Mechanism autoxidations

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

The mechanism by which an oiganic material (RH) undergoes autoxidation involves a free-radical chain reaction (3—5) ... 

Drier Mechanism. Oxidative cross-linking may also be described as an autoxidation proceeding through four basic steps induction, peroxide formation, peroxide decomposition, and polymerization (5). The metals used as driers are categorized as active or auxiUary. However, these categories are arbitrary and a considerable amount of overlap exists between them. Drier systems generally contain two or three metals but can contain as many as five or more metals to obtain the desired drying performance. 

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 ... 

Free-radical chain oxidation of organic molecules by molecular oxygen is often referred to as autoxidation (see Section 12.2.1). The general mechanism is outlined below. 

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. 

A question which inevitably arises on surveying the enormous sucess of the Amoco catalyst is why the combination Co/Mn/Br in acetic acid In order to answer this question we must first examine the mechanism of free radical chain autoxidations of alkylaromatics (ref. 4). 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

The complete mechanism of the autoxidation of methylaromatics mediated by the Amoco, Co/Mn/Br catalyst cocktail is depicted in Figure 18. The development of such a complex, elegant system must surely be considered a work of art. 

Autoxidations in alkaline media can also proceed by a different mechanism R-H + base R + O2 ROO"... 

When catechol was oxidized with Mn04 under aprotic conditions, a semiquinone radical ion intermediate was involved. For autoxidations (i.e., with atmospheric oxygen) a free-radical mechanism is known to operate. 

The first term is identical with that observed for autoxidation of U(IV) and a similar mechanism involving hydrolysis is probable for the initial stage, although the authors prefer an initial three-equivalent oxidation of Ti(Ill) to give Ti(Vl), OH- and OH . 

Although the kinetic studies summarised here are useful guides to the gross features of mechanism it is evident from apparently closely related autoxidations, e.g. those of V(III) and U(IV), that subtle factors operate. Fallab has pointed out that these reductants give similar kinetics and possess similar reduction potentials, yet differ in autoxidation rate by a factor of 3 x 10 , and has discussed differences of this type in terms of the stereochemistry of the electron-transfer process in the coordination sphere. 

Because of the foregoing we will not attempt in this article to compile every observation recorded so far in the field of induced reactions only the results where the mechanism is sufficiently clear will be referred to. Also, the wide and technically important field of autoxidation processes will be omitted, because they can be considered more properly later in the series. 

PEDRIELLI p, PEDULLi G F and SKIBSTED L H (2001a) Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-braining reaction of quercetin and epicatechin in autoxidation of methyl linoleate, JAgric Food Chem, 49, 3034-40. 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

In conclusion, oxidation of carotenoids by molecular oxygen, the so-called autoxidation process, is a complex phenomenon that is probably initiated by an external factor (radical, metal, etc.) and for which different mechanisms have been proposed. The autoxidation of a carotenoid is important to take into account when studying antioxidant activity because it can lower the apparent antioxidant activity of a carotenoid. ... 

Shikama K. 1998. The molecular mechanism of autoxidation for myoglobin and hemoglobin A venerable puzzle. Chem Rev 98 1357. 

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... 

Inhibit autoxidation of organic materials by interfering with free radical reactions that lead to incorporation of oxygen into macromolecules in a chain mechanism consisting of two interacting cyclical processes (Scheme II.1). 

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

Chiueh, C.C., Miyake, J., Peng, M.T. Role of dopamine autoxidation, hydroxyl radical generation, and calcium overload in underlying mechanisms involved in MPTP-induced parkinsonism. Adv. Neurol. 60 251, 1993. 

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