Peroxyl dissociation

Oxygen dissociation 2. Peroxyl dissociation 3. Hydrogen peroxide dissociation... 

Enthalpies of Formation AH (gas, 298 K) of Hydroperoxides and Peroxyl Radicals and C—02 Bond Dissociation Energies in Peroxyl Radicals... 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

The phenoxyl radical has an increased electron density in the ortho- and pura-positions and adds dioxygen similar to alkyl radicals. However, the C—00 bond is weak in this peroxyl radical and back dissociation occurs rapidly. Therefore, the formation of quinolide peroxide occurs in two steps, which was studied for the 2,4,6-tris(l,l-dimethylethyl)phenoxyl radical [100,101],... 

The dissociation energy of the O—H bond of H03 is 350.4 kJ mol-1 [112]. It can be anticipated that, like peroxyl radicals, ozone reacts with inhibitors (phenols) by the reaction [113] ... 

Since the peroxyl radicals derived from alcohol dissociate to a carbonyl compound and H02 (see Chapter 8), two reactions in which the aminyl radicals formed from amine are reduced, occur in parallel under the conditions of the alcohol oxidation ... 

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... 

In accord with this mechanism, free peroxyl radical of the reaction product hydroperoxide activates the inactive ferrous form of enzyme (Reaction (1)). Then, active ferric enzyme oxidizes substrate to form a bound substrate radical, which reacts with dioxygen (Reaction (4)). The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

The thermochemistry of sulfur radicals in the gas phase has been reviewed. Methylsulfonyl radicals and cations have been produced by femtosecond collisional electron transfer in the gas phase. When formed by vertical collisional electron transfer from cation CH3SO2+, radical CH3S02 dissociates to CH3 and SO2. Radical CH30S0 exists as a mixture of syn (19a) and anti (19b) isomers which are stable when formed by collisional electron transfer to the corresponding cation. Dissociation of both isomers of CH30S0 formed CH3 and SO2 via isomerization to methylsulfonyl radical. An ab initio study on the formation of the thiyl peroxyl radical has also been reported. Julolidylthiyl radicals (20) were formed by femtosecond photo-dissociation of the corresponding disulfide and have been observed... 

Aromatic amines are known as to be efficient inhibitors of hydrocarbon and polymer oxidation (see Chapters 15 and 19). Aliphatic amines are oxidized by dioxygen via the chain mechanism under mild conditions [1,2]. Peroxyl and hydroperoxyl radicals participate as chain propagating species in the chain oxidation of amines. The weakest C—H bonds in aliphatic amines are adjacent to the amine group. The bond dissociation energy (BDE) of C—H and N—H bonds of amines are collected in Table 9.1. One can see that the BDE of the N—H bond of the NH2 group is higher than the BDE of the a-C—H bond in the amine molecule. For example, DN = 418.4 kJ mol 1 and DC H = 400 kJmol-1 in methaneamine. However, the BDE of N—H bond of dialkylamine is lower than that of the C—H bond of... 

The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

Material prepared in this manner has retained a bright orange color for over one year of storage without access of air. The stability of the product appears to depend on the washing of the crystals on the filter. Potassium peroxyl-amine disulfonate is dimeric in the solid state but is dissociated into the monomeric form in solution. In this, it bears a close structural relationship to nitrogen (IV) oxide. 

Because the peroxyl radicals derived from D-glucose rapidly eliminate H02 and because HOa is largely dissociated at neutral pH [pKa(H02 ) = 4.75]124, no chain autoxidation (reactions 119 and 120, RH = D-glucose) sets in. The 02 - radical-ion is not capable of abstracting a hydrogen atom from D-glucose, and therefore does not propagate the chain.118... 

The third class of peroxyl radicals is characterized by a dissociation into a carbocation and 02 . One example has been unequivocally established to date [reaction (16) k = 6.5 x 104 s 1 Schuchmann et al. 1990]. 

Kranenburg M, Ciriano MV, Cherkasov A, Mulder P (2000) Carbon-oxygen bond dissociation enthalpies in peroxyl radicals. J Phys Chem A 104 915-921 Lai M, Rao R, Fang X, Schuchmann H-P, von Sonntag C (1997) Radical-induced oxidation ofdithio-threitol in acidic oxygenated aqueous solution a chain reaction. J Am Chem Soc 119 5735— 5739... 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

The above mechanism would predict deactivation by uv (not observed). However, subsequent work by Freund has indicated that trapped peroxyl entities in MgO, to be expected also in this system, might be dissociated by uv into pairs of O -centers. Thus the uv activation dilemma may be resolved by postulating uv activation of a more active set of centers to compensate for the holes it recombines. 

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