Hydroxyl radical reaction rates, 245, Table

TABLE 6.15 Hydroxyl Radical Reactions Rate Constants in Aqueous Solution... 

In the interest of conserving space in this handbook, a compact tabular presentation format has been adopted. Table 5.1.5.1 lists the chemical name, and its freon number (if applicable), molecular formula, molar weight and melting and boiling points. These data are available for virtually all substances in this group. Also shown in this table is the availability, expressed as a tick mark, of data on vapor pressure, solubility in water, octanol-water partition coefficient (Kqw) and the second order reaction rate constant with hydroxyl radicals. This rate constant is the critical determinant of persistence in the atmosphere. Tables 5.1.5.2 to Table 5.1.5.5 list the compounds and give the available property data with citations. 

The key feature we wish to examine in this alternate path is the competition of CO with hydrocarbon for the hydroxyl radicals. The rate constant for OH + C3H6 suggested in Reference 24 is about equal to that suggested in Reference 50 for OH CO. We tried a wide range of (OH + CO)-rates holding the basic system at the values in Table I. Since Reaction (15) is fast, Reactions (14) and (15) were added to give the overall reaction... 

Absolute Rate Constants. Absolute rate constants for the hydroxyl radical reactions, as determined from the formation curves of the hydroxycyclohexadienyl radicals, are summarized in Table I. Detailed data for benzoate ion are shown in Table II. In all cases the rate curves fit closely to a first order rate law. A detailed examination of this case seems warranted not only as an example of the data, but because of the possible use of this reaction as a reference reaction in competition kinetics. 

Table IX.4 gives the distribution of isopentane hydroxylation products in various systems, including photochemical hydroxylation by hydrogen peroxide, where free hydroxyl radicals are known to be formed as intermediates. It is clear that the ratios of selectivities with respect to the site of attack (1° 2° 3°) in the systems involving and ions are very close to each other and to the selectivity observed for the attack by hydroxyl. This leads to the conclusion [47c] that the same active species (presumably hydroxyl radicals) participates in all these systems. This is also supported by a low isotope effect (kuJkn 1.2-1.3, when comparing reactions of C6H12 and C6D12). The yield of cyclohexane hydroxylation products depends on the nature of the solvent. The high yields in acetonitrile as compared with the other solvents are to be expected, provided the hydroxyl radicals are the intermediate active species. The rate constant of the hydroxyl radical reaction with cyclohexane is 5 lO dm mol s , with...

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).

Peroxyl radicals with a strong oxidative effect along with ROOH are continuously generated in oxidized organic compounds. They rapidly react with ion-reducing agents such as transition metal cations. Hydroxyl radicals react with transition metal ions in an aqueous solution extremely rapidly. Alkyl radicals are oxidized by transition metal ions in the higher valence state. The rate constants of these reactions are collected in Table 10.5. 

Very recently, rate constants for scavenging of hydroxyl radicals by DMPO, and by the nitrone [18c], have been determined (Marriott et al., 1980) (see Table 5). As might be expected, the figures are close to the diffusion-controlled limit. The report of this work includes a concise and informative discussion of some of the difficulties with, and limitations of, the spin trapping method, especially where these relate to reactions involving hydroxyl radicals. 

Just as the fate of H radicals is crucial in determining the rate of the H2—02 reaction sequence in any hydrogen-containing combustion system, the concentration of hydroxyl radicals is also important in the rate of CO oxidation. Again, as in the H2—02 reaction, the rate data reveal that reaction (3.44) is slower than the reaction between hydroxyl radicals and typical hydrocarbon species thus one can conclude—correctly—that hydrocarbons inhibit the oxidation of CO (see Table 3.1). 

Table HI illustrates that cobalt behaves as an extraordinary catalyst in its reaction with MCPBA increasing the rate by a factor of 400,000 and reducing the activation from 27 to 9.5 kcal/mol. However, cobalt also greatly enhances the selectivity in the system (Table HI). The yield to the desired acid increases from 89% to 100% with the expected decrease in the by-products. The thermal decomposition of MCPBA, equation 2, releases the hydroxyl radical which can easily attack the acetic acid forming carbon dioxide and methyl acetate.

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. 

Table I. Trace Gas Rate Constants and Lifetimes for Reaction with Ozone, Hydroxyl Radical, and Nitrate Radical...

Any oxidation process in which hydroxyl radical is the dominant species is defined as an advanced oxidation process (AOP). For any oxidation reaction, two factors determine the rate of reaction. First, if a reaction has a high free energy or high electrical potential, the reaction is very likely to occur and it is considered to be thermodynamically favorable. The oxidation potentials for common oxidants suitable for environmental applications are listed in Table 4.1. 

Decomposition experiments for these CPs listed in Table 14.11 were carried out by the simultaneous action of UV radiation and Fenton s reagent (Benitez et al., 2000). Table 14.11 shows the first-order rate constants and half-lives. During the photo-Fenton s reagent reaction, the single photodecomposition rate constant, ku decreased as the number of chlorine substituents increased. In addition, combined rate constants, ku are much greater than the radical reaction constants, k,. Therefore, this confirms the additional contribution of the radical reaction due to generation of the hydroxyl radicals by Fenton s... 

Table 2 Rate Constants of the Reaction Between the Hydroxyl Radical and Organic Compounds in Water...

Table 1 Rate Constants for Reaction with Hydroxyl Radical for Selected Compounds...

Rate constants have been determined for the reduction of hydrogen peroxide by iron(II) and a number of iron(II) complexes. These rate constants have been compiled in Table 2. It is immediately clear that there is not much agreement between the results of various groups. However, there is a discernable trend metal complexes with more water-accessible coordination sites react faster. Graf et al. [117] have commented upon the importance of coordinated water molecules for the Fenton reaction. It is also clear that the rate of the Fenton reaction for a chelated complex near neutral pH is much faster than that of aqueous iron(II) at low pH. The use of the low-pH value of 16M ] s l in a recent calculation [118] of the flux of hydroxyl radicals in a cell gives an estimate that is at least two orders of magnitude too low. 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

The high efficiency of AOPs is supported on thermodynamic and kinetic grounds, due to the participation of radicals. The hydroxyl radical can attack virtually all organic compounds and it reacts 106-1012 times more rapidly than alternative oxidants such as O3. In Table 2, it can be observed that, after fluorine, HO is the most energetic oxidant. Table 3 shows that the reaction constant rates of different compounds with HO are several orders of magnitude higher than those with 03. However, we must emphasize that... 

Hydroxyl radical reacts very quickly with most organic substances, including free amino acids and amino acid residues in proteins. These reactions are diffusion-controlled and their rate constants are very high (Table 5). Hydroxyl radical reacts with low specificity with all amino acid residues, although tryptophan, tyrosine, histidine, and cysteine are particularly vulnerable. 

Hydroxyl radicals are highly reactive and are involved in nonspecific reactions with a wide range of compounds. These reactions involve moderate to moderately high second-order reaction rate constants (10 -10 sec ) between the radical and compounds as shown in Table 2 and represent a promising option for the remediation of hazardous chemicals. " Fenton s reagent remediation is discussed in detail later in this entry. 

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