Radicals rate constants

No systematic studies of a number of compoimds have yet appeared to discover correlations suggestive of mechanism. This paper presents the fractional conversions and reaction rates measured under reference conditions (50 mg contaminants/m ) in air at 7% relative humidity (1000 mg/m H2O), for 18 compounds including representatives of the important contaminant classes of alcohols, ethers, alkanes, chloroethenes, chloroalkanes, and aromatics. Plots of these conversions and rates vs. hydroxyl radical and chlorine radical rate constants, vs. the reactant coverage (dark conditions), and vs. the product of rate constant times coverage are constructed to discern which of the proposed mechanistic suggestions appear dominant. 

Figure 1 kci vs. koH- Second order gas phase rate constants for the reaction of Cl atoms vs. the corresponding OH radicals rate constants for the reactions with a. n-alkanes [11] b. n-alcohols [12] c. n-ethers [12] d. chloroethenes [13] and e. 1-chloroalkanes [14],... 

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

Reactivity fraction for free radicals Rate constant Rate of reaction Time... 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

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. 

For the 2-methyl-3-phenyl-3-(diphenylphosphatoxy)-2-propyl radical rate constants were obtained for the complete set of processes, including fragmentation to the contact ion pair, collapse of the contact ion pair to the rearranged... 

Chemical/Physical. Anticipated products from the reaction of epichlorohydrin with ozone or OH radicals in the atmosphere are formaldehyde, glyoxylic acid, and C1CH20(0)0HCH0 (Cupitt, 1980). Haag and Yao (1992) reported a calculated OH radical rate constant in water of 2.9 x 10 /M-sec. 

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. 

Atkinson, R W. P. L. Carter, A. M. Winer, and J. N. Pitts, Jr., An Experimental Protocol for the Determination of OH Radical Rate Constants with Organics Using Methyl Nitrite Photolysis as an OH Radical Source, J. Air Pollut. Control Assoc., 31, 1090-1092 (1981). 

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

Tuazon, E. C., W. P. L Carter, R. Atkinson, and J. N. Pitts, Jr., The Gas-Phase Reaction of Hydrazine and Ozone A Nonphotolytic Source of OH Radicals for Measurement of Relative OH Radical Rate Constants, Int. J. Chem. Kinet., 15, 619-629 (1983a). 

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. 

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