Chain mechanisms for autoxidation

Free-radical chain inhibitors are of considerable economic importance. The term antioxidant is commonly appUed to inhibitors that retard the free-radical chain oxidations, termed autoxidations, that can cause relatively rapid deterioration of many commercial materials derived from organic molecules, including foodstuffs, petroleum products, and plastics. The chain mechanism for autoxidation of hydrocarbons is ... 

The radical chain mechanism for autoxidation, using diethyl ether as the example. 

Scheme 7 Radical chain mechanism for the autoxidation of Zr(IV) alkyls...

In the familiar hydroperoxide chain mechanism for hydrocarbon autoxidation, with propagation steps,... 

Autoxidation, the oxidation of organic compounds by air, normally occurs via a radical chain mechanism. For example, cyclohexene undergoes allylic CH abstraction by an initiator, and the resulting cyclohexenyl radical reacts with O2 to give the corresponding hydroperoxy radical that abstracts an H from cyclohexene. In this case the final product is the allylic hydroperoxide. Conversion of ethers to the hydroperoxides is another familiar example. The conversion of cumene to phenol and acetone is a commercial application of the reaction (equation 9). 

There is a radical-radical combination step in the propagation part of the chain mechanism for the autoxidation reaction. The propagation part of the SRN I mechanism (Chapter 2) consists of loss of a leaving group, addition of a nucleophile, and electron transfer. 

Also potentially hazardous are compounds that undergo autooxidation to form organic hydroperoxides and/or peroxides when exposed to the oxygen in air (see Table 3.12). Especially dangerous are ether bottles that have evaporated to dryness. A peroxide present as a contaminant in a reagent or solvent can be very hazardous and change the course of a planned reaction. Autoxidation of organic materials (solvents and otho" liquids are most frequently of primary concern) proceeds by a free-radical chain mechanism. For the substrate R—H, the chain is initiated by ultraviolet... 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

The Fe(III)/S(IV) reaction has long been of interest because of its importance in the catalytic autoxidation of S(IV). The latter reaction is known to have a complex chain mechanism, and the production of SOr radicals has been considered to be the essential chain-initiating step. It is also widely believed that the direct oxidation of S(IV) by Fe(III) is the source of SO -. There is little agreement among the various papers published on the direct reaction of Fe(III) with S(IV) with regard to its mechanism, and much of this disagreement can be traced to the potential for Fe(III) to bind several S(IV) ligands under the typical conditions of excess S(IV). 

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). 

In a free-radical chain mechanism we want to 1) produce a given product selectively, 2) simultaneously produce radical species which will further propagate the chain. Consider the autoxidation of m-chlorotoluene to m-chlorobenzoic acid in the three ways given on Figure 2. For the sake of argument, we initially start with MCPBA. We will also assume the free radical chain mechanism sequence does not contain a rate determining step. 

In a mechanism for the iron catalyzed autoxidation of sulfite, this would be a chain branching step. We have some evidence that the reaction of Fe2+ with HSO, in acid, does lead to free radicals, but it is somewhat more complex than written above (24). 

The basic mechanism of autoxidation at elevated temperatures is similar to that of room-temperature oxidation, i.e., a free radical chain reaction involving the formation and decomposition of hydroperoxide intermediates. Although relative proportions of the isomeric hydroperoxides, specific for oleate, linoleate and linolenate, vary with oxidation temperatures in the range 25°C -80°C, their qualitative pattern is the same (. Likewise, the major decomposition products isolated from fats oxidized over wide temperature ranges are those reflecting autoxidation of their constituent fatty acids (2 -6). The mechanisms and products of lipid oxidation have been extensively studied. The reader is referred to the numerous monographs, reviews and research articles available in the literature (1,A,7,8,9,10,11). 

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. 

Further possibilities for catalytic oxidation are realized in the well-known catalysis of the autoxidation of hydrocarbons and other substrates by salts of transition metals such as copper, cobalt, and manganese which exhibit more than one stable oxidation state and which catalyze oxidation through free radical chain mechanisms (16). 

Recently, ho vever, Hermans et al. [2h-m] have revisited the mechanism of cyclohexane autoxidation, and found that indeed the most efficient mechanism for chain propagation is not ... 

Transition-metal ions such as Fe(III), Cu(II), Co(II), Co(III), and Mn(II) have been shown to be effective homogeneous catalysts for the autoxidation of sulfur dioxide in aqueous solution. Hoffmann and coworkers have shown that Fe(III) and Mn(II) are the most effective catalysts at ambient concentrations for the catalytic autoxidation of S(IV) to S(VI) in cloudwater and fogwatet (Jacob and Hoffmann, 1983 Hoffmann and Jacob, 1984 Hoffmann and Calvert, 1985). Mechanisms for the homogeneous catalysis by Fe(lII) and Mn(II) that have been proposed include a free-radical chain mechanism, a polar mechanism involving inner-sphere complexation followed by a two-electron transfer from S(IV) to bound dioxygen, and photoassisted electron transfer. 

Free-Radical Chain Mechanisms. A frequently postulated initiation step for the free-radical autoxidation of S(IV) catalyzed by Fe(III) is as follows ... 

Kinetic studies were carried out in order to determine the mechanism of the autoxidation reactions. The results indicate that the reactions do not proceed via the usual type of radical-chain mechanism involving hydroperoxides, and that not all metal acetylacetonates follow the same mechanism. A relatively simple mechanism has been proposed for the destructive autoxidation of iron(III) acetylacetone that postulates an intramolecular oxidation-reduction of the chelate with the formation of stable radicals, which are intercepted by highly reactive radicals produced by the decomposition of initiators. A triketone, 2,3,4-pentanetrione, is postulated as the intermediate from which most of the reaction products are derived (4, 5). 

No further evidence has been found for an independent oxidation catalyst and recent work on autoxidation sheds no light on a chain mechanism which would explain the results. Lemberg and Legge (5) proposed that the oxidation catalyst is Hb02 itself, but this point will be discussed later. 

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