Solvent Isotope Effects on the Kinetics of Nucleophilic Addition of Water to a β-Nitrostyrene

Article abstract of DOI:10.1021/jo00167a026

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 k_i(H2O)/k_i(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 k₃^H, the rate constant for rate-controlling protonation of the resulting anion by H3O(1+).At pH 6.2, the higher KSIE (7.6) on k₃^H2O 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.