Hydroperoxide free radical-induced decomposition

Free Radical-Induced Decomposition of Allylic Peroxides and Hydroperoxides... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

The decomposition of hydroperoxides can also be induced by raising the temperature and is promoted by metal catalysis. Free radicals formed during the reaction (4) and (5) can also take part in radical induced decomposition of hydroperoxides... 

The latter observations with methyl oleate, together with thermodynamic considerations and EPR evidence for free radical intermediates, suggest an alternative explanation for the dramatic increase in oxidation rates once hydroperoxides accumulate, namely that bimolecular decomposition may be specific to allylic hydroperoxides and proceed via LOO radical-induced decomposition rather than by dissociation of hydrogen-bonded dimers (280). Reaction sequence 63 is analogous to Reactions 49 and 50a, where one slowly reacting radical reacts with a... 

It has been reported that decomposition of benzenesulphonyl azide in thiols is induced by thiyl radicals, particularly in the presence of free radical sources The major product was benzenesulphona-mide. Benzenesulphonyl azide and i-butyl hydroperoxide exhibit mutually induced decomposition in chlorobenzene at 126 7° . The rate of decomposition of the azide at the beginning and end of the reaction was that expected for the uncatalysed decomposition, but in... 

Wellington (1980) discussed the interaction of hydroperoxides with iron as a transition metal catalyst which produces more free radicals which may initiate further radical-induced decomposition of the polymer. In order to remove the oxygen from solution, reducing agents such as sulphite and bisulphite may be present however, these two may react with iron to produce radicals. For example, the reaction between bisulphite (HSO") and Fe as follows ... 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

Organic hydroperoxides, such as -butyl hydroperoxide, (0113)30—0-OH, likewise induce polymerization in vinyl monomers through the action of free radicals formed as primary intermediates in their decomposition. The following compounds, or classes of compounds, also are effective polymerization initiators at temperatures where they undergo slow thermal decomposition by mechanisms which are believed to involve the release of free radicals as indicated ... 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

The resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

Iron(III) weso-tetraphenylporphyrin chloride [Fe(TPP)Cl] will induce the autoxidation of cyclohexene at atmospheric pressure and room temperature via a free radical chain process.210 The iron-bridged dimer [Fe(TPP)]2 0 is apparently the catalytic species since it is formed rapidly from Fe(TPP)Cl after the 2-3 hr induction period. In a separate study, cyclohexene hydroperoxide was found to be catalytically decomposed by Fe(TPP)Cl to cyclohexanol, cyclohexanone, and cyclohexene oxide in yields comparable to those obtained in the direct autoxidation of cyclohexene. However, [Fe(TPP)] 20 is not formed in the hydroperoxide reaction. Furthermore, the catalytic decomposition of the hydroperoxide by Fe(TPP)Cl did not initiate the autoxidation of cyclohexene since the autoxidation still had a 2-3 hr induction period. Inhibitors such as 4-tert-butylcatechol quenched the autoxidation but had no effect on the decom-... 

There have been very few reports of azide decompositions induced by radicals. It has been found that the decomposition of phenyl azide was accelerated by thiyi radicals, particularly when it was carried out in thiols as solvents in the presence of free radical sources, such as tetraphenylhydrazine, hexaphenylethane and triphenylmethyl hydroperoxide When the decomposition was carried out in thiophenol, the major product was aniline, along with a small amount of o-aminodiphenylsulphide (232)... 

Oxidation of hydrocarbons has been known for many years to involve the formation of key intermediate hydroperoxides and dialkylperoxides ( peroxides in general) from the reaction of oxygen and hydrocarbons via free radical intermediates. At low temperatures, the peroxides formed slowly accumulate and eventually decompose either thermally or by metal-induced reactions or by ionic routes. At high temperatures, formation and thermal decomposition of the peroxides occurs rapidly. Thermal decomposition leads to the production of additional free radicals (the propagation step of the reaction) and the formation of oxygen-containing products (e.g., acids, alcohols, ketones, polar compounds, and polymeric materials) that can ultimately bring about lubricant failure. 

Radical involvement is indicated also by the inhibitory effect of radical trapping agents and product analysis, since 10-15% of the products from cumene hydroperoxide decomposition induced by the organic sulfur compounds result from free-radical processes (2). Acids will... 

Post-heating of a shaped polyethylene which induces its crosslinking may sometimes lead to an undesired deformation of the product. To overcome this difficulty, two-component redox-initiating systems producing free radicals at lower temperatures have been designed [108]. The reaction system involved cumyl hydroperoxide and a transition metal ion, whose higher and lower oxidation states differ by one electron. The decomposition of hydroperoxide proceeds by an electron transfer mechanism an electron is transferred from the metallic ion (e.g. Co ) to the peroxidic bond which splits into two fragments. 

As described previously, thermooxidative degradation of polyolefins proceeds by a typical free-radical chain mechanism in which hydroperoxides are key intermediates because of their thermally-induced hemolytic decomposition to free radicals, which in turn initiate new oxidation chains. However, since the monomolecular hemolytic decomposition of hydroperoxides into free radicals require relatively high activation energies, this process becomes effective only at temperatures in the range of 120°C and higher. 

The free radical yield/for AIBN in styrene and various solvents at 50°C is /= 0.5. Because of induced decomposition,/varies strongly with solvent in the case of BPO. If, for steric reasons, the primary free radicals cannot recombine, then the free radical yield can, according to conditions, increase up to/= 1. Thus, the start reaction is rarely a simple function of added initiator concentration, since it depends on free radical yield and may also depend on induced decomposition. Because of this, faster initiator decomposition need not necessarily produce faster polymerization. For example, dibenzoyl peroxide decomposes a 1000 times faster in benzene than cyclohexyl hydroperoxide, but only polymerizes styrene five times as fast. 

If in the chain initiated reaction when v,- = const the induction period is independent of the efficiency of retardation action of the inhibitor but is determined by its concentration, then during autoxidation the inhibitor is more slowly consumed when it more efficiently terminate chains because ROOM is more slowly accumulated and the retardation period increases. Then the initiated oxidation of hydrocarbons is retarded only by compounds terminating chains. Autoxidation is retarded by compounds decomposing hydroperoxides. This decomposition, if it is not accompanied by the formation of free radicals, decreases the concentration of the accumulated hydroperoxide and, hence, the autoxidation rate. Hydroperoxide decomposition is induced by compounds of sulfur, phosphorus and various metal complexes, for example, thiophosphate, thiocarbamates of zinc, nickel, and other metals. 

Thus, LOX-catalyzed oxidative processes are apparently effective producers of superoxide in cell-free and cellular systems. (It has also been found that the arachidonate oxidation by soybean LOX induced a high level of lucigenin-amplified CL, which was completely inhibited by SOD LG Korkina and TB Suslova, unpublished data.) It is obvious that superoxide formation by LOX systems cannot be described by the traditional mechanism (Reactions (1)-(7)). There are various possibilities of superoxide formation during the oxidation of unsaturated compounds one of them is the decomposition of hydroperoxides to alkoxyl radicals. These radicals are able to rearrange into hydroxylalkyl radicals, which form unstable peroxyl radicals, capable of producing superoxide in the reaction with dioxygen. 

Although some termination may occur via free alkoxy radicals, kinetic and isotopic labeling data are most consistent with termination via the tetroxide in the case of r-butyl hydroperoxide . The chain length for the decomposition of t-butyl hydroperoxide is approximately 6-10 , Decomposition of the hydroperoxide is induced by t-butoxy radicals which are generated from di-t-butyl-... 

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