Autoxidation theory

The development of the autoxidation theory, in which the propagating radicals, alkyl, and alkylperoxyl (R ROO ), and the hydroperoxide (ROOH) are the key intermediates, has therefore led to a comprehensive theory of antioxidant action Scheme 2 shows the two major... 

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 basic autoxidation theory of hydrocarbons, which involves a complex set of elementary reaction steps initiation, propagation, and termination, is similarly... 

In recent years Emanuel, Neiman, and their respective schools have greatly contributed to the theory of antioxidant action by studying the phenomenon of the critical antioxidant concentration in terms of a degenerate branched chain reaction. The critical antioxidant concentration, a well-established feature of phenolic antioxidants, is one below which autoxidation is autocatalytic and above which it proceeds at a slow and steady rate. Since the theory allowed not only a satisfactory explanation of the critical antioxidant concentration itself but elucidation of many refinements, such as the greater than expected activity of multifunctional phenolic antioxidants (21), we wondered whether catalyst-inhibitor conversion could be fitted into its framework. If degenerate chain branching is assumed to be the result of... 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

The ideas from the dissertation were then further developed in a paper with Kramers whose intimate knowledge of the theories of Bohr concerning atomic structure contributed essentially to the correct representation of the theory. In this paper inter alia the possible occurrence of explosive chain reactions, i.e., chain reactions in which no steady state concentrations of the intermediates is obtainable, was mentioned. The surprising results by Moureau and Dufraisse (17) concerning antioxidants were explained as being due to autoxidation proceeding as a chain reaction, the chains being broken by the antioxidant. 

Examples. In the accepted theory of autoxidation, through the formation of organic hydroperoxides, the rate-controlling step in the maxi-mum-rate stage is Equation (4). For this case, the observed rate of oxidation in Stage III is that of the key compound in question, RH. Thus, Tobolsky, Metz, and Mesrobian expressed the equation for maximum rate of oxygen uptake as Equation (7), where K stands for their k k 1 (2). 

Kahn (20) questioned the formation of a diradical and proposed direct addition of oxygen to a double bond to form a cyclic transition state, which breaks down to yield the hydroperoxide. The theory of oxidation has received little support, because it does not explain the inhibitory effect of free-radical acceptors in the initial stages of autoxidation. 

Parallel to these developments, observations made in the 19th century linked the deterioration of many organic materials, such as natural oils and fats, to the absorption of dioxygen. Around the turn of the century it was recognized that these processes involved organic peroxide intermediates. Subsequently, detailed mechanistic studies with simple hydrocarbons led to the free radical chain theory of autoxidation [7]. Following close on the heels of these mechanistic developments several important catalytic oxidation processes, in both the gas and liquid phase, were developed in the period 1945-1960. Some examples are shown in Table 1. 

However, this theory does not explain the existence of pyrophoric iron or the phenomenon of pyrophoricity at all, as shown in Table 2 by a conparison of the ratio of volume of oxide to volume of clement. Also, in analyzing the problem of autoxidation of the alkali metals, one must consider than quite different types of oxides result from burning of these metals in air, e.g. LijO, NajOj, KOj, RbOj, and CsOg. 

Einset et at. (1957) studied fish oils from 7 species and noted that the rate of autoxidation was directly correlated with the iodine value and inversely correlated with the tocopherol content. The disappearance of significant amounts of tocopherol lends support to the theory that it plays an important role in stabilizing fish oils. 

C. Engler and W. Wild supposed that in autoxidation reactions the oxygen molecule is not actually divided but is opened out into an active form —O—O—, which combines with the activator to form an unstable peroxide, e.g. with turpentine. F. Haber supposed that free radicals are formed. Modern research favours the Traube-Bach theory. ... 

That epoxide groups are also present in oxidized natural rubber has been demonstrated by Golub et al (1975) by the use of H NMR and NMR spectroscopy. The presence of occasional epoxide groups is not believed to have a great effect on the properties of natural rubber but it may be noted in passing that, in theory, it provides sites for cross-linking by such materials as amines and acid anhydrides which are well known as epoxide resin hardeners. At the present time the above reaction sequence is of more interest in that it provides a route for converting peroxy radicals into alkoxy radicals and is believed by some workers to play an important role in chain scission due to autoxidation. 

Relatively small increases in stability of chloramphenicol in polysorbate 80, Myrj 59, polysorbate 20 and Brij 35 have been observed on autoclaving [134]. Selection of a suitable concentration of polysorbate 80 and adjustment of solutions to pH 4.6 reduced to half the autoxidative degradation of methyl-prednisolone even in the presence of oxygen [135]. Contrary to electrostatic theories of stabilization the base-catalysed hydrolysis of procaine was inhibited by non-ionic, anionic and cationic micelles as shown in Table 11.10. The order of inhibition of hydrolysis was NaLS > CTAB > PLE > NDB the order of partition coefficients (P ) was found to be NaLS > CTAB > NDB > PLE (see table for abbreviations). 

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