Hydroperoxides hydrocarbon-substituted

TABLE 1. Enthalpies of formation of hydrocarbon-substituted hydroperoxides and peroxides (kJ mol )... 

PINO possesses a high reactivity in the reaction with the C—H bond of the hydrocarbon. Hence, the substitution of peroxyl radicals to nitroxyl radicals accelerates the chain reaction of oxidation. The accumulation of hydroperoxide in the oxidized hydrocarbon should decrease the oxidation rate because of the equilibrium reaction. 

In the oxidation of a hydrocarbon the substitution product is an alkyl hydroperoxide. The mechanism is rendered more complex by the fact that the oxygen molecule is diatomic and oxygen atom bivalent. 

This paper presents the results of an investigation of the oxidation of substituted olefins in the presence of hydrocarbon-soluble transition metal complexes. Results indicate that the initial interaction of oxygen with the olefin probably does not occur within the coordination sphere of the metal. The best interpretation appears to be autoxidation of the olefin, initiated either by the metal or by metal catalyzed decomposition of peroxidic impurities. The initial product of an olefin having allylic hydrogens is an allylic hydroperoxide species this is usually the case in radical initiated autoxidations. Nonetheless, with some metal complexes the product profile differs markedly from that observed when radical initiators are used. In the presence of several complexes, oxidation is... 

In addition to the usual reactions of the catalyst with intermediate hydroperoxides, the second type of reaction undoubtedly involves direct reaction of the metal catalyst with the hydrocarbon substrate and/or with secondary autoxidation products. Two possible pathways can be visualized for the production of radicals via direct interaction of metal oxidants with hydrocarbon substrates, namely, electrophilic substitution and electron transfer. Both processes are depicted below for the reaction of a metal triacetate with a hydrocarbon. 

New materials consisting of amorphous silica with regular pore structure, therefore called mesoporous molecular sieves, have recently been described [7]. Isomorphous substitution of Si by Ti has been attempted by performing the synthesis in the presence of titanium compounds. Ti-MCM have been tested for oxidation of hydrocarbons in liquid phase, using HjOj or hydroperoxides as oxidants [8-10]. 

Preliminary results with a manganese-substituted Keggin ion catalyst that has an extremely stable PWi 1039 - backbone (Figure 1), shows some promise with small hydrocarbons(l). This catalyst can be heated to 65 °C for long periods without decomposition. An initial experiment with ethane and t-butyl hydroperoxide in benzene gave 2 turnovers of ethane to ethanol in three hr at 65 °C, while with propane the turnover number was 24 and provided isopropanol and n-propanol in a 5 1 ratio (Table m). 

Table m. Carbon-Hydrogen Activation of C1-C3 Hydrocarbons with a Manganese-Substituted Keggin Ion Catalyst Using t-Butyl Hydroperoxide as the Monooxygen Transfer Reagent in Benzene a... 

The reason for this trend in going from 1,4-cyelohexadiene to 1,4-di-hydronaphthalene to 9,10-dihydroanthracene is that, while the ease of removal of the initial H-atom is expected to be about the same for each compound, ease of removal of the 3-H-atom in the resulting radical decreases with the increase in benzo substitution as shown in Table 3. Thus the removal of an H-atom from the hydroanthracene radical by oxygen is thermoneutral and the reaction is sufficiently slow so that the organic hydroperoxide route dominates. The reaction of 1,4-dihydro-naphthalene is intermediate. The reaction of the 1-hydronaphthyl radical with oxygen to form naphthalene is exothermic by 10 kcal mole 1 however, it is sufficiently slow so that, as the hydrocarbon concentration is increased, the organic peroxide product increases. 

Standard enthalpies of formation, zW°29s are calculated using the B3LYP/6-311G(d,p) calculation level of density functional theory and isodesmic reaction schemes for cancellation of errors. The recommended a//°29s of each value is the average value of data obtained from several isodesmic reactions. Comparison of the thermodynamic properties between the peroxides and corresponding hydroperoxides shows uniformity in property trends. A set oxygenated hydrocarbon groups are determined that result in consistent prediction of thermodynamic properties for the substituted alkyl and vinyl peroxides and hydroperoxides. 

In Part 2, a system based on f-butylhydropero.xide (TBHP) is described. This is similar to the above Gif sterns, but the kinetic isotope effect is very different and the selectivity for adamantane substitution is different. However, Fe is activated by TBHP to an Fe oxenoid which, after reaction with hydrocarbon, then reacts with oxygen to give hydroperoxide. So the pattern of intermediates A and B seen in Part I is maintained with TBHP. Radical chemistiy may be involved in some of the reactions tliat involve ionic coupling to saturated hydrocarbons. 

As mentioned in the previous section, R-, RO-, and RO2 radicals and hydroperoxides are the main products of hydrocarbon oxidation. For this reason, easily oxidized polymers (polyolefins, polyamides, polystyrene, etc.) are stabilized by compounds that can react directly with peroxide radicals or directly with hydroperoxides. They are substituted phenols, aromatic amines, mercaptans, organic sulfides, etc. they are called antioxidants. 

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