Temperature dependence of enthalpy, entropy, and heat capacity of activation in the neutral ester hydrolysis in 2-butoxyethanol–water solutions

Abstract

The kinetic behaviour of the neutral hydrolysis of methyl trifluoroacetate in 2-butoxyethanol–water solutions when x_w is close to 0.98 was studied and the calculated activation parameters have been compared with the kinetic data of the hydrolysis of chloromethyl dichloroacetate in the same conditions. In these hydrolyses not only ΔH^‡ and ΔS^‡ but also ΔC_p^‡ are highly temperature dependent, dΔC_p^‡/dT for x_w= 0.980 being +38 ± 2 and +67 ± 5 J mol^–1 K^–2 in the hydrolyses of methyl trifluoroacetate and chloromethyl dichloroacetate, respectively. Thus the temperature dependence of ΔC_p^‡ is less pronounced in the hydrolysis of methyl trifluoroacetate than in that of chloromethyl dichloroacetate. The temperature dependence of ΔC_p^‡ and the S-shaped Arrhenius plots in the hydrolyses studied in butoxyethanol–water mixtures can be explained on the basis of the formation of ‘moving units’ of clathrate hydratelike structures when the temperature is raised. Thus with increasing temperature the solvent effects on ΔC_p^‡, generally found in typically aqueous solutions, are surpassed by the effects of the transition to a microphase structure.