Free radical mechanism, autoxidation

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. 

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). 

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. 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

Simic MG. Free radical mechanisms in autoxidation processes. J Chem Ed 1981 58 125-131. 

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 autoxidation reactions involve three steps initiation, propagation, and termination. The initiation step leads to the formation of a hydroperoxide on a methylene group adjacent to a double bond this step proceeds via a free-radical mechanism ... 

It has been suggested that autoxidation of saturated fatty acids and aldehydes occurs through a free-radical mechanism (13, 14). Supporting evidence of a radical chain mechanism was provided by Palamand and Dieckmann (15) who studied the autoxidation of hexanal. The reaction involves peroxycarboxylic acid as an intermediate (16) and probably proceeds via the mechanism shown in Figure 1. 

The self-accelerating oxidation of hydrocarbons is called autoxidation. Its initial stage is characterised by a slow reaction with oxygen followed by a phase of increased conversion until the process comes to a standstill. The degradation is driven by an autocatalytic reaction described by the well-established free radical mechanism [1, 2], consisting of four distinct stages ... 

These observations may also be interpreted by the free radical mechanism, 02 again being formed as in step 1 of the autoxidation mechanism... 

Kinetic studies of the autoxidation of hemoglobin and myoglobin do not favor intramolecular mechanisms involving reaction between separated groups on the protein molecule, but the form of the rate equations resembles that found when two valency states of a metal ion compete for the same radical intermediate. A free radical mechanism based on competition for the HO2 radical can be developed to account for the observed results. 

The analysis of the deposit may give some indication of the processes by which the deposit was formed. The presence of oxygen or otherwise, suggests that autoxidation may or may not be involved in the mechanism. The presence of sulphur or nitrogen in the deposit could demonstrate that a free radical mechanism had been involved in the formation of deposits. 

Autoxidation is the process in foods and bulk lipids, which leads to rancidity. Rancidity is the spoiled off-flavor obtained by subjective organoleptic appraisal of food. Autoxidation is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical mechanism. This indicates that the intermediates are radicals (odd electron species) and that the reaction involves an initiation step and a propagation sequence, which continues until the operation of one or more termination steps. Autoxidation of lipid molecules is briefly described by reactions 1-3. ... 

By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [123, 124], Eq. (5.16) probably involves a free-radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of these radicals. The resulting PINO radical subsequently abstracts a hydrogen atom from the a-C-H bond of the alcohol to propagate the autoxidation chain (Eqs. (5.17)-(5.19)). 

On the other hand, participation of free radicals should be considered in most cases of oxygenations. Both of carbon and oxygen centered radicals can take part in the oxygenation processes. Apart from the apparent autoxidation process, it is not easy to differentiate explicitly metal-based mechanisms from free radical mechanisms. Different types of the radical-clock reagents have been developed for detection of radicals of different lifetime, especially in the discussions on the radical-rebound mechanisms. 

The branching ratio for this reaction has been a matter of some contention. The early and widely accepted free radical mechanism for sulfite autoxidation included only the termination step [112]. An investigation of the decomposition of HSOs" catalyzed by Ag and S20g ", however, led to calculated branching ratios, of 8.6 at 10°C, (7.3 and 8.8) at 17°C, and 9.7 at 25°C [113]. By... 

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) ... 

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). 

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