QSAR analyses of oxidation and reduction rates of environmental organic pollutants in model systems

Redox transformations of organic pollutants in the aquatic environment are often the net result of reaction of the substrate with several oxidizing or reducing agents. In order to cope with this multiplicity of reaction pathways, model systems are very useful because they allow one to evaluate the possible contributing reactions in isolation. In a suitable model system, the rate law and rate constant for a particular reaction of environmental interest can be determined. Good correlations (quantitative structure-activity relationships or QSARs) are often found between rate constants for a redox reaction of closely related organic substrates and their one-electron oxidation or reduction potentials. Such QSARs have been observed for reduction of substituted nitrobenzenes by hydroquinones and by iron porphyrins in the presence of reduced sulfur species. Under appropriate conditions, half-wave potentials are proportional to one-electron redox potentials and can be used as an alternative predictor variable in QSAR analysis of redox reactions. This has been done successfully for oxidation of substituted phenols by singlet oxygen. These QSARs can be used to predict reaction rate constants for unstudied compounds. In addition, QSARs developed from model systems are also powerful tools for evaluating the kinetics of redox reactions of organic pollutants and for developing and extending deterministic models of environmental redox reactions of organic pollutants.