Free radical autoxidation

Beyer, R.E. (1990) The participation of coenzyme Q in free radical production and autoxidation. Free Radical Biol. Med. 8 545-565. 

H. E. Hajji, E. Nkhili, V. Tomao, and O. Dangles, Interactions of quercetin with iron and copper ions complexation and autoxidation, Free Radical Research, vol. 40, no. 3, pp. 303-320, 2006. 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

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). 

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

Autoxida.tlon. The autoxidation (7) of unsaturated fatty acids in phosphoHpids is similar to that of free acids. Primary products are diene hydroperoxides formed in a free-radical process. 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

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

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

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. 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

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). 

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. 

HARDWICK W F, KALYANARAMAN B, PRITSOS C A and PARDINI R S (1988) The production of hydroxyl and semiquinone free radicals during the autoxidation of redox active flavonoids, in Simic MG, Taylor K A, Ward J F and von Sonntag C Oxygen Radicals in Biology and Medicine, Plenum Press, New York, 149-52. 

Handehnan, G.J. et al.. Characterization of products formed during the autoxidation of beta-carotene. Free Radic. Biol. Med., 10, 427, 1991. 

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). 

Zhang, H., E. Kotake-Nara, H. Ono, and A. Nagao. 2003. A novel cleavage product formed by autoxidation of lycopene induces apoptosis in HL-60 cells. Free Radic Biol Med 35(12) 1653-1663. 

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