Decomposition, of peroxides

Catalysts and Promoters. The function of catalysts in LPO is not weU understood. Perhaps they are not really catalysts in the classical sense because they do not necessarily speed up the reaction (25). They do seem to be able to alter relative rates and thereby affect product distributions, and they can shorten induction periods. The basic function in shortening induction periods appears to be the decomposition of peroxides to generate radicals (eq. 33). 

Thermal and photochemical decomposition of peroxides (4) and (5) lacking a-hydrogens (those derived from ketones) produces macrocycHc hydrocarbons andlactones (119,152,153). For example, 7,8,15,16,23,24-hexaoxatrispiro [5.2.5.2.5.2] tetracosane (see Table 5) yields cyclopentadecane and oxacycloheptadecan-2-one. 

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. 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

The luminescence reaction of coelenterazine is initiated by the peroxidation of coelenterazine at its C2 carbon by molecular oxygen (Fig. 3.3.4). Then, the peroxidized coelenterazine decomposes into coelenteramide plus CO2, producing the energy needed for the light emission. For the mechanism of the decomposition of peroxide that produces the energy, two different pathways can be considered. 

The Chemistry of Radical Polymerization Table 3.5 Selected Kinetic Data for Decomposition of Peroxides ... 

The reactions of /-butoxy radicals are amongst the most studied of all radical processes. These radicals are generated by thermal or photochemical decomposition of peroxides or hyponitrites (Scheme 3.75). 

Peroxide decomposition in aromatic and other unsaturated solvents homolytic aroniMic substitution and olefin polymerization Decomposition of peroxides in aromatic solvents leads to attack on the aromatic nucleus by radicals and hence to substitution products (for a recent summary, see Williams, 1970). In the substitution of benzene and related substrates by phenyl radicals, for example, cyclohexadienyl... 

FIGURE 15.2 Decomposition of peroxides by ions of metals (Redox mechanism). 

A detailed physical examination of the purple complex formed in alkaline solution between Fe(III), ethylenediaminetetraacetic acid (EDTA) and peroxide shows it to have a composition [Fe "(EDTA)02] (togA, 3 c =4.33). This complex catalyses decomposition of peroxide, the rate-pH profile going through a maximum at pH 9-10 . 

There are a lot of accidents involving the handling of hydrogen peroxide in partly rusted iron containers, which catalyse the explosive decomposition of peroxide. 

It gives rise to an extremely violent reaction with hydrogen peroxide. This is certainly due to the catalysis of the decomposition of peroxide caused by the phosphoric acid produced by water. 

The standard approach to reducing scorch (e.g. discoloration resulting from oxidation) involves addition of one or more antioxidants. Scorch retardants prevent premature decomposition of peroxides and cross-linking of polymers (pre-vulcanisation). 

This may occur by free-radical formation, especially in the presence of transition-metal ions such as those of iron or copper. Similar mechanisms can result in the decomposition of peroxide but there are means of controlling or avoiding this problem. 

Decomposition of Peroxides by Various Stabilizers. The efficiency of tert-butyl hydroperoxide decomposition in tert-butyl alcohol by various additives was determined (Table 9). Under the conditions of these experiments, the phenolic antioxidants and dilauryl thiodipropionate had little or, often, no effect on the hydroperoxide decomposition. The three zinc salts effectively inhibited peroxide decomposition. This effect might briefly inhibit the onset of substrate oxidation under weathering-test conditions, but the peroxide would decompose whenever its concentration reached a sufficient level to permit significant light... 

A 6000 m3 aqueous refinery effluent containing sulfides and traces of hydrocarbons was to be treated to remove sulfides before discharge. Aqueous ferric chloride was added, followed by 1800 1 of 50 wt% peroxide added over 40 min, and after a further 30 min an explosion occurred which blew off the lid of the treatment vessel. This was attributed to ignition of the explosive mixture of hydrocarbon vapours and oxygen (from iron-catalysed decomposition of peroxide) formed above the liquid surface. 

Radicals produced from the initiator either directly attack the organic compound RH (for instance, this is the case during the decomposition of peroxides) or first react with dioxygen, and then, already as peroxyl radicals, attack RH (for instance, this is the case of decomposition of azo-compounds). RH gives rise to alkyl radicals when attacked by these radicals. The reaction of dioxygen addition to an alkyl radical,... 

This dependence is the result of general occurrence of the homolytic decay of peroxide with the rate constant kd and chain decomposition of peroxide due to reactions with the radical formed from the solvent RH according to the following kinetic scheme ... 

The very intensive chain decomposition of benzoyl peroxide was found in alcoholic solutions [16,18,19]. This is the result of the very high reductive activity of ketyl radicals formed from alcohol. They cause the chain decomposition of peroxide by the following mechanism ... 

The decomposition of peroxides in the presence of stannum hydride is accompanied by the chain decomposition of peroxide [56]. Chain propagation occurs by the substitution reaction. 

Ketones play an important role in the decomposition of peroxides to form radicals in alcohols undergoing oxidation. The formed hydroxyhydroperoxide decomposes to form radicals more rapidly than hydrogen peroxide. With an increase in the ketone concentration, there is an increase in the proportion of peroxide in the form of hydroxyhydroperoxide, with the corresponding increase in the rate of formation of radicals. This was proved by the acceptor radical method in the cyclohexanol-cyclohexanone-hydrogen peroxide system [59], The equilibrium constant was found to be K — 0.10 L mol 1 (373 K), 0.11 L mol 1 (383 K), and 0.12 L mol 1 (393 K). The rate constant of free radical generation results in the formation of cyclohexylhydroxy hydroperoxide decomposition and was found to be ki = 2.2 x 104 exp(—67.8/7 7) s 1 [59]. 

An accident investigation identified the cause of the accident to be the rapid decomposition of peroxides, which formed in the ether while the bottle sat in storage. It is hypothesized that some of the peroxides crystallized in the threads of the cap and exploded when the cap was turned. 

A hydrogen peroxide still is used to concentrate peroxide by removing water. The still is of high-purity aluminum, a material that is noncatalytic to the decomposition of peroxide vapor. The still is designed to produce 78% hydrogen peroxide. It will explode spontaneously at about 90%. Illustrate some recommended design features for this still. 

Use transition metal sequestrants such as phosphonates or nitrilotriacetic acid in the formulation containing peroxygen bleaches, since transition metals can catalyze the decomposition of peroxide. 

A large quantity of discoloured (and peroxidised) turpentine was heated with fuller s earth to decolourise it, and it subsequently exploded. Fuller s earth causes exothermic catalytic decomposition of peroxides and rearrangement of the terpene molecule. 

Thermal decomposition of peroxides generates a variety of radical groups which can covalently react with CNTs. This strategy has been employed in particular by Peng and co-workers who functionalized sidewalls with benzoyl, lauroyl and carboxyalkyl derivatives [38]. 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

A wide variety of peroxides have been used to produce alkyl radicals, either directly as fragments of the decomposition of peroxides, or indirectly by hydrogen abstraction from suitable solvents. The production of alkyl radicals used in homolytic alkylation has been accomplished by thermal or photochemical homolysis and recently also by redox reactions due to the possibilities offered by alkylation in acidic aqueous solution. 

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