We and others have used the alkyl lactates methyl lactate and ethyl lactate during studies of lactate dehydration to acrylics for ease of delivery of lactate to gas phase reactors. We noted that aqueous solutions of these alkyl lactates spontaneously hydrolyze at room temperature.
Kinetic description of the hydrolysis of alkyl lactates has been previously 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. Both methyl lactate and ethyl lactate show the same kinetic behavior although slower reactions were observed for ethyl lactate samples versus the same concentration methyl lactate sample. A dual kinetic regime best describes the observed hydrolysis behavior. At lower lactate concentrations, a kinetic model where the reaction was catalyzed by a solvated proton from dissociated lactic acid fits the data best, while at higher lactate concentrations, the best fitting kinetic model includes an additional water molecule participating in the rate-determining step by facilitating the formation and/or collapse of the tetrahedral intermediate.
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