Formation of peroxy radicals

The oxidation of hydrocarbons is the result of the rapid reactions of the carbon radicals with oxygen, viz. 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

The thermochemistry of the reaction of oxygen with carbon radicals has been evaluated for the gas phase [31] and these data would be expected to be valid also for the liquid phase. Table 1 contains averages of the thermodynamic values estimated by Benson [31]. 

Values of AH0 correspond to the C—02- bond strength and indicate that this bond can dissociate readily to the corresponding carbon radical and 02. Comparison of the equilibrium constants for both alkyl and allyl/ benzyl systems show that the value for the latter system is considerably smaller although both decrease with temperature. The bond strengths of the allyl and benzyl peroxy radicals are weaker than those of the alkyl peroxy radicals by approximately the resonance stabilization associated with the carbon radical [67]. The fraction of carbon radicals which would be oxygenated if other loss mechanisms for the radicals were not impor- 

Thermodynamics and equilibrium constants for reactions of oxygen with carbon radicals [31] 

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The above-mentioned mode of reactions changes when the irradiation is carried out in the presence of gases such as oxygen. In this case, energy transfer, the reaction of oxygen with polymer radicals [32] (leading to the formation of peroxy radicals) and other reactions may affect the type and concentration of products formed [33]. The same can be said for certain additives mixed into the elastomer for one or the other purpose. 

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. 

As mentioned in the introduction, there are conflicting views as to the contributions made to polymer degradation by various initiating species. Among these species, in addition to ketones, hydroperoxides are some of the more important chromophores. As it is known, the photolysis of hydroperoxides yields alkoxy and hydroxy radicals. In polymers, in the presence of oxygen, these radicals lead to the secondary formation of peroxy radicals. The latter in turn are converted by hydrogen abstraction into new hydroperoxides (Scheme I) ... 

Molecular oxygen plays a special role in radical polymerizations. It is known to react very rapidly with hydrocarbon radicals with the formation of peroxy radicals ... 

Through the process of hydrogen abstraction, and consequential formation of peroxy radicals and alkyl radicals, fuel components can be completely consumed... 

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. 

Oxygenated Products. The formation of peroxy radicals at higher temperature depends on the competition between the reactions (1, 3, 13,14) ... 

The H atoms formed in reaction 15a can react with 02 (reaction 11) to form H02. The stabilized Criegee intermediate (CH200) can participate in further reactions, some of which will result in the formation of peroxy radicals. Larger alkenes react with ozone to produce organic peroxy radicals. 

At American University, Harriet Frush and Horace Isbell worked on the mechanism of oxidation of carbohydrates by peroxides. They discovered that, in aqueous alkaline hydrogen peroxide, aldoses are quantitatively degraded to formic acid, so that hexoses produce six moles of this acid and pentoses produce five moles. A detailed study of the mechanism of the reaction revealed that degradation takes place by several pathways, the most rapid one involving the formation of peroxy radicals and hydroxy radicals. Thus, when a hydroperoxide-aldose adduct reacts with hydrogen peroxide, a peroxy radical is formed, which decomposes to a hydroxy radical, formic acid, and the next lower aldose. It was also found that, under basic conditions, hydroxy radicals oxidize alditols and aldonic acids to carbonyl compounds in much the same way they do with Fe2+ in the Fenton reaction. During the years she spent at American University, Dr. Frush was able to publish 10 papers without help from any research assistant or laboratory technician. This brought her total to more than 70 papers. 

The formation of peroxy-radicals and their subsequent reactions play an important role in the oxidation of hydrocarbons at moderate temperatures. The reaction (17) which in the solid phase must strictly involve a third body (the matrix M) has been studied with the rotating cryostat. 

In contrast to the mechano-radical, the PE radicals formed by ionizing radiation react not readily with oxygen but rather easily decay before formation of peroxy radicals by warming experiments in the presence of oxygen. In polypropylene case the observed ESR spectrum from the PP sawdust cut in liquid nitrogen is heavily superposed with the asymmetric spectrum from a peroxy radical, as shown in Fig. 

The first reaction involves formation of peroxy radicals, which undergo internal hydrogen abstraction followed by decomposition, viz. 

The chemistry which leads to most of the interesting cool-flame, two-stage ignition and related phenomena is centred around the reversible formation of peroxy radicals, RO2, by reaction (1). At higher temperatures the equilibrium is over to the left, so that peroxy radicals and their rate enhancing reactions are not important. The equilibrium is of course also... 

Step (A) represents the liberation of atomic hydrogen and the formation of peroxy radicals. In step (B), two peroxy radicals combine to form a tetroxide, and in step (C) the tetroxide decomposes to form phosgene, oxygen, and atomic chlorine. Finally, in step (D), the atomic chlorine of (C) reacts with the atomic hydrogen of (A) to form hydrochloric acid. 

On the other hand, as already noted, homolytic cleavage yields biradicals 51 and 53, and their reaction with molecular oxygen results in formation of peroxy radicals 52 and 54, whose structure depends on the resonance form of the biradical blocked by O2. The most important contributors are believed to be 51 and 53 (Scheme 16). 

Since there are synergistic effects between antioxidants, commercial preparations usually contain mixtures of these antioxidants. As oxidative rancidity is strongly catalyzed by some heavy metal ions, in particular QT+, antioxidant mixtures often contain sequestrants (e.g., citric acid and ethylenediaminetetraacetic acid (EDTA)) in order to complex these ions. Reductants such as ascorbic acid, which decrease the local concentration of oxygen, are also able to decrease the formation of peroxy radicals. 

Small carbonyl compounds are formed during the photochemical oxidation of many volatile organic compounds (VOC s), in urban as well as in rural areas. Photolysis and reaction with the OH radical are the most important initiation reactions for the atmospheric degradation of these compounds, and lead to the formation of peroxy radicals in the former case and either stable molecules and/or free radicals in the latter case (Finlayson-Pitts and Pitts, 1999). 

The intensity of chemiluminescence can be determined as the peak-top intensity /cL-max or the area of the peak,, 4n max. In the absence of antioxidants, the rate formation of peroxy radicals, d[POO ]/dt, can be expressed in terms of the initiation rate of oxidation, Rt. 

However, what is more important, oxygen also takes part in the formation of peroxy-radicals [Ingold, 1969 Singh, 1989], which leads to the radiation-induced oxidative degradation [Spinks and Woods, 1990 Kashiwabara and Seguchi, 1992 Williams, 1992, Soebiantio et al., 1996], viz. ... 

The hydroxyl radical is key to building up higher concentrations of ozone. It can react with volatile organic compounds and carbon monoxide leading to the formation of peroxy radicals. Consider methane as a simple example of a contributory VOC ... 

Table 19.2 Rate constants n, ki2 and (in cm molecule s ) for the formation of alkyl radical channel (II), formation of peroxy radical channel (12), formation of m-cresol, formic acid and O2 channel involved in the reaction of 2,3-dimethylphenol with OH radical.

Bohn, B. Formation of peroxy radicals from OH — tolucme adducts and O2. J. Phys. Chem. A 105, 6092-6101 (2001)... 

TGA showed that the LDPE samples all degraded around 480 °C. For the LDPE stabilised with corn starch and pro-oxidant (LDPE-MB) polymer a second shoulder appeared around 318-327 °C. The mechanism for photo-oxidation and thermooxidation starts with the formation of free radicals on the main chain, with subsequent formation of peroxy radicals. Later, intra- and inter-molecular reactions lead to the formation of hydroperoxides. 

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