PROTON INVENTORY STUDY OF A WATER-CATALYZED HYDROLYSIS

The neutral water reaction of bis(4-nitrophenyl) carbonate in H2O-D2O was subjected to a H+ inventory analysis. A plot of observed rate constants vs. the atom fraction of 2H is linear, indicating that a single H+ contributes to the solvent isotope effect of 2.24. This result is not consistent with a one-water mechanism nor with a two-water cyclic concerted mechanism but does agree with a two-water general base mechanism in which the transition state is reached at an early stage. Urea, tetraalkylammonium salts, KBr and other structure-making or structure-breaking additives cause only minor changes in the hydrolysis rates. Hydrolytic enzymes do not apparently achieve appreciable rate accelerations by perturbing the water structure at their active sites.