Nitroxyl radical , decomposition

The addition of an acceptor decreases the rate of POOH decomposition. The increase of added [InH] creates a tendency for k-% to decrease to the kA value, i.e., Ax —> A d at [InH] —> DC. Acceptors, which do not react with hydroperoxide groups, were used sterically hindered phenols and stable nitroxyl radicals (TEMPO) were found to be efficient acceptors. The ratio kinA(2kt)m can be calculated from the values Ax and A d according to the formula ... 

Aminyl radicals react with peroxyl radicals by two ways with the formation of either N—O or C—O bonds [63], The decomposition of the resulting unstable peroxides gives rise to the nitroxyl radical and iminoquinone, respectively. 

The resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

The fact that hydroxylamine has been found among the products of transformations of nitroxyl radical during the oxidation of hydrocarbons is the evidence in support of this mechanism. Both mechanisms described earlier are realized in parallel and supplement each other. The result of the competition between them depends primarily on the temperature, because the thermal decomposition of alkoxyamine AmOR requires a fairly high activation energy (see Table 19.11). 

The oxidation of primary and secondary alcohols by stable organic nitroxyl radicals has been reviewed.111 The kinetics of reactions of alkanes and arenes with peroxynitrous acid suggest the participation of the same active oxidizing species in both gas and aqueous phase HOONO or its decomposition product OONO. 112 The oxidation of the alkaloids reserpine and rescinnamine by nitric acid has been studied.113... 

The redox reaction also extends to the participation of hydroperoxides, but their efficient decomposition depends on the formation of a non-radical product such as an alcohol. Another example of a redox couple is found in the behaviour of the nitroxyl radical (R NO )- Depending on the structure of R, these are efficient radical scavengers and a redox couple between the mdical and the hydroxyl amine (R NO /R NOH) is formed (which is analogous to the galvinoxyl radical G-/GH). It is noted that the hindered amine stabilizers (e.g. Tinuvin 770 and the monomeric and polymeric analogues) are ineffective as melt antioxidants, possibly because of reaction with hydroperoxides or their sensitivity to acid. 

Analysis of reaction rate constants in model systems shows that at room temperature, the main reaction leading to regeneration of nitroxyl radicals is their interaction with peroxide radicals, (Reaction 11) and at elevated temperatures (more than 80°) the main reaction is that of hydroxylamine ether decomposition (Reaction 15) (53). 

Figure 13. Isolated products and nitroxyl radical species from the TNT thermochemical decomposition process.35,36...

PMMA, which in itself is stable on exposure to NO2, enters into reactions after previous irradiation by UV light at 293 K [16]. The photolysis of PMMA induces the formation of double bonds as a result of ester group decomposition. The ESR spectrum observed after exposure of samples to NO2 is shown in Figure 2. The spectrum represents the superposition of the signals of two nitroxyl radicals at low frequencies of rotational mobility (10 s < Tc < 10 s) ... 

As can be seen, the hydroperoxide consumption rate is initially low and then sharply increases. The observed character of the kinetic curves cannot be explained by reactions (74) or (75). According to the ESR data, the decomposition of PP hydroperoxide in an NO atmosphere gives dialkylnitroxyl radicals. It was shown that the induction periods for the hydroperoxide decomposition and nitroxyl radical accumulation are very sensitive to the presence of trace amounts of higher nitrogen oxides. This leads to the conclusion that the interaction of hydroperoxide with NO is more likely to proceed according to the scheme ... 

For those systems where Ri = R2 = H or Ri = H, R2 = CH3, i.e. where the number of alkyl groups at C, is <1, and R3 = H to NO3, the alkoxynitroxyl radicals formed according to Eq. 7 under steady-state-ESR or pulse radiolysis conditions do not give rise to nitrobenzene radical anions. This means that the rate constants for heterolysis of the nitroxyls are < 10 s . This is not only true in weakly acidic (pH 4) or neutral but also in strongly alkaline solution (pH 13-14). The latter observation means that the nitroxyls are not susceptible to base catalyzed heterolysis. From this the rate constant for OH catalyzed decomposition can be estimated to be < 10 M s [19]. This low number for... 

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