Isobaric and Isochoric Activation Parameters for the Water-Catalyzed Hydrolysis of p-Methoxyphenyl 2,2-Dichloropropionate in Typically Aqueous Solutions

Article abstract of DOI:10.1021/ja00387a038

Isobaric thermodynamic activation parameters (ΔG_p<*>, ΔH_p<*>, ΔS_p<*>) and volumes of activation (ΔV<*>) have been measured for the water-catalyzed hydrolysis of p-methoxyphenyl 2,2-dichloropropionate (1) in water and in t-BuOH-H2O (n_H2O = 0.950 and 0.925) and 2-n-butoxyethanol-H2O (n_H2O = 0.980) at 25 deg C.These data, in combination with the isobaric thermal expansivities (α) and the isothermal compressibilities (β) of the solvent, were used to calculate the isochoric activation parameters ΔU_v<*> and ΔS_v<*>.It is found that the solvent dependence of the sets (ΔH_p<*>, ΔS_p<*>) and (ΔU_v<*>, ΔS_v<*>) is widely different as a result of the large changes in α/β and in ΔV<*>.The results are interpreted mainly in terms of hydrophobic interactions between 1 and the organic cosolvent and the large electrostriction, as expressed in the strongly negative ΔV<*> values.The analysis is facilitated by consideration of thermodynamic parameters for transfer of a model substrate (p-nitrophenyl propionate) from water to the t-BuOH-H2O system.It is concluded that both isobaric and isochoric activation parameters contain fundamental information about the solvation changes during the activation process.There is no compelling evidence to prefer either of these sets for an analysis of kinetic solvent effects on the hydrolysis of 1 in the solvent systems used.