Radicals formation/production

Homogeneous Sonochemistry Bond Breaking and Radical Formation. The chemical effect of ultrasound on aqueous solutions have been studied for many years. The primary products are H2O2 there is strong evidence for various high-energy intermediates, including HO2,... 

The reaction rates and product yields of [2+2] cycloadditions are expectedly enhanced by electronic factors that favor radical formation. Olefins with geminal capto-dative substituents are especially efficient partners (equations 33 and 34) because of the synergistic effect of the electron acceptor (capto) with the electron donor (dative) substituents on radical stability [95]... 

Acetylene Ion. No evidence for the contribution of ion-molecule reactions originating with acetylene ion to product formation has been obtained to date. By analogy with the two preceding sections, we may assume that the third-order complex should dissociate at pressures below about 50 torr. Unfortunately, the nature of the dissociation products would make this process almost unrecognizable. The additional formation of hydrogen and hydrogen atoms would be hidden in the sizable excess of the production of these species in other primary acts while the methyl radical formation would probably be minor compared with that resulting from ethylene ion reactions. The fate of the acetylene ion remains an unanswered question in ethylene radiolysis. 

The observation (Porter ef a ., 1972) that added BrCCla almost completely suppresses the polarization of the olefin, while leaving the polarization of trans-4 unalfected, points to the secondary radical pair as the principal immediate precursor of a-methylstyrene. A rate constant for the decomposition of thediazenyl radical of 10 -10 sec has been estimated. Cage collapse and free-radical formation are also thought to occur and appropriately polarized products have been identified (see above). 

KRISTENSEN D, krOger-ohlsen m V and SKIBSTED L H (2002) Radical formation in dairy products Prediction of oxidative stability based on electron spin resonance spectroscopy, in Morello M J, Shahidi F Ho CT Free Radicals in Food, Chemistry, Nutrition and Health Effects, ACS Symposium Series 807, Washington D C, 114-25. 

It is significant that in the absence of O2 (solid points in Figure 4) almost no radicals were formed the amount reported is close to the detection limit of the instrument. In one sense, this observation provides an explanation for the positive effect that O, has on the rate of reaction between NO and CH4 [3,4] i.c., O2 enhances CH,- radical formation. However, the results also indicate that NO itself is not very effective in generating active sites which are responsible for CH,- radical production. This means that the reaction of NO with CH4, in the absence of added O, may occur via a nonradical pathway. 

Unexpectedly, the rate of Nj formation in the presence of CH4 and Oj went through a maximum at 700 °C. Furthermore, in the temperature range 700 °C - 800 °C, the conversion of CH4 approached nearly 100%. Although it is not possible to measure directly the amount of Oj under reaction conditions, one may presume that some remains even at temperatures of ca. 700 C. Nevertheless, at higher temperatures as Oj is consumed its positive effect diminishes. Moreover, as shown in Figure 3, the production of CH,- radicals reached a maximum at 750°C. The decrease in oxygen concentration and the maximum in CHj- radicals formation may cause the maximum in the rate of Nj formation that is described in figure 7. 

Quantum yields for the formation of symmetrical and unsymmetrical (mixed) products were determined as a function of solvent viscosity. If perchance expulsion of CO were concerted, yielding two benzyl radicals, formation of mixed combination products may reflect the ability of the radicals to escape from the solvent cage. If this were true, variation of the solvent viscosity should alter the rate of escape of these radicals and the ratio of symmetrical to unsymmetrical products should change. The results found in this study are presented in Table 4.6. The data in Table 4.6 indicate that the reaction is... 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

The reaction mechanism in the irradiated flue gas is probably quite complex, but basically the EB excites the gas molecules and promotes reactions that convert the oxides to acids. These then react with ammonia or calcium compounds to give solid products that are removed by the filter. The initiation reaction is believed to be brought about by radical formation, such as OH,... 

It is seen that this difference ranges from 137 to 204 kJ mol-1, which is very significant. The high chemical reactivity of free atoms and radicals in various chemical reactions is one of the reasons for which the radical chain reactions occur much more rapidly than the direct molecular transformation of reactants into products. Although the radical formation is an endothermic reaction but, when appearing in the system, radicals rapidly enter into the reaction, and each radical induces the chain of transformations. 

Free radical formation in oxidized organic compounds occurs through a few reactions of oxygen bimolecular and trimolecular reactions with the weakest C—H bond and double bond (see Chapter 4). The study of free radical generation in polymers (PE, PP) proved that free radicals are produced by the reaction with dioxigen. The rate of initiation was found to be proportional to the partial pressure of oxygen [6,97]. This rate in a polymer solution is proportional to the product [PH] x [02]. The values of the apparent rate constants (/ti0) of free radical formation by the reaction of dioxygen (v 0 = k 0[PH][O2]) are collected in Table 13.8. 

At 300 K and below, when hydroperoxides are stable, the decay of PMP peroxyl radicals gives rise to low-molecular-weight products, namely, water, acetone, and isobutyric aldehyde. The formation of these products can be explained by the breakdown of various peroxyl radicals with production of hydroxyl ion and cleavage of the C—C bond. 

Hence, the copper surface catalyzes the following reactions (a) decomposition of hydroperoxide to free radicals, (b) generation of free radicals by dioxygen, (c) reaction of hydroperoxide with amine, and (d) heterogeneous reaction of dioxygen with amine with free radical formation. All these reactions occur homolytically [13]. The products of amines oxidation additionally retard the oxidation of hydrocarbons after induction period. The kinetic characteristics of these reactions (T-6, T = 398 K, [13]) are presented below. 

Two forms of xanthine oxidoreductase namely XO and XDH are present in many human and animal cells and plasma, XDH and XO are the predominant species in cytoplasma and serum, respectively [39]. Damaging effects of XO-catalyzed superoxide production in post-ischemic tissues were demonstrated by many authors. For example, Chambers et al. [40] and Hearse et al. [41] have shown that the suppression of superoxide production by the administration of XO inhibitor allopurinol or SOD resulted in the reduction of infarct size in the dog and of the incidence of reperfusion-induced arrhythmia in the rat. Similarly, Charlat et al. [42] has also shown that allopurinol improved the recovery of the contractile function of reperfused myocardium in the dog. However, the use of allopurinol as the XO inhibitor has been questioned because this compound may affect oxygen radical formation not only as a XO inhibitor but as well as free radical scavenger [43]. Smith et al. [44] also showed that gastric mucosal injury depends on the oxygen radical production catalyzed by XO and iron. 

Calcium oxalate monohydrate responsible for the formation of most kidney stones significantly increased mitochondrial superoxide production in renal epithelial cells [42], Recombinant human interleukin IL-(3 induced oxygen radical generation in alveolar epithelial cells, which was suppressed by mitochondrial inhibitors 4 -hydroxy-3 -methoxyacetophe-none and diphenylene iodonium [43]. Espositio et al. [44] found that mitochondrial oxygen radical formation depended on the expression of adenine nucleotide translocator Anti. Correspondingly, mitochondria from skeletal muscle, heart, and brain from the Antl-deficient mice sharply increased the production of hydrogen peroxide. 

Recent studies suggest that many factors may affect hydroxyl radical generation by microsomes. Reinke et al. [34] demonstrated that the hydroxyl radical-mediated oxidation of ethanol in rat liver microsomes depended on phosphate or Tris buffer. Cytochrome bs can also participate in the microsomal production of hydroxyl radicals catalyzed by NADH-cytochrome bs reductase [35,36]. Considering the numerous demonstrations of hydroxyl radical formation in microsomes, it becomes obvious that this is not a genuine enzymatic process because it depends on the presence or absence of free iron. Consequently, in vitro experiments in buffers containing iron ions can significantly differ from real biological systems. 

---