Reaction mechanism autoxidations

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 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. 

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).

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. 

Either the monomolecular or the dimolecular decomposition serves to feed new radicals into the reaction to initiate the chain reaction of autoxidation. These radicals may further react through different paths. They may follow a radical chain mechanism or other well-known radical reactions, such as coupling or disproportionation. 

Linoleic acid and its esters were frequently used in the study of autoxidation Being a 1,4-diene, linoleic acid has one particularly weak a-methylenic C... H bond, so that, at least in the initial stages, the hydroperoxide which becomes conjugated is practically the only oxidation product. The generally accepted reaction mechanism is the chain reaction... 

A reaction mechanism for the metal-ion-catalyzed autoxidation of ascorbic acid, involving the formation of an intermediate ternary ascorbate-metal ion-dioxygen complex, is illustrated in Scheme 2. Although the bonding between the metal ion and the dioxygen in the intermediate... 

In order to explain the autoxidation processes, one generally has recourse to the chain-reaction mechanism, depending on the existence of highly unstable intermediates often of a hypothetical nature only. Examples of such unstable compounds are found in the papers by Bodenstein on the autoxidation of acetaldehyde (2) and by Haber and Franck on the oxidation of sodium bisulfite (26). This mode of explanation was not pursued in the present work, as it would have necessitated the addition of new hypotheses to explain the incorporation of the ozone molecule into the proposed chain reactions. But one may consider, in a simplified manner, that ozone starts the chain by means of its active oxygen. It has been stated (27) that peroxides, especially benzoyl peroxide, act as autoxidation catalysts. 

The simplicity of the easily surveyed reaction equation is strongly misleading. The reaction mechanism of the autoxidation of alkyl-substituted aromatic compounds consists of several complex steps - free-radical chain reactions triggered by oxidation catalysts. In general, two initiation steps can be distinguished [5, 6, 10] ... 

The modification of the initiation step and the reaction mechanism is based on the high reactivity of the catalytic active transition metal complexes toward peroxides and peroxy radicals which are formed by autoxidation or from benzylic radicals which have been trapped by oxygen. To elucidate the combined complex reaction mechanisms, the impact of the single catalysts will be discussed separately. 

Molecular oxygen is the major cause of irreversible deterioration of hydrocarbon substrates, leading to the loss of useful properties and to the ultimate failure of the substrate. The oxidation process of hydrocarbons is autocatalytic oxidation starts slowly, sometimes with a short induction period, followed by a gradual increase in the rate, concomitant with the build up of hydroperoxides, which eventually subside, giving rise to a sigmoidal oxidation curve. When initiators such as peroxides are present, the length of the induction period is absent, or very short, but it can be prolonged by antioxidants, as shown in Fig. 1. The basic autoxidation theory of hydrocarbons involves a complex set of elementary reaction steps in a free radical-initiated chain reaction mechanism the basic tenets of this theory apply equally to polymer oxidation. 

The idea that the processes, which occur during the combustion of a fuel in an engine cylinder, take place by a chain reaction mechanism comparable to that associated with the liquid phase autoxidation of benzal-dehyde, has led to the proposal that knock suppressors act by destroying the chains and reducing the rate at which the flame front accelerates. It is known that surfaces or solid bodies suppress chain reactions, in fact one of the criteria for a chain reaction in gaseous combustion processes is the decrease in rate of reaction caused by the increase in surface exposed to the gases. However, the mode by which the chains are broken or... 

The di-iron site and formulation of the 02-binding reaction Mechanism of dioxygen binding Autoxidation Cooperative hemerythrins Mixed-valent forms Synthetic models... 

Russell, G.A. Pathways in autoxidation, in Peroxide Reaction Mechanisms, pp. 107-128 (1962) (edited by J. O. Edwards), Interscience Publ., New York. 

The mechanism of action of ascorbic acid and its derivatives is given in Section 5.14.6.1.4, and the mechanism of action of tocopherols is described in Section 5.4.6. Phenolic compounds (Ar-OH) as primary antioxidants may interfere with the oxidation of hpids (R-H) in a reaction in competition with the propagation phase reaction of autoxidation. They react with hydroperoxyl radicals... 

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