A detailed thermodynamic analysis of the activation parameters for the simple hydrolysis of acetic anhydride in an acetonitrile/water cosolvent system is presented. The activation parameters are obtained using regression analyses of the Eyring rate equation under iso-mole fraction and isodielectric conditions. The iso-mole fraction Eyring plots are linear, indicating the activation enthalpy and entropy are both temperature independent under these conditions. However, the isodielectric Eyring plots are non-linear, and the analysis shows both the activation entropy and activation enthalpy are strongly temperature dependent under isodielectric conditions. The thermodynamic analysis is complemented by linear solvent energy analysis and computational studies of possible transition structures, the latter showing that a concerted six-member ring structure with a single water molecule is the likely transition state.

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This article presents the derivation of the thermodynamic expressions for the activation free energy and reaction rate constant for cosolvent reaction systems. These expressions account for the factors that are specific to solution-phase reactions, which include isotropic electrostatic effects and close-range solvent−solute interactions. This article discusses the idea that electrostatic effects can be correlated with the isotropic relative permittivity, and solvent−solute interactions can be correlated with the cosolvent mole fraction. This article also shows that this type of thermodynamic analysis is necessary for understanding certain nuances of solution-phase reaction processes not tractable by other types of analyses. K E Y W O R D Sactivation free energy terms, electrostatic effects, Eyring plots, kinetics, solvent-solute interactions Int J Chem Kinet. 2018;50:873-879.

Studies have shown that aqueous reactions generating a change in pH can be accurately monitored using a fast-response pH electrode. This technique has been successfully applied in this work to the aqueous hydrolysis of acetic anhydride, which is a reaction that has been studied using a variety of techniques for nearly one hundred years. Many of these techniques involve elaborate equipment and sophisticated analyses, making the pH technique an attractive alternative. Studies here have focused on the temperature effects of the simple hydrolysis and acetatecatalyzed hydrolysis reactions. Data analyses suggest the notion that if simple hydrolysis occurs by a two-step mechanism, it does so only at low temperatures, whereas acetate-catalyzed hydrolysis occurs almost assuredly by a single step mechanism. Results of this work yield the following values for the activation parameters for simple hydrolysis (subscripted with a "w") and acetate-catalyzed hydrolysis (subscripted with an "a") at atmospheric pressure: ΔH { w ¼ 39:9 0:7 ð ÞkJÁmol À1 ; ΔS { w ¼ À227 2 ð ÞJÁK À1 Ámol À1 ; ΔH { a ¼ 49:7 0:3 ð ÞkJÁmol À1 and ΔS { a ¼ À157 1 ð ÞJÁK À1 Ámol À1 . Implications of these results are discussed in this article.