Homolytic decomposition of hydroperoxides

Propagation. Propagation reactions (eqs. 5 and 6) can be repeated many times before termination by conversion of an alkyl or peroxy radical to a nonradical species (7). Homolytic decomposition of hydroperoxides produced by propagation reactions increases the rate of initiation by the production of radicals. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

Due to the unimolecular and bimolecular homolytic decomposition of hydroperoxide, such as... 

The homolytic decomposition of hydroperoxides was proved to be catalyzed by Bronsted as well as Lewis acids (for example, BF3, A1C13, SbCls) [230]. Experimental data on acid catalysis of the homolytic decomposition of hydroperoxides are collected in Table 10.9. 

Acid Catalysis of the Homolytic Decomposition of Hydroperoxides (Experimental Data)... 

The formation of the H0S 02 radical was demonstrated by EPR spectroscopy [51]. The homolytic decomposition of hydroperoxide catalyzed by S02 underlies an oscillating pattern of the M-hcxylbenzcne oxidation inhibited by thiophene or BaS04 [48]. Radicals can also be... 

In the early stages of oxidation, the concentration of RO2H may be very low. Initiating events other than the homolytic decomposition of hydroperoxides are critical at this early stage. Once decomposition of RO2H has occurred, the rate of hydrogen abstraction from... 

Thus, it has been proposed that the homolytic decomposition of hydroperoxides can be induced by sulfenic acid (12,13). There is evidence that various carboxylic acids can promote radical formation from hydroperoxides at elevated temperatures (II, 14). The intermediate thiosul-furous acid (Reaction 7) itself may function as the source of radicals, since sulfinic acid is known to initiate the radical polymerization of vinyl monomers at 20°C (15). Based on the AIBN-initiated oxidation of cumene, Koelewijn and Berger (16) proposed that pro-oxidant effects arise from catalysis of the radical decomposition of hydroperoxides by intermediate compound formation between the hydroperoxide and sulfoxide. However, under our conditions hydroperoxide was stable in the presence of sulfoxide alone. 

Since homolytic decomposition of hydroperoxides into free radicals (stage 4) requires relatively high activation energies, this process is rather slow and becomes effective only at temperature of about 120°C or higher. [20] However, in the presence of catalytic amoimts of certain metal ions, hydroperoxides decompose at a lower temperature and/or at higher rate. This degradation mechanism involves a series of redox processes in which the most active catalysts are those derived by metals easy to oxidize or to reduce by one-electron, such as a series of transition metals like Fe, Co, Mn, Cu, Ce, V. Then several competitive termination couplings between different radicals (step 5) or the decompositions of pero radicals (step 6) lead to radical destruction. 

Antioxidants also lose their efficiency at elevated temperatures because of homolytic decomposition of hydroperoxides formed by reversing reaction (4) and by producing a chain-carrying peroxyl radical. However, the homolytic decomposition of hydroperoxides producing alkoxyl radicals by reaction (10) is more important than reaction (-4), because it requires less energy to cleave an LO-OH bond (- 44 kcal/mol) than an LOO-H bond (90 kcal/mol). Cleavage by reaction (10) is also facilitated by metal catalysts and by heat (Chapter 4). 

Sheldon has considered the competing process of homolytic decomposition of hydroperoxides during the epoxidation of olefins with tert-h xty hydroperoxide in the presence of molybdenum complexes. It was found that homolytic decomposition of the hydroperoxide is initiated by electron transfer reactions of Mo(V) and Mo(VI) complexes with the hydroperoxide giving rise to free radical species. Reaction rates and products of hydroperoxide decomposition were dependent on the solvent and on the presence or absence of an olefin. The rates and selectivities of epoxidation were highest in polychlorinated hydrocarbons and very poor in coordinating solvents such as alcohols or ethers [387]. 

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