Autoxidation reactions, mechanisms

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

METAL ION CATALYZED AUTOXIDATION REACTIONS KINETICS AND MECHANISMS... 

The common element of Schemes 1-3 is that they each postulate direct interaction between the metal center and dioxygen. Although it is not stated explicitly, Eqs. (3) and (11) most likely proceed via an inner-sphere mechanism. Thus, the metal-dioxygen interaction implies spin pairing between the reactants when the metal ion is paramagnetic. As a consequence, the formation of the M-O2 type intermediates circumvents the restriction posed by the triplet to singlet transition which seems to be the major kinetic barrier of autoxidation reactions (5). 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

The results surveyed in this chapter demonstrate the composite nature of autoxidation reactions and the potential problems involved in exploring the intimate details of the appropriate mechanisms. The kinetic observatiuons have been interpreted to different depths, but there is still plenty of room for improving available kinetic models. Most... 

Reliable mechanisms can serve as the basis for the design of efficient new catalysts for autoxidation reactions. A systematic analysis of the effects of the non-participating ligands on the kinetics of the overall reaction and on the catalytic activity of the metal center(s) could be a... 

Mechanism of Autoxidation Reactions in Olefine and Polyolcfinic Substances, including Rubber. Trans. Faraday Soc. 38, 348 (1942). 

Metal Ion Catalyzed Autoxidation Reactions Kinetics and Mechanisms Istvdn Fabian and Viktor Csordds INDEX... 

The effects of heteroatoms on autoxidation reactions are reviewed and discussed in terms of six phenomena (1) the effect on reactivity of a-hydrogens in the hydroperoxide chain mechanism in terms of electron supply and withdrawal (2) the effect on a-hydrogen acidity in base-catalyzed oxidation (3) the effect on radical ion stability in base-catalyzed redox chains (4) the possibility of heteroatom hydrogen bond attack and subsequent reactions of the resulting heteroradical (5) the possibility of radical attack on higher row elements via valence expansion (6) the possibility of radical addition to electron-deficient II and III group... 

The [Fen(CN)5NO]3 ion can be oxidized back to NP in the presence of oxygen (see the reactions of NP with thiolates below) (57,60). Detailed kinetic studies on this important bioinorganic oxidation reaction are not available. In a more general context, the mechanism of the autoxidation reactions of NO complexes awaits a systematic study, which could in principle be afforded with the known (MX5(NO + ) series (M = Fe, Ru or Os X = amines, polypyridines, etc.), if appropriate reductants were used to generate the NO complexes. 

The work of Fallab and his collaborators has shown how the coordination act may bring the reactants together in autoxidation reactions. In several instances coordination furnishes a catalytic path for these reactions. Specific examples include the autoxidation of Fe+2 in the presence of sulfosalicylic acid (28), the autoxidation of 1-hydrazinophthalazine by iron (II) (27, 83), and the autoxidation of a formazyl-zinc complex (11). It is probable that the importance of this kind of a mechanism will be more widely realized as more and more detailed kinetic studies are made on metal-catalyzed autoxidation reactions. Some other... 

Metal catalysis, which is claimed to have an important role in initiating autoxidation, appears to be so complex that in some systems catalysts are converted to inhibitors when their concentrations are increased. The additives examined include the N-butylsalicylaldimino and N-phenylsalicylaldi-mino chelates of cobalt(ll), copper(11), nickeVJl), and zinc as well as a number of 3,5-diisopropylsalicylato metal chelates. Some were autoxidation catalysts, some were inhibitors, and some exhibited catalyst-inhibitor conversion. Reaction mechanisms which account for most of the observed phenomena are proposed. The scope for developing metal chelates as antioxidants and the implications concerning the critical antioxidant concentration are outlined. 

If this mechanism is strictly followed the chain length and hence the value of 02-uptake (see below) increases linearly with the substrate concentration and (initiation rate)"1/2 (i.e., in radiolytic studies the dose rate) and in charged polymers also on the pH (cf. Ulanski et al. 1996a). In polymers, the chain reaction may mainly proceed intramolecularly (Ulanski et al. 1996a Janik et al. 2000). An example for an efficient intramolecular autoxidation is poly(acrylic acid) [reactions (34)-(36) Ulanski et al. 1996a], In these autoxidation reactions, hydroperoxides are formed which, in some cases, are quite unstable [e.g. reaction (37) see also Leitzke et al. 2001],... 

Analysis of autoxidation reactions within cells showed that there were distinct mechanisms between intracellular (membrane) and aqueous solutions (215). The autoxidation reaction was suggested to result in N02 formation in the membrane, but not in aqueous solution, supporting the previous study (211). This exemplifies the importance of reaction conditions and kinetics on biological responses. 

Fig. 14.37. Regioselective Baeyer-Villiger rearrangement of an electron-poor aromatic aldehyde. This reaction is part of the autoxidation of benz-aldehyde to benzoic acid. Both alternative reaction mechanisms are shown the [1,21-rearrangement (top) and the /3-elimination (bottom).

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... 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

With purified hydrocarbons where such trace impurities have been scrupulously removed, a long induction time precedes metal-catalyzed autoxidation. The mechanism of formation of initial R02H in the absence of a radical chain is not known in any detail, but metal ions do not seem to be involved. The most important point to note in this context is that various types of metal-dioxygen complexes have been isolated and fully characterized. However, such complexes do not seem to play any role in metal ion initiated autoxidation reactions. 

Entwistle et al. (21) orginally adapted a general free radical mechanism to the autoxidation of alkali cellulose. The reaction mechanism presented below for the interaction of cellulose with oxygen was proposed by Shafizadeh and Bradbury (22) ... 

The basic reaction mechanism for the autoxidation process just presented provides a useful frame of reference for a discussion of this reaction system. However, much needs to be learned about the interaction of cellulose with a composite system that includes oxygen, water, and metallic contaminants. A better understanding of this reaction system is essential for the development of practical measures for the inhibition of the autoxidative degradation of paper. 

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