Hydroxy radicals rate constants

Hydrolysis stable aqueous hydrolysis rate at pH 5, 7, pH 9 measured hydroxy radical rate constant for mecoprop k = 9.0 x 1012 M-1 Ir1 (Armbrust 2000)... 

Heberger K and Fischer H 1993 Rate constants for the addition of 2-hydroxy-2-propyl radical to alkenes in solution Int. J. Chem. Kin. 25 913-20... 

Under these conditions, a component with a low rate constant for propagation for peroxy radicals may be cooxidized at a higher relative rate because a larger fraction of the propagation steps is carried out by the more reactive (less selective) alkoxy and hydroxy radicals produced in reaction 4. 

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... 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

Chemical/Physical. Ozonation in water at 60 °C produced 7-formyl-1-indanone, 1-indanone, 7-hydroxy-l-indanone, l-indanone-7-carboxylic acid, indane-l,7-dicarboxylic acid, and indane-1-formyl-7-carboxylic acid (Chen et al, 1979). Wet oxidation of acenaphthene at 320 °C yielded formic and acetic acids (Randall and Knopp, 1980). The measured rate constant for the gas-phase reaction of acenaphthene with OH radicals is 8.0 x 10 " cmVmolecule-sec (Reisen and Arey, 2002). 

Chemical/Physical. In the gas phase, cycloate reacts with hydroxyl and NO3 radicals but not with ozone. With hydroxy radicals, cleavage of the cyclohexyl ring was suggested leading to the formation of a compound tentatively identified as C2H5(Cff0)NC(0)SC2H5. The calculated photolysis lifetimes of cycloate in the troposphere with hydroxyl and NO3 radicals are 5.2 h and 1.4 d, respectively. The relative reaction rate constants for the reaction of cycloate with OH and nitrate radials are 3.54 x lO " and 3.29 x 10 cm /molecule-sec, respectively (Kwok et al., 1992). 

Ghta, T. and Ghyatna, T. A set of rate constants for the reactions of hydroxy radicals with aromatic hydrocarbons. Bull Chem. Soc.Jpn., 58(10) 3029-3030, 1985. 

Table 4-4 Reaction rate constants for the reaction of hydroxy] radical with inorganic carbon.

Photolysis of 5,6-dihydro-5-selenophenyl-dTyd (Tallman et al. 1998) also affords the C (6) H -adduct, and in the presence of 02 the corresponding peroxyl radical [reactions (170) and (171)]. The latter may undergo H02 -elimination giving rise to Thd [reaction (172)] or, in the presence of tributyltinhydride, yields 5,6-dihydro-5-hydroxy-dTyd [reaction (173)]. The ratio of these two products depends on the tributyltinhydride concentration, and from such data the ratio of the rate constants of reactions (172) and (173) has been calculated at 1.3 x 10 2 mol dm-3. 

The reactions of butane-2,3-diol by HCF in alkaline medium using Ru(III) and Ru(VI) compounds as catalysts leads to similar experimental rate equations for both the reactions. The mechanism involves the formation of a catalyst-substrate complex that yields a carbocation for Ru( VI) or a radical for Ru(III) oxidation. The role of HCF is in catalyst regeneration. The rate constants of complex decomposition and catalyst regeneration have been determined.89 A probable mechanism invoving formation of an intermediate complex has been proposed for the iridium(III)-catalysed oxidation of propane- 1,2-diol and of pentane-1,5-diol, butane-2,3-diol, and 2-methylpentane-2,4-diol with HCF.90-92 The Ru(VIII)-catalyzed oxidation some a-hydroxy acids with HCF proceeds with the formation of an intermediate complex between the hydroxy acid and Ru(VIII), which then decomposes in the rate-determining step. HCF regenerates the spent catalyst.93... 

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) ... 

The latter reaction which has the activation energy about 85 kJ/mol seems to be more probable at higher temperatures at lower temperatures hydroxy radicals may appear in other reaction pathways as in the monomolecular decomposition of hydroperoxides, etc. Hydroxyl which are very mobile and very reactive propagate the reaction site only to a short distance (the rate constant of transfer reaction of HO radicals with CH3 group of ethane is equal 108 dm3 mol-1 s approximately at 20 °C). 

The data shown in Scheme 12.7 confirm that addition of (substituted) benzoyl radicals is slower than that of counter radicals (Scheme 12.7b), and therefore the benzoyl adducts have weak signals in the TR ESR spectra. A similar conclusion was obtained by FT TR ESR study of the photolysis of IRG2959 in the presence of NBA (Schemes 12.1 and 12.3)." ° ESR signals of two products of photolysis of IRG2959, namely, 2-hydroxy-2-propyl radical and substimted benzoyl radical, demonstrate quite different time dependences in the presence of acrylate. The signal of 2-hydroxy-2-propyl radical disappears faster with an increase in acrylate concentration, whereas benzoyl radical is practically unaffected by the presence of acrylate in concentrations up to 0.1 M." ° However, one can obtain only estimations of kinetic rate constants A add... 

Evidently, r of Pis can participate in multiple reactions, and the rate constants of many of these reactions have been measured.. sz.ssc.eo 2-Hydroxy-2-propy 1 radical reacts with TEMPO with a rate constant of 10 M s , a value that is somewhat lower than the rate constant Ardiff expected for diffusion-controlled reactions. In our opinion, it is quite possible that this reaction is limited by diffusion, but an absolute value is less than A diff due to steric limitations—screening off a reactive oxygen atom by methyl groups of TEMPO (pseudodiffusion reactions. )... 

The second-order rate constants for the reaction of OH radical with meta- and para-isomers of hydroxy-, methoxy-, chloro- and nitro-benzaldehydes are in the range (2.8 to 12) x 10 dm mol s . The higher rate in methoxybenzaldehyde is attributed to the activation of the ring by the electron-donating -OCH3 group. 

The 1,3-cyclohexadiene could not be prepared with higher purity than 98% and hence the analysis based on the final products is less meaningful. The yield of 3- and 4-hydroxycyclohexenes show that only 31% (0.18 pmolJ /0.58 pmolJ ) of the OH radicals add to the double bonds. There is no information about the missing 44% (100% — 25%—31%). Von Sonntag and coworkers suggested that the yield of hydroxy cyclohexenes is not indicative of the OH addition to the double bonds due to non-quantitative reaction of the ally lie radical 1 (equation 10) with RSH. Since, in the case of 1,4-cyclohexadiene, they found complete material balance, they concluded that the alky lie radical formed in reaction reacts quantitatively with the thiolic compound. Thus, radical 2 formed in reaction (11) will react quantitatively with RSH. The inefficiency of the reduction of the allylic radical by the thiol is probably due to the weak ally lie C—H bond which leads to a six orders of magnitude lower rate constant for the RSH-I- allylic radical reaction compared with the RSH-I- alkyl radical reaction. If all the material imbalance is due to incomplete reduction of the allylic radical, its formation is the main path of reaction of OH with 1,3-cyclohexadiene. 

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