Kinetics of reactions of chlorine dioxide (OCIO) in water-II. Quantitative structure-activity relationships for phenolic compounds

Chlorine dioxide (OCIO) rapidly oxidizes substituted phenoxide anions in aqueous solution and second-order rate constants for this reaction have recently been reported for 25 compounds. Analysis of these data for correlations reveals several quantitative structure-activity relationships (QSARs) that can be used to predict rate constants for the reaction of additional phenolic compounds with chlorine dioxide. The rate constants for oxidation of phenoxide anions give good Hammett correlations to σ^- constants and Marcus correlations using half-wave oxidation potentials. Ortho- as well as meta- and para-substituted phenoxides have been included in both types of correlations and the only outliers are compounds that exhibit strong intramolecular hydrogen bonding. The results of the Marcus correlation analysis are consistent with the expected mechanism of reaction where the rate-limiting step is single electron-transfer from the phenoxide anion to chlorine dioxide. The second-order rate constants for oxidation of the undissociated forms of substituted phenols are about six orders of magnitude smaller than the corresponding values for phenoxide anions, indicating that only the reaction of phenoxide anions will be significant under the conditions of water treatment with chlorine dioxide. The rate constants for undissociated phenols also have much greater relative uncertainties than those for the phenoxide anions, and give much less satisfactory correlations.