Journal of Organic Chemistry p. 3294 - 3297 (1983)
Update date:2022-08-17
Topics:
Crowell, Thomas I.
The hydrolysis rate of 3,4-(methylenedioxy)-β-nitrostyrene (S) has been measured in H2O and in 99percent D2O buffer solutions from pH -0.9 to 10.6.The kinetic solvent isotope effect (KSIE), k(H2O)/k(D2O), is 6.2 at pH 2.5, where k is independent of pH in both solvents and attains a higher value, approximately 22, at about pH 6.2.The isotope effects ki(H2O)/ki(D2O) on the rate constants for the individual steps of the mechanism have been determined and their contributions to the overall KSIE evaluated.Accordingly, the KSIE on the pH-rate plateau at pH 1-4 can be separated into two factors: 5.0 for K12, the ionization constant of S as a pseudo-base in water; and 1.4 for k3H, the rate constant for rate-controlling protonation of the resulting anion by H3O(1+).At pH 6.2, the higher KSIE (7.6) on k3H2O becomes important while the uncatalyzed addition of water to the double bonds is partly rate controlling.The rate at the midpoint of a proton-inventory plot (49.5percent D2O) shows a negative deviation from linearity of 18percent.
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