Peroxy radicals hydrogen atom transfer from

Hydrogen Atom Transfer from Hydroperoxides to Peroxy Radicals. The reaction of cumylperoxy radicals with Tetralin hydroperoxide (Reaction 10) can be studied at hydroperoxide concentrations below those required to reduce the oxidation rate to its limiting value. The rate of oxidation of cumene alone can be represented by ... 

Table VI. Effect of tert-Butyl Alcohol on Hydrogen Atom Transfer from Tetralin Hydroperoxide to Cumyl Peroxy Radicals at 30°C.

Hydrogen Atom Transfer from Hydrocarbons to Peroxy Radicals. The ready conversion of one chain carrier to another in hydrocarbon oxidations by the addition of a hydroperoxide is illustrated in Table VII. 

The active enzyme abstracts a hydrogen atom stereospecifically from the intervening methylene group of a PUFA in a rate-limiting step, with the iron being reduced to Fe(II). The enzyme-alkyl radical complex is then oxidized by molecular oxygen to an enzyme-peroxy radical complex under aerobic conditions, before the electron is transferred from the ferrous atom to the peroxy group. Protonation and dissociation from... 

R00, is formed in the propagation step by rapid reaction with oxygen. In the absence of an efficient antioxidant, the peroxy radical is converted to the hydroperoxide, ROOH, by hydrogen-atom abstraction from the polymer chain giving rise to another polymeric radical, R , and peroxy radical R00. Propagation via chain transfer is also promoted by homolysis of ROOH to RO and 0H in the absence or depletion of the antioxidant AH. 

Elimination of HOO- from peroxy radicals having a-hydrogens is another common pathway for the formation of superoxide. For example, ethanol reacts with OH by hydrogen atom transfer to give a mixture of several radicals dominated by 5, which rapidly reacts with oxygen to form a peroxy radical that eliminates OOH (with the concomitant production of acetaldehyde). This reaction is not as efficient with the peroxy radical of t-butanol, which has no a-hydrogens (von Sonntag, 1987). 

More recently it has been shown (6, 7) that zinc dialkyl dithiophosphates also act as chain-breaking inhibitors. Colclough and Cunneen (7) reported that zinc isopropyl xanthate, zinc dibutyl dithiocarbamate, and zinc diisopropyl dithiophosphate all substantially lowered the rate of azobisisobutyronitrile-initiated oxidation of squalene at 60°C. Under these conditions, hydroperoxide chain initiation is negligible, and it was therefore concluded that inhibition resulted from removal of chain-propagating peroxy radicals. Also, consideration of the structure of these zinc dithioates led to the conclusion that no suitably activated hydrogen atom was available, and it was suggested that inhibition could be accounted for by an electron-transfer process as follows ... 

The first suggestion on the mechanism of the reaction between phenols and peroxy radicals emerged from experiments on the antioxidant effect of phenols, by Bolland and ten Have (1947a, b). These authors found a fair correlation between the increase of chain-terminating efficiency and the decrease of the redox potential of phenols and suggested a mechanism in which the phenolic hydrogen atom is transferred to the peroxy radical, i.e.,... 

Radical trapping. To allow for stabilizaton by this mechanism, another reaction (number 49) was included to allow easy abstraction of a hydrogen atom from an additive (QH) by a peroxy radical to form a hydroperoxide and a harmless adduct. With the same value of the rate constant as for energy transfer and for concentrations as low as 10 M, the photooxidation process was efficiently slowed. Figure 9 shows the linear dependence of the time to failure (5% oxidation) as the concentration of QH is altered. Note that the trap is consumed in the process and the apparent induction time is associated with its removal. The stabilization is less effective for higher intensity (and probably higher temperature) because the faster photo (or thermal) decomposition of ROOH continues the degradation process. 

The salts of alkyl xanthates, A/,A/ -di-substituted dithio-carbamates and dialkyidithiophosphates [26] are effective peroxide decomposers. Since no active hydrogen is present in these compounds, an electron-transfer mechanism was suggested. The peroxide radical is capable of abstracting an electron from the electron-rich sulfur atom and is converted into a peroxy anion as illustrated below for zinc dialkyl dithiocarbamate [27] ... 

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