Autoxidation mechanism

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. 

The literature dealing with the autoxidation mechanism involved in lipid deterioration has been concerned with investigations on pure unsaturated fatty acids and their esters. The reactions involved, however, are representative of those occurring in lipids and lipid-containing food products. 

In contrast to catalytic hydrogenation, where no reaction takes place in the absence of a catalyst, catalytic oxidations with molecular oxygen are complicated by the fact that oxygen reacts with organic substrates even in the absence of a catalyst. This involves the so-called free radical autoxidation mechanism with the following as key steps ... 

We will not go in depth into the subject of antioxidants (12), which is more a part of preformulation than a stress test, but the autoxidation mechanism does suggest that oxidation can be inhibited by peroxy radical scavengers (chain-breaking antioxidants) like phenol antioxidants, by heavy metal chelating agents, and by peroxide inactivating substances (preventive antioxidants). 

The degradation pattern of an organic molecule due to autoxidation is often a complex process, and is one of the reasons why molecular oxygen is seldom the preferred reagent of the synthetic organic chemist. Now that the main features of the autoxidation mechanism have been stated, it is appropriate to analyze the fate of the reactive species afforded by the reaction, in order to understand the whole degradation profile. 

There are two basic methods for making polymer materials photo-chemically degradable.1,2 One method is to chemically incorporate a chromo-phore into the polymer chains. One commercially successful chromophore is the carbonyl group.1,2,7 Absorption of UV radiation leads to degradation by the Norrish type I and II processes or by an atom abstraction process (Scheme 1). Note that once radicals are introduced into the system, chain degradation occurs by the autoxidation mechanism (Scheme 2). 

The browning of proteins by glucose is catalysed by transition metal ions and by air. The AGEs in tissue proteins best characterised are CML and pentosidine, products of a combination of glycation and oxidation of hexoses or ascorbate. In view of facts such as these, the AGE hypothesis evolved to accommodate a role for oxidative stress.399 Multiple autoxidative mechanisms are involved in the formation of AGEs, as illustrated in Scheme 8.4 for the formation of CML from glucose, including ... 

Alkylperoxy radicals play vital roles in both propagation and termination processes. Hydroperoxides, R02H, are usually the primary products of liquid phase autoxidations [reaction (4)] and may be isolated in high yields in many cases. Much of the present knowledge of autoxidation mechanisms has resulted from studies of the reactions of alkylperoxy radicals30-33 and the parent hydroperoxides,348-d independently of autoxidation. Thus, the various modes of reaction of organic peroxides are now well-characterized.35 -39... 

Hayou, E., A. Treinin, and J. Wilf (1972). Electronic spectra, photochemistry and autoxidation mechanism of sulfite-bisulfite-pyrosulfite systems. J. Am. Chem. Soc. 94,47-57. 

In the cyclohexane oxidation route cyclohexane is oxidized with air at 125-126°C and 8-15 bar in the liquid phase using Co or Mn naphthanates as the catalyst. This affords a mixture of cyclohexanol and cyclohexanone via a classical free radical autoxidation mechanism. Cyclohexane conversion is limited to 10-12% in order to minimize by-product formation via further oxidation. The selectivity to cyclohexanol/cyclohexanone is 80-85%. 

E. N. Frankel, Chemistry of Autoxidation, Mechanism, Products and Flavor Significance, Flavor Chemistry of Fats and Oils, American Oil Chemists Society Publication, Peoria, Illinois, 1985. 

Probable Autoxidation Mechanism under Ordinary Conditions. For some time it was believed that this reaction proceeded in the following manner Oxygen adds to the aldehyde, forming a very imstable peroxide. This peroxide isomerizes to a peracid (in the present case perbenzoic acid), which is stable enough for assay. The steps are represented as follows ... 

Autoxidation mechanism, Cross-activation, Nitric radicals. Reactors, Sulphoxy radicals Introduction... 

At the National Institute of Chemistry (NIC), in the frame of CMD subproject of EUROTRAC-2, experimental studies of the role of soluble constituents of atmospheric aerosols in the aqueous-phase autoxidation mechanisms of S(IV) was studied. The research focused on atmospheric water droplets (clouds, fog), where soluble constituents of atmospheric particles may be important in aqueous SO2 oxidation under non-photochemical conditions. In the frame of CMD project laboratory experiments in a semi-batch continuous stirred tank reactor under controlled conditions (T, air flow rate, stirring), were made in order to study the autoxidation of S(IV)-oxides catalyzed by transition metal ions (Fe(III), Fe(II), Co(II), Cu(II), Ni(II), Mn(II)). These studies were carried out at the National Institute of Chemistry. 

Firstly, compared to the same complex in free solution or as a crystalline solid, the zeolite encapsulated material displays quite remarkable stability towards autoxidation and peroxo-dimer formation(IO). For example the half-life for the oxygen adduct epr signal at room temperature in air is 4weeks when entrapped in the zeolite compared to several minutes in free solution (10) (in the crystalline Co-salen solid, peroxo-dimers are formed exclusively (10)). This is a manifestation of the extremely effective site-isolation achieved by entrapping the complexes inside the pores of the zeolite leading to elimination of the normal autoxidation mechanism. 

The reactive species that initiate free-radical oxidation are present in trace amounts. Extensive studies (11) of the autoxidation mechanism have clearly established that the most reactive materials are thiols and disulfides, heterocyclic nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further complicates the control of oxidation. It is difficult to avoid some metals, particularly iron, in fuel systems. 

Frankel, E.N. (1985) Chemistry of autoxidation Mechanism, products and flavor significance, in Flavor Chemistry of Fats and Oils (eds D.B. Min and T.H. S mouse), AOCS Press, Champaign, IL, pp. 1-37. 

TBA-I catalyzed the oxidation of cyclohexane with 1 atm molecular oxygen at 365 K. The main products wa e cyclohexanol and cyclohexanone and an induction period was observed. The selectivities changed little with time. A small amount of dicyclohexyl, which is formed by the reaction of two cyclohexyl radicals, was observed. Neither acids nor oxoesters were observed. The induction period and the formation of dicyclohexyl suggest that the reaction involves a radical-chain autoxidation mechanism. The... 

These observations may also be interpreted by the free radical mechanism, 02 again being formed as in step 1 of the autoxidation mechanism... 

Gerothanassis, I. R, Momenteau, M., Loock, B. (1989). Hydrogen-bond stabilization of dioxygen, conformation excitation, and autoxidation mechanism in hemoprotein models as revealed by 0 NMR spectroscopy, J. Am. Chem. Soc., Ill 7006. 

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