient storage. The model also shows that empty micelles are without any kinetic importance, since they are formed at the end of reaction after the ester is completely depleted.