Radical reactions rate constants

Kwok ESC, R Atkinson, J Arey (1995) Rate constants for the gas-phase reactions of the OH radical with dichlorobiphenyls, 1-chlorodibenzo-p-dioxin, 1,2-dimethoxybenzene, and diphenyl ether estimation of OH radical reaction rate constants for PCBs, PCDDs, and PCDFs. Environ Sci Technol 29 1591-1598. 

Kwok, W.S.C., Atkinson, R. (1995) Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure reactivity relationship an update. Atmos. Environ. 29, 1685-1695. 

Throughout this work, we will refer to the heats of reactions and the methods used for the kinetic determinations. We have collected background information for discussion in this section. A list of important bond dissociation energies is followed by brief descriptions of the more important kinetic methods used for determining radical reaction rate constants discussed in this review. 

FIGURE 3-13 Relations between conversion of nitric oxide to nitrogen dioxide and ozone, atomic oxygen, and hydroxyl-radical reaction rate constants. Reprinted with permission from Grosjean. ... 

Atkinson, R. Estimation of OH radical reaction rate constants and atmospheric lifetimes for polychlorinated biphenyls, dibenzo-/rdioxins, and dibenzofmans. Environ. Sci Technol, 21(3) 305-307, 1987a. 

Hodson J. 1988. The estimation of the photodegradation of organic compoundsby hydroxyl radical reaction rate constants obtained from nuclear magneticresonance spectroscopy chemical shift data. Chemosphere 17 2339- 2348. 

Baxley, J. S M. V. Henley, and J. R. Wells, The Hydroxyl Radical Reaction Rate Constant and Products of Ethyl 3-Ethoxypropionate, hit. J. Chem. Kinet., 29, 637-644 (1997). 

Hodson, J., The Estimation of the Photodegradation of Organic Compounds by Hydroxyl Radical Reaction Rate Constants Obtained from Nuclear Magnetic Resonance Spectroscopy Chemical Shift Data, Chemosphere, 17, 2339-2348 (1988). 

Kwok, E. S. C., and R. Atkinson, Estimation of Hydroxyl Radical Reaction Rate Constants for Gas-Phase Organic Compounds Using a Structure-Reactivity Relationship An Update, Atmos. Enriron., 29, 1685-1695 (1995). 

Oberg, T. (2005) A QSAR for the hydroxyl radical reaction rate constant validation, domain of application, and prediction. Atmos. Environ., 39, 2189—2200. 

OH radical reaction rate constants for phenol were estimated by De et al. (1999). 

The effect of pH on the hydroxyl radical reaction rate constants was studied in buffered solution at pH = 3.5, 7.0, and 11.0. As the pH increases above 9, the presence of alternate scavenger HOy will react with hydroxyl radicals, and the rate constant is nearly 300 times that of H202. The data for haloben-zenes showed that the reactivity of hydroxyl radical with these derivatives does not vary much among the compounds studied, and an average rate constant of 5.0 x 109 M 1 s 1 could be used for these compounds. 

De, A.K., Chaudhuri, B., Bhattacharjee, S., Dutta, B.K., Estimation of OH radical reaction rate constants for phenol and chlorinated phenols using UV/H202 photo-oxidation, /. Haz. Mat., 64, 91-104, 1999. 

Estimation Methods for OH Radical Reaction Rate Constants... 

Based on direct spectroscopic measurements of OH radical concentrations at close to ground level, peak daytime OH radical concentrations are typically (3-10) x 106 molecule cm-3 (see, for example, Brauers et al., 1996 Mather et al., 1997 Mount et al., 1997). A diur-nally, seasonally, and annually averaged global tropospheric OH radical concentration has been derived from the emissions, atmosphere concentrations, and OH radical reaction rate constant for methyl chloroform (CH3CC13), resulting in a 24-hr average OH radical concentration of 9.7 x 10s molecule cm 3 (Prinn et al., 1995). 

For the majority of gas-phase organic chemicals present in the troposphere, reaction with the OH radical is the dominant loss process (Atkinson, 1995). The tropospheric lifetime of a chemical is the most important factor in determining the relative importance of transport, to both remote regions of the globe and to the stratosphere, and in determining the possible buildup in its atmospheric concentration. Knowledge of the OH radical reaction rate constant for a gas-phase organic compound leads to an upper limit to its tropospheric lifetime. 

To date, OH radical reaction rate constants have been measured for 500 organic compounds (Atkinson, 1989, 1994, 1997). However, many more organic chemicals are emitted into the atmosphere, or formed in situ in the atmosphere from photolysis or chemical reactions of precursor compounds, for which OH radical reaction rate constants are not experimentally available. Thus the need to reliably calculate OH radical reaction rate constants for those organic compounds for which experimental data are not currently available. 

The method is based on the observations that gas-phase OH radical reactions with organic compounds proceed by four reaction pathways, assumed to be additive H-atom abstraction from C-H and O-H bonds, OH radical addition to >C=C< and -C=C-bonds, OH radical addition to aromatic rings, and OH radical "interaction" with N-, S-, and P-atoms and with more complex structural units such as ->P=S, >NC(0)S- and >NC(0)0- groups. The total rate constant is assumed to be the sum of the rate constants for these four reaction pathways (Atkinson, 1986). The OH radical reactions with many organic compounds proceed by more than one of these pathways estimation of rate constants for the four pathways follow. Section 14.3.5 gives examples of calculations of the OH radical reaction rate constants for the "standard" compounds lindane (y-hexachlorocyclohexane), trichloroethene, anthracene, 2,6-di-ferf-butylphenol, and chloropyrofos. 

As examples of the calculation of OH radical reaction rate constants using the method discussed above (Kwok and Atkinson, 1995), the OH radical reaction rate constants for lindane [y-hexachlorocyclohexane cyclo-(-CHCl-)6], trichloroethene (CHC1=CC12), 2,6-di-tert-butylphenol, and chloropyrofos appear below. As the section dealing with OH radical addition to aromatic rings mentions, at present the rate constant for the reaction of the OH radical with anthracene (and other PAH) cannot be estimated with the method of Kwok and Atkinson (1995). In carrying out these calculations, one first must draw the structure of the chemical (the structures are shown in the appendix to Chapter 1). Then one carries out the calculations for each of the OH radical reaction pathways which can occur for that chemical. 

To date, no experimentally measured OH radical reaction rate constant for lindane at room temperature is available in the literature for comparison. 

The OH radical reaction rate constant for 2,6-di-ferf-butylphenol is given by ... 

OH radical reaction with chloropyrofos is anticipated to proceed via OH radical addition to the pyridine ring, OH radical "interaction" with the P=S group [with a group rate constant k >P=S (Table 14.4)], and H-atom abstraction from the two -OCH2CH3 groups bonded to the P atom. The total OH radical reaction rate constant is given by ... 

Becker, K.H., Biehl, H.M., Bruckmann, P., Fink, E.H., Filhr, F., Klopffer, W., Zellner, R., Zetzsch, C. (1984) Hydroxyl radical reaction rate constants and tropospheric lifetimes of selected environmental chemicals. Kernforschungsanlage. Jillich, GmbH. November 1984, ISSN 0343-7639. 

Oxidation photooxidation t,/2 = 2.4-6.0 d, based on estimated rate constant for the vapor-phase reaction with hydroxyl radicals in the atmosphere (Atkinson 1985 quoted, Howard 1991) measured hydroxy radical reaction rate constant for dicamba 4.8 x 1012 M 1 /h (Armbrust 2000). Hydrolysis t,/2 > 133 d for 2 pg mL 1 to hydrolyze in dark sterile pond water at 37-39°C (Scifres et al. 1973 quoted, Muir 1991) ... 

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