Alkanes by oxygen

Oxidation of n-butane. In the presence of oxygen, Co(l 1) is converted into Co(lll), the actual catalyst for oxidation of alkanes by oxygen thus oxidation of n-butane by Co(lll) ion at 100° at a pressure of 17-24 atm. gives acetic acid (83.5% yield) together with traces of n-butyric acid, propionic acid, and methyl ethyl ketone. Oxidation of n-pentane under similar conditions gives acetic acid (48% yield) and propionic acid (27% yield). Isobutane is relatively inactive. The reaction involves electron transfer in which cobalt ions function as chain carriers. 

Considering in a general way the activation of light alkane by oxygen, the ammoxidation of propane has certainly not to be forgotten. This process is already under industrial development. 

Scheme 6.1. A cartoon depicting a possible path for the removal of a hydrogen from an alkane by oxygen, producing a carbon radical and the hydrogen peroxide radical, followed by recombination of the radicals to yield an alkylhydroperoxide.

Davda, R. R. Shabaker, J. W. Huber, G. W. Cortright, R. D. Dumesic, J. A., A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueous-phase reforming of oxygenated hydrocarbons over supported metal catalysts. Applied Catalysis B 2005,56,171. 

J. M. Thomas, R. Raja, G. Sankar, and R. G. Bell, Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen. Nature 398,227 (1999) J. M. Thomas, Designing a molecular sieve catalyst for the aerial oxidation of n-hexane to adipic acid, Angew. Chem. Int. 

Notably, once the oxygenated hydrocarbons have been converted into synthesis gas, it is then possible to carry out the subsequent conversion of synthesis gas into a variety of liquid products by well-established catalytic processes, such as the production of long-chain alkanes by Fischer-Tropsch synthesis and/or the production of methanol. 

An alternate method of dehydrogenation is by reaction of alkane with oxygen ... 

Data on alkyl radical oxidation between 300° and 800°K. have been studied to establish which of the many elementary reactions proposed for systems containing alkyl radicals and oxygen remain valid when considered in a broad framework, and the rate constants of the most likely major reactions have been estimated. It now seems that olefin formation in autocatalytic oxidations at about 600°K. occurs largely by decomposition of peroxy radicals rather than by direct abstraction of H from an alkyl radical by oxygen. This unimolecular decomposition apparently competes with H abstraction by peroxy radicals and mutual reaction of peroxy radicals. The position regarding other peroxy radical isomerization and decomposition reactions remains obscured by the uncertain effects of reaction vessel surface in oxidations of higher alkanes at 500°-600°K. 

All hydrocarbons are attacked by oxygen at elevated temperatures. If enough oxygen is present, complete combustion to carbon dioxide and water occurs. This is particularly important since alkanes are the major constituents of natural gas, and the principal use of natural gas is in combustion. 

Not only alkenes and arenes but also other types of electron-rich compound can be oxidized by oxygen. Most organometallic reagents react with air, whereby either alkanes are formed by dimerization of the metal-bound alkyl groups (cuprates often react this way [80]) or peroxides or alcohols are formed [81, 82]. The alcohols result from disproportionation or reduction of the peroxides. Similarly, enolates, metalated nitriles, phenolates, enamines, and related compounds with nucleophilic carbon can react with oxygen by intermediate formation of carbon-centered radicals to yield dimers (Section 5.4.6 [83, 84]), peroxides, or alcohols. The oxidation of many organic compounds by air will, therefore, often proceed faster in the presence of bases (Scheme 3.21). 

M. domestica also hydroxylates and metabolizes alkenes and alkanes to oxygenated derivatives. The epoxide and ketone derivative of (Z)-9-tricosene, (9,10)-epoxytricosane and (Z)-14-tricosene-10-one are derived from the C23 alkene by a cytochrome P450 enzyme which epoxidizes at the 9,10 position and hydroxylates at carbon 10 from the other end of the molecule, and the secondary alcohol is then oxidized to the ketone (Ahmad et al., 1987). 

Hill and co-workers have reported that TMSP is effective for the epoxidation of alkenes and the oxygenation of alkanes by use of f-BuOOH and PhIO as oxidants [67]. The characteristics of TMSP in alkene epoxidation, compared with those of metalloporphy-rins, Schiff base complexes, and triflate salts, are as follows [67] ... 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

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