Hydroxyl free radical reactivity toward aqueous chlorinated phenols

Second-order kinetic constants (kOH,S) for the hydroxyl free radical attack on a series of nine chlorophenols (2-chlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,4,5-trichlorophenol, 2,4,6-trichlorophenol, 2,3,5,6-tetrachlorophenol, 2,3,4,5-tetrachlorophenol, 2,3,4,6-tetrachlorophenol, and pentachlorophenol) in aqueous solution were determined by means of the competitive kinetics method. Experimental values ranged from 3.5×1009 M−1s−1 for pentachlorophenol to 8.2×1009 M−1s−1 for 2-chlorophenol. A general trend of lower kinetic constant values with higher degree of chlorination was observed and tested with four different correlations using Hammett's σ values, the number of chlorine substituents on the aromatic ring, and estimated diffusion coefficients. These correlations were statistically significant (α=0.01), although the regression coefficients were moderate (R2<0.6). Statistical analysis indicated that all correlations were equally valid. This report includes second-order kinetic constants not previously reported for six chlorophenols, and also provides the means to estimate constants for other chlorophenols for which no experimental data are available.