Effect of pressure on the hydrolysis of methyl acetate and ethylene oxide in acetone + water mixtures: quantities of activation at constant volume, and the isokinetic pressure

Abstract

The extension of the concept of isokinetic conditions into the dimension of pressure is discussed. The use of the enthalpy and entropy of activation at constant pressure and of the energy and entropy of activation at constant volume to characterize activated processes is discussed, and it is concluded that neither set can be rigorously justified as easier to understand, but that the constantvolume parameters are probably to be preferred most of the time. The effect of pressure up to 3 kbar on the rate of the acid-catalyzed hydrolysis of methyl acetate and ethylene oxide in acetone+ water mixtures has been measured. The constant-volume parameters of activation for the acidcatalyzed hydrolysis of methyl acetate vary with solvent in a less complicated way than the constantpressure parameters, the minima in the constant-pressure parameters not being present in the constant-volume parameters. The existence of the minima in the constant-pressure parameters is ascribed mainly to the large change of thermal expansivity when an organic liquid is added to water.The effect of solvent on the rate of hydrolysis of methyl acetate is dependent on the pressure, and reverses sign at about 2 kbar. The implications of this observation in the determination of reaction mechanisms is discussed.

Effect of pressure on the rate of solvolysis of t-butyl chloride in ethanol-water mixtures

“…and a verification is desirable. It seems likely, however, as was concluded earlier for some other reactions (1,2), that the effect of solvent on the volume of activation cannot be accounted for by dielectric theory, unless saturation occurs.…”

“…1480 bar. The only comparable measurements of the effect of solvent and pressure on the activation volume are those on the solvolysis of benzyl chloride at 50 "C in ethanol-water mixtures by Hyne; Golinkin, and Laidlaw (10) and in glycerol-water by Gay and Whalley (29), and there is a partial analogy with the acid-catalyzed enolization of acetone and acetophenone in ethanol-water mixtures (I), and with the acidcatalyzed hydrolysis of methyl acetate and ethylene oxide in acetone-water mixtures (2). Spectacular effects of the kind described here occurred in none of these measurements.…”

High Pressure

“…For example, the effect of solvent on a reaction can be studied under pressure, and sometimes this leads to increased insight. The effect of solvent (acetone-water) concentra tion on the rate of the acid-catalyzed hydrolysis of methyl acetate changes sign at about 2 kb [Baliga et al (84)]. which shows that simple conclusions from the effect of solvent at 1 bar have little mechanistic value for this reaction, and, by implication, for other reactions also.…”

“…Conditions (pressure, temperature, solvent composition, etc.) under which particular reactions are isokinetic with respect to solvent composition have been observed (84), and isokinetic conditions with respect to pressure can be inferred in mixed solvents from the positive volume of activation of the reaction iso-Pr1 + 1311-� iso-Pr 1311 + 1-WHALLEY in 2-butanone and acetone, and the negative volumes in methanol, ethanol, and cyclohexanone [Ershov et al (85,86) …”

Effect of Pressure on the Alkaline Hydrolysis of Ethyl Acetate in Acetone–Water Solutions. Parameters of Activation at Constant Volume

“…In a number of papers from this laboratory, the effect of solvent composition in aqueous organic solvents on the enthalpy and entropy of activa-I tion for the neutral hydrolysis of benzyl (4a, 5,6) and tert-butyl (4b) chlorides, the acid-catalyzed 1 enolization of acetone and acetophenone (7), the acid-catalyzed hydrolysis of methyl acetate and ethylene oxide (8), and the base-catalyzed decomposition of diacetone alcohol (9, lo), have been found to appear simpler after the coordinate transformation from constant-pressure conditions to constant-volume conditions. In ~articu-lar, the minima that occur in the constant-pressure parameters are much smaller or are nonexistent in the constant-volume parameters.…”