Rate constant peroxy radical reactions

The most useful chain-breaking antioxidants react with peroxy radicals. Their rate constants are in the range 104 to 106 liters per mole per second for this reaction. The radicals formed from the antioxidants (if any) must be reactive towards other free radicals but unreactive in chain transfer. 

Oxygen has two possible interactions during the polymerization process [94], and these reactions are illustrated in Fig. 2. The first of these is a quenching of the excited triplet state of the initiator. When this quenching occurs the initiator will absorb the light and move to its excited state, but it will not form the radical or radicals that initiate the polymerization. A reduction in the quantum yield of the photoinitiator will be observed. The second interaction is the reaction with carbon based polymerizing radicals to form less reactive peroxy radicals. The rate constant for the formation of peroxy radicals has been found to be of the order of 109 1/mol-s [94], Peroxy radicals are known to have rate constants for reaction with methyl methacrylate of 0.241/mol-s [100], while polymer radicals react with monomeric methyl methacrylate with a rate constant of 5151/mol-s [100], This difference implies that peroxy radicals are nearly 2000 time less reactive. Obviously, this indicates that even a small concentration of oxygen in the system can severely reduce the polymerization rate. 

This has significant implications for the interpretation of the observed peroxy radical decay rate constant. If reaction (44) is very fast (an estimated rate constant of 2.5 x 10 cm s was obtained from modeling the product study), then four, as opposed to two, peroxy radicals are effectively removed for every reactive encounter between two CF3O2 molecules, implying that the actual self-reaction rate constant is half the observed value. A more careful analysis by both studies gives the corrected selfreaction rate constant as A = 1.8 x 10 cm s . ... 

The addition of the peroxyl radical to the double bond is governed by the electron density in the alkene bond and by electrophility of the radical. The rate constants of addition reactions increase with an increase of electron density on the double bond and with the increase of the electrophilic character of a radical (Table 6). The considerably larger electrophility of acyl peroxy radical (CH3CO3, C6H5CC>3) may explain by 5 orders faster addition of acyl peroxyl radicals [69] to a-methyl styrene at 20 °C. Electrophility of radicals leads to the marked reduction of activation energy of addition to alkenes methyl peroxyl radical has 47 kJ/mol, while acetyl peroxyl radical has 19 kJ/mol [70]. 

Oxidation rate constant k, for gas-phase second order rate constants, Icqjj for reaction with OH radical, k os with NO3 radical and ko3 with O3 or as indicated, data at other temperatures see reference k < 4 X 10 M h for singlet oxygen, 1.1 x 10 M fh for peroxy radical at 25°C (Mabey et al. 1982) photooxidation = 77-3840 h in water, based on reported reaction rate constants for ROj radicals with the phenol class (Mill Mabey 1985 selected, Howard et al. 1991) photooxidation ty, = 8.0 h in air, based on reaction with photochemically produced hydroxyl radical in air (GEMS 1986 selected, Howard 1989) koH = 71.5 X 10 cm molecule- s- at 296 2 K (Atkinson 1989)... 

Surface water photooxidation ty, = of 77-3840 h in water, based on reported reaction rate constants for RO2 radical with the phenol class (Mill Mabey 1985 selected, Howard et al. 1991) ti/j 58 d, estimated for photooxidation via peroxy radicals and ty, = 2600 yr for reaction with singlet oxygen in water (Howard 1991)... 

Oxidation the free radical oxidation rate constant k = 18 M-s for reaction with peroxy radical (Wolfe et al. 1980a) ... 

The intermediates formed in AOPs sometimes are more toxic than the parent compounds and are required to be decomposed completely using either combination of AOPs or combination of AOP and some other treatment methods such as adsorption and biodegradation. Carbonyl compounds, particularly aldehydes, are quite toxic, and some of the secondary compounds formed from aldehydes, especially peroxyacylnitrates are more dangerous than the parent compounds. Organic peroxy radical (ROj) reactions are of significance because they represent an important class of intermediates formed in the oxidation process of hydrocarbons (15). Intermediates such as ethers and alcohols have enhanced reactivity toward hydroxyl radical. The rate constant of oxidation of these compounds is of similar order of magnitude as of the alkanes. 

Self-Reaction Kinetics. Of all peroxy radical reactions, the self-reaction between two identical peroxy radicals is perhaps the most studied. The measurement of peroxy radical UV absorption cross sections, discussed above, often occurs under the assumption that all the chlorine or fluorine atoms produced by photolysis are converted quantitatively into peroxy radicals however, this assumption must be corrected for by the loss of peroxy radicals from self-reaction. Furthermore, studies of RO2 -b NO or RO2 -f HO2 reactions usually take place at sufficiently high RO2 concentrations to require knowledge of the self-reaction rate constant, in order to interpret the results of the kinetics measurements. Both concerns make laboratory studies of peroxy self-reaction kinetics an important issue. In contrast, the steady-state atmospheric concentrations of HCFC-based peroxy radicals are probably too small for their self-reactions to be relevant to atmospheric chemistry. In this context, the most important peroxy-peroxy radical reactions would be between the HCFC-based peroxy radicals and CH3O2, but such reactions have not been studied to date. 

Table 6 collects the available HCFC-based peroxy radical self-reaction rate constants. There are two important points to keep in mind regarding this table. The first is that the peroxy radical self-reaction proceeds by two major channels ... 

The data in Table 6 suggest a few general trends. The room-temperature HCFC-based peroxy radical self-reaction rate constants all lie in the rather... 

Rate constants for FC(0)0, reactions are listed in Table 11. The reaction above has been studied by Wallington and co-workers [83]. The rate coefficient at 296 + 2 K for this reaction is 2.5 + 0.8 x 10" cm s" which is consistent with the measured rate coefficient for other peroxy radical reactions with NO [9]. 

Very few data are available for self-reactions of secondary alkylperoxy radicals. Our main contribution in this case is the study of cycloalkylperoxy radicals C-C5H9O2 and C-C6H11O2 radicals. The rate constants are 40 times larger than the only linear secondary peroxy radical studied so far, /-C3H7O2 and exhibit a slight positive temperature dependence. More data would be necessary for a better description of such reactions but, due to their relatively low rate constants (< 5 X lO" " cm molecule s ) their contribution to atmospheric chemistry is negligible. 

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. 

These ESR spectra are in good agreement with ESR spectra of ozonized PP published previously (30) The rapid formation of peroxy radicals indicates that ozone reacts with PP without induction period. In the initial stage of reaction the hydroperoxide groups (POOH) concentration increases and the rate of POOH formation is linearly dependent on the ozone concentration (Fig.2). After prolonged ozonization the concentration of POOH remains almost constant. 

Oxidation rate constant k, for gas-phase second order rate constants, k0H for reaction with OH radical, kN03 with N03 radical and k0j with 03 or as indicated, data at other temperatures see reference k = 4 x 107 M-1 h-1 for singlet oxygen and k = 5 x 103 M-1 h-1 for peroxy radical (calculated, Mabey et al. 

Other important aromatic amines such as chlorpromazine (26) have also been subjected to oxidation studies using oxidants produced by pulse radiolysis. Typical among these is the use of chloroalkylperoxyl radicals formed by pulse radiolysis in a variety of solvents. These oxidants yield the corresponding radical cation. The rate constants (Table 3) for these reactions were determined42. Other studies have determined the reactivity between chlorpromazine and BiV- in H2O/DMSO in varying proportions. The rate constants for the formation of the radical cation of chlorpromazine were similar in value to those obtained from the peroxy radical reactions4. 

Reported rate constants for the reaction of acetone with OH radicals in the atmosphere and in water are 2.16 x lO and 1.80 x 10 cm /molecule-sec, respectively (Wallington and Kurylo, 1987 Wallington et al., 1988a). Between 20 and 100 mmHg, reaction of acetone with OH radicals revealed no significant pressure dependence. Reaction products likely to form include acetic acid, methanol, methyl- and peroxy radicals (Wollenhaupt et al., 2000). 

It has been speculated that aqueous solutions of aromatic amines can be oxidized by organic radicals, but there are no actual data on reaction rates. Based on a study of reaction rate data for compounds with structures similar to 3,3 -dichlorobenzidine, an estimate of the half-life of aromatic amines in water is approximately 100 days, assuming a peroxy radical concentration of 10 mole/L in simlit, oxygenated water (EPA 1975). Based on the oxidation rates of similar compounds, the direct oxidation of 3,3 -dichlorobenzidine by singlet oxygen in solution may be treated as a first-order reaction, to arrive at an estimated reaction constant of <4xlOVmole-hour (Mabey et al. 1982). The oxidation rate constant with... 

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