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 wit hout any mixing solvent. Alkanoate anions and ethanol are the products of t he reaction. A dynamic model is proposed that describes quantitatively this kinetic behavior. The model includes the main processes occurring in the b iphasic medium and the corresponding thermodynamic calculations of the aver age size and stoichiometry of the molecular aggregates. Modeling indicates that salting-in and solvent effects caused by the alkanoate anions and etha nol determine the autocatalytic kinetics in the hydrolysis of C-4 ethyl est er where no aggregation occurs. In the C-5 to C-8 experiments, ester-contai ning micelles (ECM) are mainly formed by cooperative aggregation of alkanoa te anions with ester molecules. ECM is formed only after a threshold concen tration of the alkanoate anion has been reached. In a phase-transfer-like p rocess, ECM carries ester molecules into the aqueous phase, where hydrolysi s 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 t ransient storage of alkanoate anions in a reservoir has been assumed to exp lain this delay. Collective adsorption of alkanoate anions at the oil-water interface, which occurs without any threshold concentration, could play th e role of such a transient storage. The model also shows that empty micelle s are without any kinetic importance, since they are formed at the end of r eaction after the ester is completely depleted.