Hydroperoxyl radicals

NO generally catalyses tliel consumption by transfomiing hydroperoxyl radicals into highly-reactive 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). 

Measurements of tropospheric HO and H02 concentrations have been accomplished by both direct and indirect means. Direct techniques are based on the measurement of the hydroxyl or hydroperoxyl radical using some physical property of the radicals themselves e.g. optical absorption. Indirect techniques refer to methods based on the measurement of compounds that are uniquely and/or quantitatively formed from or destroyed by HO or HO2. Examples of these techniques for both [HO ] and [HO2 ] will be given. 

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

Under conditions of liquid-phase oxidation, alkylperoxyl radicals are stable and react rapidly with RH. They do not decompose into olefin and hydroperoxyl radical. However, some peroxyl radicals have a weak C—OO bond and decompose back to R and dioxygen ... 

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

The interatomic distances in peroxyl radicals were calculated by quantum-chemical methods. The experimental measurements were performed only for the hydroperoxyl radical and the calculated values were close to the experimental measurements (see Table 2.5). The length of the O—O bond in the peroxyl radical lies between that in the dioxygen molecule (r0—o= 1.20 x 10-10m) and in hydrogen peroxide (r0—o= 1-45 x 10-lom). 

Chain propagation in oxidized 1,2-substituted ethylenes proceeds via addition of dioxygen followed by the elimination of the hydroperoxyl radical [156] ... 

Due to the high initiation rate and low (room) temperature, chains for oxidation of alkanes are short and many products are formed by disproportionation of peroxyl and hydroperoxyl radicals. The G values of the products of radiolytic oxidation of four alkanes are given in the following table [233] ... 

The rate of chain decomposition of hydroperoxide v = (kp/v/2kd)[ROOH]i i1/2. Dioxygen reacts with ketyl radicals with the formation of hydroxyperoxyl radicals. The latter is decomposed into ketone and H02 . Hydroperoxyl radicals also possesses reducing activity and induce the chain decomposition of hydroperoxide [121,143]. 

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

In addition, hydroperoxyl radicals become the active chain termination agents in the presence of ions and salts of transition metals (see Chapter 17). 

The alkylhydroxyperoxyl and hydroperoxyl radicals formed from alcohol possess a reducing activity and attack hydroperoxide with the formation of the alkoxyl radical. This radical is very active and propagates the chain reacting with hydrocarbon. 

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... 

Enthalpies, Activation Energies, and Rate Constants of the Reduction of Peroxides by Ketyl, Semiquinone, and Hydroperoxyl Radicals Calculated by the I PM Model [68]... 

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

As a result, two different peroxyl radicals take part in chain propagation in oxidized alcohol, namely, alkylhydroxyperoxyl and hydroperoxyl radicals. 

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 mechanism of H02 formation from peroxyl radicals of primary and secondary amines is clear (see the kinetic scheme). The problem of H02 formation in oxidized tertiary amines is not yet solved. The analysis of peroxides formed during amine oxidation using catalase, Ti(TV) and by water extraction gave controversial results [17], The formed hydroperoxide appeared to be labile and is hydrolyzed with H202 formation. The analysis of hydroperoxides formed in co-oxidation of cumene and 2-propaneamine, 7V-bis(ethyl methyl) showed the formation of two peroxides, namely H202 and (Me2CH)2NC(OOH)Me2 [16]. There is no doubt that the two peroxyl radicals are acting H02 and a-aminoalkylperoxyl. The difficulty is to find experimentally the real proportion between them in oxidized amine and to clarify the way of hydroperoxyl radical formation. 

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) ... 

High values of the inhibition coefficient (/= 12-28) were detected for the first time in the oxidation of cyclohexanol [1] and butanol [2] inhibited by 1-naphthylamine. For the oxidation of decane under the same conditions, /= 2.5. In the case of oxidation of the decane-cyclohexanol mixtures, the coefficient / increases with an increase in the cyclohexanol concentration from 2.5 (in pure decane) to 28 (in pure alcohol). When the oxidation of cyclohexanol was carried out in the presence of tetraphenylhydrazine, the diphenylaminyl radicals produced from tetraphenylhydrazine were found to be reduced to diphenylamine [3]. This conclusion has been confirmed later in another study [4]. Diphenylamine was formed only in the presence of the initiator, regardless of whether the process was conducted under an oxygen atmosphere or under an inert atmosphere. In the former case, the aminyl radical was reduced by the hydroperoxyl radical derived from the alcohol (see Chapter 6), and in the latter case, it was reduced by the hydroxyalkyl radical. 

The oxidation of primary and secondary alcohols in the presence of 1-naphthylamine, 2-naphthylamine, or phenyl-1-naphthylamine is characterized by the high values of the inhibition coefficient / > 10 [1-7], Alkylperoxyl, a-ketoperoxyl radicals, and (3-hydroxyperoxyl radicals, like the peroxyl radicals derived from tertiary alcohols, appeared to be incapable of reducing the aminyl radicals formed from aromatic amines. For example, when the oxidation of tert-butanol is inhibited by 1-naphthylamine, the coefficient /is equal to 2, which coincides with the value found in the inhibited oxidation of alkanes [3], However, the addition of hydrogen peroxide to the tert-butanol getting oxidized helps to perform the cyclic chain termination mechanism (1-naphthylamine as the inhibitor, T = 393 K, cumyl peroxide as initiator, p02 = 98 kPa [8]). This is due to the participation of the formed hydroperoxyl radical in the chain termination ... 

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,... 

The formed hydroperoxyl radicals react with quinone. In addition to H02, the aminoperoxyl radicals can apparently reduce quinones also. 

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 ... 

Oxidation of the formed alcohol with production of hydroperoxyl radicals (see Chapter 7). 

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 latter rapidly reacts with peroxyl radicals to produce hydroperoxyl radicals possessing the high reducing activity (see Chapter 16). 

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