Radicals by oxygen

The observed formation of isobutyrate (Figs. 5 and 6) would appear to be one of the possible reasons for the slow decrease in the nitroxide concentration. The formation of isobutyrate can be seen as a reaction competing with the capture of the acyl radicals by oxygen. The absence of isopropyl ether in the reaction mixture is explained by its immediate cleavage - following its formation analogous to isobutyrate - to nitroxide by oxygen-centered radicals (mainly acyl peroxy radicals). 

Owing to their liquid or semisolid nature, monomers are easy to process into polymers. For radical polymerization the use of solid AIBN for liquid monomers at room temperature and liquid MEKP for semisolid monomers or a mixture of liquid and semisolid monomers with some heating is convenient. During the course of curing at 85- 100°C for 22 h the problem of surface inhibition of free radicals by oxygen from the air can be avoided by inert-gas blanketing. 

For anion-radicals, air (i.e., oxygen, carbon dioxide, and water [moisture]), on the whole, is an active component of the medium and so it should be removed before conducting reactions. Understandably, air inhibits anion-radical reactions The anion-radicals primarily formed are consumed at the expense of oxidation, carboxylation, and protonation. Certainly, oxidation can take place only if the acceptor organic molecule possesses a lower affinity for an electron than oxygen does or if one-electron oxidation of the anion-radical by oxygen proceeds more rapidly than the anion-radical decomposition into a radical and an anion (RX R + X ). 

These reactions probably occur either by electron transfer (ET) from the silicon-silicon a-bond to oxygen, or by trapping silyl radicals by oxygen. It was also pointed out that the... 

The acid represented by the formula so modified no longer comes within the definition of the lactic series. It is carbo methylic acid, and difibrs essentially from glycoUic ncid and the lactic series in general, inasmuch as the carbon of its chlorous radical, oxat 0, is linked to the carbon of the basylous radical by oxygen. ... 

Data on alkyl radical oxidation between 300° and 800°K. have been studied to establish which of the many elementary reactions proposed for systems containing alkyl radicals and oxygen remain valid when considered in a broad framework, and the rate constants of the most likely major reactions have been estimated. It now seems that olefin formation in autocatalytic oxidations at about 600°K. occurs largely by decomposition of peroxy radicals rather than by direct abstraction of H from an alkyl radical by oxygen. This unimolecular decomposition apparently competes with H abstraction by peroxy radicals and mutual reaction of peroxy radicals. The position regarding other peroxy radical isomerization and decomposition reactions remains obscured by the uncertain effects of reaction vessel surface in oxidations of higher alkanes at 500°-600°K. 

Since the efficiency of fluorescence quenching of the sensitizer paralleled the oxidizability of the arene in a series of substituted alkyl benzenes, the reaction was thought to proceed through electron transfer followed by protonation and trapping of the radical by oxygen. 

However, Kupletskaya and co-workers (100) have shown that the products of photodecomposition of CpFe(CO)2R (R = Ph, 1-naphthyl, 1-azulenyl, or acenaphthyl) in air-saturated solution were those expected from the trapping of Ar radicals by oxygen. These results could be construed as evidence for photoinduced Fe—Ar bond homolysis, but it is more likely that these products arise from a free radical reaction initiated by the oxygen present in solution. A similar free radical-initiated homolysis of an Fe—(tj -CsHs) bond has been postulated for CpFe(CO)2(V CsH5) 101). 

Above ca. 400—450 °C abstraction of a hydrogen atom from alkyl radicals by oxygen to yield the conjugate alkene and hydroperoxy radical... 

The plot for the free-radical-produced polymer shows an intercept at zero time of 2 scissions per molecule. This indicates that there are weak links in this polymer that degrade very rapidly at all temperatures studied. These are believed to be peroxide groups in the polymer chain, formed by the rapid scavenging of the styryl radical by oxygen during polymerization, and are primary initiation sites for thermal degradation. 

Replacement of C-3 in 5-hexenyl radicals by oxygen or nitrogen accelerates 5-exo cyclizations and, therefore, some of the most utilized radical cyclizations are employed for the formation of heterocycles. The stereoselectivity of the majority of such cyclizations follows the Beckwith... 

The reason for this trend in going from 1,4-cyelohexadiene to 1,4-di-hydronaphthalene to 9,10-dihydroanthracene is that, while the ease of removal of the initial H-atom is expected to be about the same for each compound, ease of removal of the 3-H-atom in the resulting radical decreases with the increase in benzo substitution as shown in Table 3. Thus the removal of an H-atom from the hydroanthracene radical by oxygen is thermoneutral and the reaction is sufficiently slow so that the organic hydroperoxide route dominates. The reaction of 1,4-dihydro-naphthalene is intermediate. The reaction of the 1-hydronaphthyl radical with oxygen to form naphthalene is exothermic by 10 kcal mole 1 however, it is sufficiently slow so that, as the hydrocarbon concentration is increased, the organic peroxide product increases. 

X lO M/sec at 25°C) [82,83]. This presumably explains both the slow rate of photolytic decomposition under strictly anaerobic conditions as well as the marked acceleration of the photolytic decomposition rate in the presence of air due to trapping of the radicals by oxygen. In the latter case the final products are cobalt(III) (i.e. aquocobalamin from methylcobalamin) and formaldehyde, with traces of methanol, methane and formic acid. 

Wavelengths of 500 and 600 nm caused photobleaching rather than photo-yellowing of the polymer. The chemistry of this phenomenon, also reported by others [29], is not fully understood. Quenching of polyenyl radicals by oxygen [84,85] and possible reaction of polyenes with hydrogen chloride formed during photodehydrochlorination, have been proposed [86,87] as possible mechanisms. 

The direct formation of free radicals by oxygen attack,... 

These products could be accounted for by assuming the addition of an amino radical to the styrene, followed by capture of the intermediate radical by oxygen and the reductive breakdown of the resulting hydroperoxide to the alcohol. A reaction of that type has been discussed by Minisci and Galli (28). 

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