Hydroperoxyl radical, reactions

The hydroperoxyl radical reacts more slowly with other species than does the hydroxyl radical. The kinetics and mechanisms of hydroperoxyl radical reactions are difficult to study because it is hard to retain these radicals free of hydroxyl radicals. 

Formation of Hydrogen Tetroxide. The reaction of hydrogen atoms withHquid ozone at — 196°C proceeds through the intermediate formation of hydroperoxyl radicals forming hydrogen tetroxide, which decomposes on warming to produce equimolar amounts of and O2 (53). 

The oxidation of such nonsaturated compounds proceeds through hydroperoxyl radical formation by the reaction [13] ... 

The latter generates hydroperoxyl radicals possessing the reducing activity. Hydroper-oxyl radicals reduce hydroperoxide and accelerate chain termination by the reactions ... 

The experimental data on the reactions of ketyl radicals with hydrogen and benzoyl peroxides were analyzed within the framework of IPM [68]. The elementary step was treated as a reaction with the dissociation of the O—H bond of the ketyl radical and formation of the same bond in acid (from acyl peroxide), alcohol (from alkyl peroxide), and water (from hydrogen peroxide). The hydroperoxyl radical also possesses the reducing activity and reacts with hydrogen peroxide by the reaction... 

In addition to the abstraction reaction, alkylhydroxyperoxyl radicals are decomposed into carbonyl compound and hydroperoxyl radical. 

A different situation in the oxidation of these two alcohols is seen. The hydroperoxyl radical is the main chain propagating species in oxidized 2-propanol the portion of alkylhydroxy-peroxyl radicals in this reaction is less than 2.5%. In oxidized cyclohexanol, on the contrary, the stationary state concentrations of both radicals are close and both of them take important part in chain propagation. 

In the presence of specially added hydrogen peroxide, the stationary concentration of hydroperoxyl radicals increases due to the exchange reaction. 

The thermodynamic functions (AH, AS, AG(298 K)) of hydrogen peroxide reactions with transition metal ions in aqueous solutions are presented in Table 10.1. We see that AG(298K) has negative values for reactions of hydroxyl radical generation with Cu1+, Cr2+, and Fe2+ ions and for reactions of hydroperoxyl radical generation with Ce4+, Co3+, and Mn3+. 

It is rather surprising that the induced decomposition of POOH of PP occurs with long chains in a dioxygen atmosphere. The most probable reaction of tertiary hydroperoxyl groups of PP decay is that of hydroperoxyl radicals formed from POOH (see Chapter 19) ... 

FIGURE 16.1 The dependence of activation energy E on reaction enthalpy A He for reaction of hydrogen atom abstraction by aminyl radical from the C—H bond of alkylperoxyl radical and O—H bond of hydroperoxyl radical calculated by IPM method (see Chapter 6). The points fix the reactions with minimum and maximum enthalpy among known aromatic aminyl radicals. 

We see, at first, that the reaction enthalpy for quinone abstraction reactions with the C—H bond of alkylperoxyl radicals is higher than with the O—H bond of the hydroperoxyl radical. The second important factor is different triplet repulsions in these two types of abstraction reactions. Indeed, the reaction with R02 proceeds via TS of the C H O type. Such reaction is characterized by the high thermally neutral activation energies Eeo = 62.9 kJ mol-1. The value of Ee0 for the reaction involving the O H O TS reaction center is much lower (27.3 kJ mol-1). With the rate constants have a very low value, the reaction Q + R02 cannot influence the oxidative chain termination in comparison with the interaction of two R02 radicals. Indeed, the rate constant for the latter is 105—107 L mol-1 s-1 and, in these cases, the inequality (2k6v )1/2 2k[Q] always holds. The reason for such high Ee0 values and, hence,... 

Nitro compounds, like quinones, terminate chains in oxidizing compounds where hydroperoxyl radicals are formed. This was proved for the oxidation of polyatomic esters [37] and PP [38], Nitrobenzene retards the initiated oxidation of the following esters tetrapropionate of pentaerythritol, propionate of 2,2-dimethylbutanol, and dipropionate of 2,2-dimethylpro-panediol terminating chains by the reaction with peroxyl radicals [37]. The hydroperoxyl radicals were supposed to be formed as a result of the following reactions ... 

The cross-disproportionation of nitroxyl and hydroperoxyl radicals is an exothermic reaction. For example, the enthalpies of disproportionation of TEMPO radical with H02, Me2C(0H)02, and cydo-C(,Y 10(OH)O2 radicals are equal to 109, —92, and 82 kJ mol-1, respectively. The Ee0 value for the abstraction of an H atom from the O—H bond in ROOH by a nitroxyl radical is 45.6 kJ mol 1 and AHe min = —58 kJ mol-1. Since AHe < AHe min, (see Chapter 6), the activation energy of such exothermic reactions for these reactions is low (E 0.5RT), and the rate constant correspondingly is high [31 34]. Therefore, in the systems in which hydroperoxyl, hydroxyperoxyl, and aminoperoxyl radicals participate in chain propagation, the cyclic chain termination mechanism should be realized. 

The rate constants for two-electron reaction (8) (A+) and one-electron reaction (19) (k 19) are cited in Table 22.3. As seen from Table 22.3, ks values are about 100 1000 times greater than kig values therefore, the production of superoxide (or hydroperoxyl radical) by peroxidases might play an insignificant role compared to the two-electron oxidation of hydroperoxides. 

High values of reaction rates for the two dismutation steps confirm the ability of both nitroxides TPO and 3-CP to be SOD mimics. However, as follows from the above mechanism, hydroperoxyl radical and not superoxide must participate in the first dismutation step (Reaction (5)). (As expected, a rate constant for the reaction of nitroxides with superoxide is very low <103 1 mol 1 s-1 [27].) Therefore, superoxide had to be protonated before participating in Reaction (5), which will diminish the total catalytic process at physiological pH and increase it at lower pH values. 

The superoxide anion radical (O2 ) is produced when oxygen accepts one electron. This radical has a short lifetime in aqueous solutions, where it mainly undergoes spontaneous dismutation to hydrogen peroxide and oxygen (Reaction 1). The superoxide radical is in equilibrium with its conjugated acid, the hydroperoxyl radical (HO2), which is a stronger oxidant and generally more reactive than O2... 

Davis, D. D., W. A. Payne, and L. J. Stief. The hydroperoxyl radical in atmospheric chemical dynamics Reaction with carbon monoxide. Science 179 280-282. 1973. 

The fact that reaction (12) is much slower than reaction (8), implies that Fe is faster depleted from the solution. As a result, Fenton process is halted because the redox chain cannot be supported itself. In addition, it is accepted that (Pignatello 1992 Boye et al. 2003) the hydroperoxyl radical (HO2 ) has a much lower oxidant power than OH. In the presence of organics, Fenton chemistry is even more complex because hydroxyl radical, both iron cations and the oxidation products enter into a series of consecutive and parallel reactions. An example of the complexity of these reactions is discussed elsewhere (Gozzo 2001) but a brief description is given here. The initial step for an organic substrate (R-H) oxidation starts with the interaction of itself with OH, according to (Walling and Kato 1971) ... 

The CBS-QB3 potential energy surface accounts for the various experimentally observed products, including hydroperoxyl radical, propene, HO, propanal, and oxirane (c-CsHgO). The activation barrier for simultaneous 1,4-H transfer and HO2 expulsion, obtained via calculations, compares well to the experimentally observed barrier (26.0kcal/mol) of DeSain et al. This work provides some ramifications for larger alkylperoxy radicals multiple conformers of long alkylperoxy radicals are likely to play a role in the overall oxidation chemistry and dictate consideration for correct treatment of thermochemistry at lower temperatures T< 500 K), unimolecular reactions dictate peroxy radical chemistry. 

As is the case with sulfur dioxide, nitric oxide undergoes a complex variety of reactions by which it is converted to nitrogen dioxide. For example, it too reacts with hydroperoxyl radicals to form nitrogen... 

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