Free-radical mechanism, alkane oxidation

This was also accomplished with BaRu(0)2(OH)3. The same type of conversion, with lower yields (20-30%), has been achieved with the Gif system There are several variations. One consists of pyridine-acetic acid, with H2O2 as oxidizing agent and tris(picolinato)iron(III) as catalyst. Other Gif systems use O2 as oxidizing agent and zinc as a reductant. The selectivity of the Gif systems toward alkyl carbons is CH2 > CH > CH3, which is unusual, and shows that a simple free-radical mechanism (see p. 899) is not involved. ° Another reagent that can oxidize the CH2 of an alkane is methyl(trifluoromethyl)dioxirane, but this produces CH—OH more often than C=0 (see 14-4). ... 

Carboxylic acids are oxidized by lead tetraacetate. Decarboxylation occurs and the product may be an alkene, alkane or acetate ester, or under modified conditions a halide. A free radical mechanism operates and the product composition depends on the fate of the radical intermediate.267 The reaction is catalyzed by cupric salts, which function by oxidizing the intermediate radical to a carbocation (Step 3b in the mechanism). Cu(II) is more reactive than Pb(OAc)4 in this step. 

CIS-[Ru(H20)2(dinso) ] is made from as-RuClj(dmso) and Ag(BF ) in aq. EtOH. The system c/s-[Ru(H20)j(dmso) ] Vaq. Na(ClO) or TBHP/CH Cl oxidised alkanes such as adamantane, cyclo-octane, -heptane and -hexane to the corresponding alcohols and ketones as did [Ru(Hj0) PWjj(0)3g ] . A free-radical mechanism may be involved for the TBHP oxidations, but those with (C10) probably involve oxoruthenate(VI) or oxoruthenate(IV) intermediates [823], The oxidative destruction of a-chlorinated alkenes by CM-[Ru(HjO)2(dmso) ] Vaq. Oxone /Me(CH3) jN(HSO ) MCj to carboxylic acids and ultimately to CO and HCl was reported [946],... 

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

It is clear from a recent review of the mechanisms of metal-catalyzed oxidations of hydrocarbons (89) that by far the most extensive studies have been on the oxidation of alkenes and aromatic compounds relatively little work on alkane oxidation is to be found. The studies on these reactions show that, if the reactivity is enhanced by a hard metal, it is often because the metal becomes involved in the free-radical reactions and generates further free radicals by the chain decomposition of hydroperoxides (39) ... 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

Mill, T. and D.G. Hendry. 1980. Kinetics and mechanism of free radical oxidation of alkanes and olefins. In C.H. Bamford and C.F. Tipper, Eds., pp. 1-83, Comprehensive Chemical Kinetics, Vol. 16, Chapter 1. Elsevier, New York. 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

Hydrogen peroxide is also the oxidant in the halogenations of alkanes under Gif conditions. In these systems, developed by Barton and his coworkers312, alkanes are selectively transformed into alkyl chlorides or bromides by polyhaloalkanes and H202 in the presence of FeCypicolinic acid catalyst in pyridine/acetic acid solvent313-315. It has clearly been established that the reaction mechanism does not involve a free-radical intermediate. 

The above analysis shows that the formation of free radicals in the interaction of alkane molecules with the surface of oxides may prove to be energetically preferable as compared to any other mechanisms of their activation. Furthermore, this process requires only one type of single active centers and it proceeds in a single step. The combination of these factors may render this process the most favorable. This conclusion is experimentally confirmed by... 

Kinetics and Mechanisms of Free Radical Oxidation of Alkanes and Olefins in the Liquid Phase... 

The major objective of this chapter is to provide a critical review of the kinetics and mechanisms of free radical oxidation of alkanes and alkenes and the techniques for their measurement and determination under mild conditions in the liquid phase. A brief discussion of photooxygenation (singlet oxygen) reactions is included for completeness. Literature has been reviewed carefully through 1975 and updated with references to mid-1978. 

The free-radical substitution of H for OOH in alkanes is called autoxidation. ( Autoxidation is a misnomer, because the substrate is not oxidizing itself O2 is oxidizing the substrate ) Autoxidation proceeds by a free-radical chain mechanism. Note that the mechanism for oxidation includes a very rare radical-radical combination step in the propagation part. The radical-radical combination step doesn t terminate the chain in this particular reaction because O2 is a... 

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