Peroxy radicals, disproportionation

SCHEME 10.5 Proposed pathway for the nonenzymatic conversion of BHTOOH to BHT-QM in keratinocytes. BHTOOH is oxidized to a peroxy radical that spontaneously loses oxygen. Two BHT phenoxy radicals then undergo disproportionation. 

The recombination of the cyclohexyl peroxy radicals produced in one of these two reaction pathways gives rise to cyclohexanol, cyclohexanone and oxygen ( disproportionation ) ... 

Oxygen increased the destruction of glutamate residues (G = —3.5), most of the increase arising from the increase in peptide-bond destruction described above. An unusual reaction of PGA in O2 described by Sokol et al. (35) might account for a major part of the enhanced destruction. They found pyruvic acid to be a major product (G 1) when 0.15% solutions of PGA at neutral pH s were irradiated under 1 atm. O2. The mechanism proposed for the formation of pyruvic acid was based on a bi-molecular disproportionation of two y-peroxy radicals. 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

In this example, primary and secondary peroxy radicals form intermediates which disproportionate to non-radical degradation products. In contrast, tertiary peroxy radicals may either combine to give di-tertiary alkyl peroxides or undergo a... 

It has been long known that free-radical chain oxidation of hydrocarbons is followed by chemiluminescence (CL). In the oxidation of hydrocarbons with a single oxidizable group, excited-state generation comes about from the disproportionation of peroxy radicals (ROO ). This termination step of the oxidation chain, which proceeds through the tetroxide intermediate, yields three products, namely, ketone (R.h=0), alcohol (ROH) and molecular oxygen (Eq. 1). 

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. 

Polymer "moiety"-polymer "moiety" reactions. Chemical groups formed on the backbone of polymer chains such as peroxy radicals, hydroperoxides, ketones, etc. can undergo bimolecular reactions such as disproportionation, energy transfer, etc. The effect of the polymeric medium apparently is to reduce the bimolecular rates by a factor of about 10 4 relative to fluid solution rates, due to the reduced mobility of both reactants. 

Alkyl radicals (c) may either combine or disproportionate, and so cause no further immediate damage, or, during irradiation in air, may combine with dissolved oxygen in the polymer to give peroxy radicals R02 [88, 191,193]. 

For the doubly hindered phenoxy radical products, there are no active hydrogens to be scavenged by the peroxy radicals further, disproportionation of the dimer back to the parent phenol and a quinone methide must be assumed to be slow (19). 

This reaction could explain the products observed after the irradiation of aqueous solutions of salts of the chlorinated phenylacetic acid herbicide fenac (36) or 1-naphthaleneacetic acid (NAA, IV) (37) (Figure 3). Removal of the electron in its solvent cage could generate a radical (V) which reacts rapidly with oxygen the resulting peroxy radical could disproportionate by well-known routes to provide the transiently... 

The main reactions of phenolic antioxidants take place with peroxy radicals, oxygen, singlet oxygen, phenoxy radical disproportionation, etc. 

They admitted that the terminating radical pair must belong to the same macromolecule (intramolecular process) or to two immediate neigh " bouring polsmer molecules and that the termination reaction is predominantly a disproportionation of a primary or a secondary peroxy radical with another peroxy radical [reaction (8)]. 

For /-butyl peresters there is also a variation in efficiency in the series where R is primary secondary>tertiary. The efficiency of /-butyl peroxypentanoate in initiating high pressure ethylene polymerization is >90%, that of /-butyl peroxy-2-ethylhexanoate ca 60% and that of/-butyl peroxypivalate ca 40%.196 Inefficiency is due to cage reaction and the main cage process in the case where R is secondary or tertiary is disproportionation with /-butoxy radicals to form /-butanol and an olefin.196... 

A striking example is control of the steady state level of superoxide radical anion (Oj"). Although superoxide anions are not very reactive, they playa prominent role in oxidative stress by triggering formation of peroxy nitrite. The control of their steady state level is thus vital for cells. In most living organisms the well-known metalloenzyme superoxide dismutase (SOD), present in almost all aerobic cells catalyzes the disproportionation of into HjOj and Oj ... 

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