Nitroxyl radicals disproportionation

The reaction of AmO with H02 occurs with AH < A//c min and, subsequently, with a low activation energy (E=0.5RT) and a high rate constant. The latter is higher than 2kt for peroxyl radicals (see Chapter 6), which is important for cyclic chain termination. The inverse situation takes place in reactions of nitroxyl radical disproportionation with alkylperoxyl radicals. For these reactions we observe inequality AH > A//c min and, subsequently, relatively a high activation energy (E> 0.5RT) and a low rate constant. The latter are lower than 2kt for... 

In acid medium, nitroxyl radicals disproportionate to produce hydroxylamines and oxoammonium salts. 

The cross-disproportionation of nitroxyl and hydroperoxyl radicals is an exothermic reaction. For example, the enthalpies of disproportionation of TEMPO radical with H02, Me2C(0H)02, and cydo-C(,Y 10(OH)O2 radicals are equal to 109, —92, and 82 kJ mol-1, respectively. The Ee0 value for the abstraction of an H atom from the O—H bond in ROOH by a nitroxyl radical is 45.6 kJ mol 1 and AHe min = —58 kJ mol-1. Since AHe < AHe min, (see Chapter 6), the activation energy of such exothermic reactions for these reactions is low (E 0.5RT), and the rate constant correspondingly is high [31 34]. Therefore, in the systems in which hydroperoxyl, hydroxyperoxyl, and aminoperoxyl radicals participate in chain propagation, the cyclic chain termination mechanism should be realized. 

Principally, nitroxyl radicals can disproportionate with alkylperoxyl radicals. 

Why are the activation energies of the reactions of nitroxyl radicals with O—H bonds lower than those in their reactions with C—H bonds As in the case of the reaction of R02 with quinones, the difference in E values occurs as a result of the different triplet repulsions in TS [23]. When a TS of the O H O type is formed (the AmO + H02 reaction), the triplet repulsion is close to zero because the O—O bond in the labile compound AmOOH is very weak. Conversely, the triplet repulsion in the reaction of AmO with the C—H bond is fairly great, due to the high dissociation energy of the AmO—R bond. This accounts for the difference between the activation energies and between the rate constants for the reactions considered above. Thus, the possibility of the realization of a cyclic chain termination mechanism in the reactions of nitroxyl radicals with peroxyl radicals, incorporating O—H groups, is caused by the weak triplet repulsion in the TS of such disproportionation reactions... 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

Under thermal conditions, hydroxylamine ethers can reversibly decompose (Reaction 15). The radicals formed disproportionate to eliminate olefins and yield hydroxylamine (Reaction 16). In the presence of sufficiently effective acceptors of alkyl radicals (e.g., oxygen), the reaction rate of peroxy radical formation is much higher than that of hydroxylamine formation. Thus, in the process of polymer photooxidation, nitroxyl radicals regenerate and can break multiple oxidative chains. 

There is extensive history on the use of stoichiometric and catalytic organic nitroxyls for alcohol oxidation, wherein the key step involves a reaction between the alcohol and an JV-oxoammonium salt (Scheme 15.5, featuring TEMPO) [18, 20]. The JV-oxoammonium salt can be formed in situ from the corresponding nitroxyl radical using various oxidants, such as NaOCl or NO2 (Scheme 15.5, top left), or by acid-induced disproportionation of the nitroxyl into N-oxoammonium and hydroxylamine species (Scheme 15.5, bottom left) [21]. Stable Af-oxoammonium salts have also been isolated and used directly as reagents or catalysts for alcohol oxidation [22]. The pH-dependent mechanism of the reaction of the Af-oxoammonium salt with alcohols has been studied... 

Nitroxyl radicals react with polymer alkyl radicals by disproportionation or termination mechanism, and subsequently, they are regenerated in reaction with peroxyl radicals (Scheme 12.13). 

D.G. Pobedimskii showed that the homolytic reaction with the formation of free radicals occurs in parallel with heterolytic transformation. Manifestations of this phenomenon are diverse. The acceptor of free radicals, stable nitroxyl radical, is consumed in the reaction of hydroperoxide with phosphite. This reaction in ethylbenzene in the presence of oxygen is accompanied by chemiluminescence appeared by the disproportionation of peroxyl radicals. Chemical polarization of nuclei was found... 

In general, nitroxyl radicals are mild oxidation reagents containing the N,N-disubstituted NO-group with one unpaired electron. At strongly acidic conditions (pH < 2) the radical disproportionates to the oxoanunonium cation and its hydroxyl-amine counterpart. Above pH 3 the reverse reaction occurs which affords two molecules of the nitroxyl radical (Scheme 7.81). 

Nonchain mechanisms, though, often involve radical-radical combinations and disproportionations. Nitroxyls can be used to trap alkyl radicals that may be present in a reaction medium by a radical-radical combination reaction, giving an electron-sufficient species that is more easily studied than the free radical. The photochemical reactions of carbonyl compounds often involve radical-radical combination and disproportionation steps. 

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