Hydroxyl rate constant

Figure 3a Maximum conversion in a single pass reactor and the initial rate (no TCE) vs. literature second order hydroxyl rate constants. (Same symbols as (2a)).

Jolly, G.S., Paraskevopoulos, G., Singleton, D.L. (1985) Rate of OH radical reactions. XII. The reactions of OH with c-C3H6, c-C5H10, and c-C7H14. Correlation of hydroxyl rate constants with bond dissociation energies. Int. J. Chem. Kinet. 17, 1-10. 

Jolly, G. S., G. Paraskevopoulos, and D. L. Singleton, Rates of OH Radical Reactions. XII. The Reactions of OH with c-C3H, c-C Hl(l, and c-C7Hu. Correlation of Hydroxyl Rate Constants with Bond Dissociation Energies, Int. J. Chem. Kinet., 17, 1-10... 

The pseudo-first order rate constants (resp. coefficients) for the direct reaction of some compounds may almost be in the order of typical hydroxyl rate constants (kR > 10 M s ), due to high concentrations of the pollutants as well as mass transfer enhancement. For example, Sotelo et al. (1991) measured values of 6.35 106 and 2.88 106 M l s"1 for the dissociating hydroxylated phenols, resorchinol (1,3-dihydroxybenzene) and phlorogluci-nol (1,3,5-tn hydroxybenzene) respectively (pH = 8.5 and T= 20 °C). 

The oxidation rates for bromoform were slower than the oxidation rates of unsaturated chlorinated aliphatic compounds, including the TCE. Because the hydroxylation rate constant of TCE is 109 Mr1 s 1 and the hydrogen abstraction of bromoform is 1.1 x 108 M 1 s aromatics and alkenes react more rapidly by hydroxyl addition to double bonds than does the more kinetically difficult hydrogen atom abstraction. No oxidative destruction of chloroform by Fenton s reagent was experimentally observed an explanation for this is that both H202 and Fe2+ have rate constants about one magnitude higher with respect to hydroxyl radicals than chloroform. 

Reactions with the Primary Species. In aqueous solution the monomers are exposed to the action of the species formed from the water in the primary act. The rate constants published for the reactions of the hydroxyl radicals and hydrated electrons are included in Table I. Most of the hydroxyl rate constants were measured using thiocyanate and are therefore subject to the usual uncertainties of this method (5). No rate constants appear to have been published for the reactions of the hydrogen atoms. 

Rate constants for methanol and ethyl alcohol relative to those for benzoate ion, phenylacetate ion and p-nitrobenzoate ion are shown in Table III. Each value in the table consists of experiments at five separate concentration ratios. The random uncertainty in each value is less than 10%. In determining these rate constants from optical density ratios it was necessary to make a small correction for the contribution to the optical density by the H-adduct free radical. The molar extinction coefficients at 340-350 m/x for the H-adduct and OH-adduct are similar for benzoic acid (22) and were assumed to be comparable for the other two aromatic ions in the table. The correction is necessary since the rate constants for the reaction of hydrogen atoms with the alcohols used are two orders of magnitude lower than the rate constants for hydrogen atom addition to the aromatic ring, while the analogous hydroxyl rate constants are roughly comparable. 

If the reaction is run in aqueous solutions, OH from the water can replace the halide as the aryl substituent, thus forming phenols 129). The reaction of the 2-benzoylphenyl radical to form the corresponding phenol is slower than the reaction forming the halo-aryl. The hydroxylation rate constant equals... 

Hydroxyl rate constant is the reaction rate constant of the solvent wifli hydroxyl radicals in the atmosphere. 

Hydroxyl radicals. The acid ionization constant of the short-lived HO transient is difficult to determine by conventional methods but an estimate can be made because HO, but not its conjugate base, O -, oxidizes ferrocyanide ions HO + Fe(CN) — OH- + Fe(CN)g . Use the following kinetic data26 for the apparent second-order rate constant as a function of pH to estimate Ka for the acid dissociation equilibrium HO + H20 =... 

Table I. Trace gas rate constants and lifetimes for reaction with ozone, hydroxyl radical, and nitrate radical. Lifetimes are based upon [O3]=40ppb [HO ]=1.0x10 molecules cm (daytime) [NO3 ]=10ppt (nighttime).

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

The lifetime of T is an inverse function of the hydroxyl radical concentration [HO>] and the rate constant kj for its reaction with a particular trace gas... 

These equations identify the dominant source and loss processes for HO and H02 when NMHC reactions are unimportant. Imprecisions inherent in the laboratory measured rate coefficients used in atmospheric mechanisms (for instance, the rate constants in Equation E6) can, themselves, add considerable uncertainty to computed concentrations of atmospheric constituents. A Monte-Carlo technique was used to propagate rate coefficient uncertainties to calculated concentrations (179,180). For hydroxyl radical, uncertainties in published rate constants propagate to modelled [HO ] uncertainties that range from 25% under low-latitude marine conditions to 72% under urban mid-latitude conditions. A large part of this uncertainty is due to the uncertainty (la=40%) in the photolysis rate of 0(3) to form O D, /j. 

Figure 8. Influence of the reaction rate constants on the isocyanate and hydroxyl decrease during curing model calculations.

The dominant transformation process for trichloroethylene in the atmosphere is reaction with photochemically produced hydroxyl radicals (Singh et al. 1982). Using the recommended rate constant for this reaction at 25 °C (2.36x10 cm /molecule-second) and a typical atmospheric hydroxyl radical concentration (5x10 molecules/cm ) (Atkinson 1985), the half-life can be estimated to be 6.8 days. Class and Ballschmiter (1986) state it as between 3 and 7 days. It should be noted that the half-lives determined by assuming first-order kinetics represent the calculated time for loss of the first 50% of trichloroethylene the time required for the loss of the remaining 50% may be substantially longer. 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

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 3a presents the maximum conversion and the initial rate of each compoimd versus the literatme second order rate constant for contaminant reaction with hydroxyl radicals. The conversion increases with increasing values of koH, but with considerable scatter. 

The conversion and initial rate in the presence and absence of TCE versus the product of the second order rate constant and the dark adsorption appear in Figure 4a,b. Figure 4a shows considerable scatter in the data, reveahng only broad, general trends between conversion or rate and hydroxyl second order rate constant. However, the plots of enhanced conversion and initial rate vs. the corresponding chlorine second order rate constant multiplied by the dark adsorption data are smoother (Figure 4b). 

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

Anbar, M. and Neta, P. (1967). A compilation of specific biomolecular rate constants for the reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds in aqueous solutions. Int. J. Appl. Radiat. Isot. 18, 493-497. 

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