Solvent Effects on the Enthalpy and Entropy of Activation for the Hydrolysis of β-Lactones

Abstract

The hydrolysis of β-propiolactone and β-butyrolactone in binary water + dioxane mixtures was investigated by kinetic studies. The following conclusions were reached: First, β-propiolactone is more reactive than β-butyrolactone across the range of water + dioxane compositions. This observation was rationalized in terms of the electric charge flow caused by the β-butyrolactone’s methyl substituent. Second, hydrolysis of these lactones is essentially enthalpy controlled. Third, an increase in the dioxane percentage, which relaxes the intermolecular hydrogen bonds in the ordered structure of water, reduces the enthalpy of activation ΔH# and simultaneously increases the entropy of activation ΔS#(absolute value) for solvent compositions up to 60% dioxane. Fourth, plotting ΔH#/ΔS# against the solvent composition yields an N-shaped curve. This results is a consequence of the quadratic and cubic terms appearing in the expressions of ΔH# and ΔS# as functions of the solvent media composition. Fifth, an ABC classification was set up to characterize the behavior of ΔH#/ΔS# for the solvolysis of these lactones.