| Abstract
| - Phosphorothioate esters are phosphate esters in which one of the nonbridging oxygen atoms hasbeen replaced by sulfur. In the comparative hydrolysis reactions of phosphorothioate and phosphateesters, the sulfur substitution accelerates the rates of the monoesters while slowing the rates ofdiesters and of triesters. Previously measured enthalpies and entropies of activation for thehydrolysis reactions of the monoesters, p-nitrophenyl phosphate and p-nitrophenyl phosphorothioate,were compared to the activation parameters measured herein for the diesters, ethyl p-nitrophenylphosphate and ethyl p-nitrophenyl phosphorothioate, and the triesters, diethyl p-nitrophenylphosphate and diethyl p-nitrophenyl phosphorothioate. A consistent trend of a greater ΔH⧧ for thephosphorothioate analogue was found in all three classes of ester. In the monoester case, a morepositive ΔS⧧ arising from a mechanistic difference (DN + AN for the phosphorothioate versus ANDNfor the phosphate) compensates, resulting in a lower ΔG⧧ for the phosphorothioate monoester.Spectroscopic investigations indicate there is no significant difference in bond order to the leavinggroup in phosphates, as compared to their phosphorothioate analogues, ruling this out as acontribution to the consistently higher enthalpies of activation.
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