Hydroxyl radical rate constants

Bakken, G. A., Jurs, P. C. Prediction of hydroxyl radical rate constants from molecular structure. Chem. Inf. Comput. Sci. 1999, 39,1064-1075. 

Winer, A.M., Damall, K.R., Atkinson, R. Pitts, Jr., J.N. (1979) Smog chamber study of the correlation of hydroxyl radical rate constants with ozone formation. Environ. Sci. Technol. 7, 622-626. 

Klamt, A. (1993) Estimation of gas-phase hydroxyl radical rate constants of organic compounds from molecular orbital calculations. Chemosphere 26, 1273-1289. 

Armbrust K.L. Pesticide hydroxyl radical rate constants measurements and estimates of their importance in aquatic environments. Environ. Toxicol Chem., 19(9) 2175-2180, 2000. 

Klamt, A. Estimation of Gas-Phase Hydroxyl Radical Rate Constants of Organic Compounds from Molecular Orbital Calculations, Chemosphere, 26, 1273-1289 (1993). 

Rate constants for the reaction of hydroxyl radicals with different compounds were determined by Haag and Yao (1992) and Chramosta et al. (1993). In the study of Haag and Yao (1992) all hydroxyl radical rate constants were determined using competition kinetics. The measured rate constants demonstrate that OH0 is a relatively nonselective radical towards C-H bonds, but is least reactive with aliphatic polyhalogenated compounds. Olefins and aromatics react with nearly diffusion-controlled rates. Table 4-3 gives some examples comparing direct (kD) and indirect (kR) reaction rate constants of important micropollutants in drinking water. 

Hammett s equation was also established for substituted phenols from the elementary hydroxyl radical rate constants. The Hammett resonance constant was used to derive a QSAR model for substituted phenols. The simple Hammett equation has been shown to fail in the presence of electron-withdrawing or electron-donating substituents, such as an -OH group (Hansch and Leo, 1995). For this reason, the derived resonance constants such as o°, cr, and o+ were tested in different cases. In the case of multiple substituents, the resonance constants were summed. Figure 5.24 demonstrates a Hammett correlation for substituted phenols. The least-substituted compound, phenol, was used as a reference compound. Figure 5.24 shows the effects of different substituents on the degradation rates of phenols. Nitrophenol reacted the fastest, while methoxyphenol and hydroxyphenol reacted at a slower rate. This Hammett correlation can be used to predict degradation rate constants for compounds similar in structure. 

Hammett s equation for rate constants of substituted phenols reacting with hydroxyl radical Experimental conditions elementary hydroxyl radical rate constants (l/m sec 108), pH 9. 

Correlation of hydroxyl radical rate constants for aromatics in water vs. Hammett s o constants. (From Haag, W.R. and Yao, C.C.D., Environ. Sci. Technol., 26(5), 1005-1013, 1992. With permission.)... 

Davenport, J. and Mill, T., An SAR for estimating hydroxyl radical rate constants for organic chemicals, Natl. Mtg. Am. Chem. Soc., Div. Environ. Chem., 26(2), 156-161, 1986. 

Figure 9.25 shows the correlation coefficient of 0.9636 between the hydroxyl radical rate constants and UV/Ti02 rate constants for chlorinated alkanes. Therefore, the reaction mechanism for the degradation of chlorinated alkanes by UV/Ti02 is similar to the reaction with hydroxyl radical. This correlation suggests that the reaction proceeds via hydroxyl radical attack on the chlorinated alkane. 

Correlation between hydroxyl radical rate constants and UV / 1i02 rate constants for substituted alcohols. 

Bakken, G.A. and Jurs, PC. (1999a). Prediction of Hydroxyl Radical Rate Constants from Molecular Structure. J.Chem.lnf.Comput.ScL, 39,1064-1075. 

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