Radicals, oxygen scavenging

The reaction is retarded by the addition of the radical chain scavengers 2,6-di-/-butyl-4-methylphenol and hydroquinone. The oxidation rate is not affected by the oxygen pressure (Table 2), indicating that re-oxidation of Ce(III) in 1 is not rate limiting. The oxidation rate is first order in formaldehyde and goes through a maximum with increasing concentration of 1. All these phenomena are consistent with a chain radical mechanism of oxidation (11,12). 

Packer, J.E., Mahood, J.S., Mora-Arellano, V.O., Slater, T.F., Willson, R.L., and Wolfenden, B.S. 1981. Free radicals and singlet oxygen scavengers Reaction of a peroxy-radical with P-carotene, diphenyl furan and 1,4-diazobicyclo (2,2,2)-octane. Biochem. Biophys. Res. Commun. 98 901-906. 

Ozone also reacts with ethane in the gas phase at room temperature. Rather than a direct molecular reaction, however, evidence points to the initiation of radical-chain reactions by the very small O-atom concentrations present in ozone at room temperature. Added oxygen scavenges the radicals and slows the build-up, leading to induction periods which may be in excess of 3 h. Recent advances in mechanistic investigations of gas-phase ozonolysis of alkanes have been reviewed. Oligomeric peroxides dominate the products of oxidation of nitrotoluenes with ozone in acetic acid. °... 

Antioxidants are not oxygen scavengers or absorbers. They function by reaction with free radicals. 

In the presence of hydroquinone the rate of dehydrochlorination of poly (vinyl chloride) in oxygen is reduced considerably (20, 24). The presence of an oxygen scavenger in lieu of a radical scavenger or antioxidant would be expected to have an even greater effect. 

Hanasaki Y, Ogawa S, Fukui S. 1994. The correlation between active oxygens scavenging and antioxidative effects of flavonoids. Free Radic Biol Med 16 845-850. 

Specific research, including with radical scavengers, showed that it is not the H02 particle that participates in the bleaching reaction, but a radical oxygen anion 0214. It is presumed that this particle is formed from H02 which, owing to the localised charge on the terminal oxygen, is unstable and further decomposes ... 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

For a number of carbohydrate-Gly melanoidins (170 °C, 20 min, dialysed), free-radical content (EPR) and oxygen scavenging ran parallel,81 except for maltotriose, where oxygen scavenging was greater than expected on the basis of such a relationship. Yet, the overall implication is that the oxygen-consuming properties are based on a radical mechanism. 

B. Cammerer, I. J. Fuchs, and L. W. Kroh, Antioxidative activity of melanoidins — radical and oxygen scavenging properties, in Melanoidins in Food and Health, Vol. 2, J. M. Ames (ed), European Communities, Luxembourg, 2001, 159-164. 

Experimental evidence for the presence of radical intermediates is provided by the identification of expected products from radical rearrangements, by the use of appropriate radical probes and by direct detection by electron spin resonance (ESR). Other mechanistic evidence includes inhibition by radical traps, such as di-t-butylnitroxide (DTBN), TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), galvinoxyl and oxygen, and by radical anion scavengers such as p-dinitrobenzene (p-DNB). 

This assignment is corroborated by the observation that the absorption is not affected by the presence of oxygen, an electron and radical anion scavenger, ruling out PCB radical anion as the species contributing to the absorption. This also rules out any alkyl or chloroalkyl radical from C2H4C12, since they react with 02. The yields measured in terms of Gs (absorption units) depend strongly on the concentration of the PCB, the absorption... 

The other commercial stabilizer (UV-3), a hindered-amine type, is presumed to afford photochemical protection by functioning primarily as a free radical and/or oxygen scavenger (13). Thus protection should only occur in the primary substrate, the cellulose acetate film, and not in a secondary substrate such as the blue wool fabric. The results shown in Table II verified this assumption the AE values obtained, irrespective of the concentration of UV-3 used in the film, were within experimental error of those observed with the unprotected fabric. 

The spectral changes observed during oxidation provide a means for monitoring the kinetics of the oxidation process. The reactions follow a rate law that is first order in [Ni] and first order in [02] Rate = fc[Ni(L)CN] [02] (79). The reaction is relatively slow and not very sensitive to the nature of the N-substituent (k = 1.4-3.1 X 10 2 M 1 s-1 in dimethylformamide (DMF) at 30 °C). The reaction rates are independent of the presence of a singlet oxygen scavenger or radical traps. The activation parameters AH and AS were measured by using the temperature dependence of the second-order rate constants in DMF... 

Singlet Oxygen Scavengers and/or Quenchers — /3-carotene, a-tocopherol. histidine, tryptophan, guanine, uric acid Hydroxyl Radical Scavengers — most organic compounds k 8 10 M- s- ... 

This scheme of interrelated primary photochemical and subsequent radical reactions is comphcated by the back reaction of hydrogen atoms and hydroxyl radicals with formation of water (Fig. 7-16, reaction 2) or the dimerization of the latter with formation of hydrogen peroxide (Fig. 7-16, reaction 3). Furthermore, hydroxyl radicals are scavenged by hydroperoxyl radicals with formation of oxygen and water (Fig. 7-16, reaction 5) or by hydrogen peroxide to yield hydroperoxyl radicals and water (Fig. 7-16, reaction 4). In addition, hydroxymethyl radicals (HOCH ) formed by reaction 1 (Fig. 7-16) are able to dimerize with formation of 1,2-ethane-diole (Fig. 7-16, reaction 7) or they disproportionate to yield methanol and formaldehyde (Fig. 7-16, reaction 8). 

Toxic Derivatives of Molecular Oxygen Such as Superoxide Radical Are Scavenged by Protective Enzymes... 

Radical formation is a concern for all aerobic organisms. Oxygen is a ground-state diradical and is very reactive and responsible for oxidative cell damage. Cells have radical-specific scavengers that minimize the damage that radicals can do. 

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