Dual-Regime Reaction Kinetics of the Autocatalytic Hydrolyses of Aqueous Alkyl Lactates

Abstract

Kinetic description of the hydrolysis of alkyl lactates has been limited to acid-catalyzed conditions despite the spontaneous hydrolysis of methyl lactate and ethyl lactate in aqueous solution. As the reaction progresses, generated lactic acid further catalyzes ester hydrolysis, while the rate of the reverse esterification reaction also increases with the accumulation of acid product. The reaction sequence of lactate hydrolysis is described in three kinetic stages: initiation/neutral hydrolysis, autocatalytic hydrolysis, and equilibrium. The evolution of lactate hydrolysis was measured for varying temperatures (1.5 to 40 °C) and initial concentrations of methyl or ethyl lactate (1 to 40 mol %) to quantify the kinetic transitions between reaction stages. Lower temperatures resulted in a distinct induction period where negligible hydrolysis was observed. The effect of initial concentration on the length of the induction period was nonmonotonic and was divided into dilute (below about 6 mol % lactate) and concentrated (above about 6 mol %) regimes. Solutions of either lower or higher lactate concentration corresponded to longer induction periods and slower reactions. A dual kinetic regime best describes the observed hydrolysis behavior. For hydrolysis of alkyl lactates below 10 mol %, a rate law derived from the conventional ester hydrolysis mechanism effectively modeled behavior, while at higher lactate concentrations, an additional water molecule must be included in the rate-determining step to appropriately capture the hydrolysis behavior.