Oxygen-centered radicals rate constants

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

Nitrogen- and oxygen-centered radicals offer some specific features. The rate constant of the reaction of dialkylaminyl radicals with (TMSlsSiFl is close to the analogous reaction of secondary alkyl radicals. In the reactions of... 

Rate Constants for Reactions of Oxygen-Centered Radicals with Silicon Hydrides... 

The rate constants for the oxygen-centered radical and nitrogen-centered radical (aminyl radical and aminium radical) are also shown in Figure 1.17. 

The aryloxyl radicals formed in the initial antioxidant reaction of phenols (equation 1) may undergo several different kinds of secondary reactions, including Type (1), rapid combination (termination) with the initiating oxygen-centered radicals (equation 11) Type (2), self-reactions Type (3), initiation of new oxidation chains by H-atom abstraction from the substrate, the so-called prooxidant effect and Type (4), reduction or regeneration by other H-atom donors resulting in synergistic inhibition. The relative importance of these secondary reactions will be considered briefly here, since they may affect the overall efficiency of the antioxidant, which includes the antioxidant activity, as measured by the rate constant, (equation 10), and the number of radicals trapped, n. 

Alkylperoxy radicals are generated by the reactions of carbon-centered radicals with oxygen and in the induced decomposition of hydroperoxides (Scheme 3.82). Their reactions have been reviewed by Howard452 and rate constants for their self reaction and for their reaction with a variety of substrates including various inhibitors have been tabulated.453... 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

The role of oxygen in radical and other polymerizations has been reviewed by Rhanu and Kishore.187 Rate constants for the reaction of carbon-centered radicals with oxygen are extremely fast, generally - 1 d" M 1 s 1.181,188 The initially formed... 

The rate constants for the reactions of carbon-centered radicals with oxygen are not very sensitive to the extent of spin delocalization. For example, the rate constant (in dm3 mol"1 s ) for ferf-butyl is 4.9 x 109, benzyl 2.4 x 109, cyclohexa-dienyl 1.6 x 109,4-nitrobenzyl 9 x 108, and diphenyl methyl 7.5 x 108, respectively. 

One of the most important implications of these experiments is the conclusion that, due to the high polarity of the C=0 bond, organoelement a-ketones R3MCOR (M = Ge, Sn) are extraordinary effective radical traps. The electronegative oxygen atom of their carbonyl group could be attacked not only by element-centered radicals R3M (M = Ge, Sn), but also by thiyl radicals SR " and phosphorus-centered radicals. Indeed, the experimental estimates of the absolute reaction rate constants of the element-centered radicals with... 

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