Autoxidation suppression

Abstract One of the most important yet complex chemistries is the spontaneous reaction of oxygen with organic matter, generally referred to as autoxidation. Suppressing this chemistry is important in many industries and critical when it comes to the performance of lubricants. The chapter summarizes kinetic parameters for reaction of the potent sulphur-containing antioxidants with both cumylperoxide radicals and cumene hydroperoxide. It also demonstrates that antioxidants can be formulated that have synergistic activity. One example of this phenomenon is the combination in one molecule of phenolsulphide and aminosulphide moieties, which results in an antioxidant with superior activity for the critical hydroperoxide decomposition process compared to molecules containing either moiety alone. 

Dopamine, a strong water-soluble antioxidant, was identified in banana fruit (Musa cavendishii) by Kanazawa and Sakakibara (2000). Banana fruit contained high levels in the pulp and peel 2.5-10 mg/100 g and 80-560 mg/100 g, respectively. A banana water extract was reported to suppress the autoxidation of linoleic acid by 65-70% after a 5-day incubation in an emulsion system, as determined from peroxide value and thiobarbituric acid reactivity (Kanazawa and Sakakibara 2000). 

Mochizuki et al. also demonstrated that the uncatalyzed autoxidation of catechins is suppressed in the presence of a sufficient amount of borate buffer (44). This was interpreted by considering the adduct formation between borate ion and the catechins. The oxidation resumes on addition of Cu(II) in the presence of the borate ions. This may indicate that Cu(II) forms more stable complex(es) with the catechins than the borate ion, but direct oxidation of the catechin-borate complex by Cu(II) cannot be excluded either. 

The reaction rate is half-order in palladium and dimeric hydroxides of the type shown are very common for palladium. The reaction is first order in alcohol and a kinetic isotope effect was found for CH2 versus CD2 containing alcohols at 100 °C (1.4-2.1) showing that probably the (3-hydride elimination is rate-determining. Thus, fast pre-equilibria are involved with the dimer as the resting state. When terminal alkenes are present, Wacker oxidation of the alkene is the fastest reaction. Aldehydes are prone to autoxidation and it was found that radical scavengers such as TEMPO suppressed the side reactions and led to an increase of the selectivity [18],... 

Although autoxidation of Ru(sar) + has similar characteristics in acidic solution, in base hydrogen atom transfer from Ru(sar) + to O2 leads to a deprotonated Ru(III) species which is oxidized to relatively stable Ru" (sar-2 H+) + Ref. 175. The strong deviation from linearity for semi-log plots, with a large excess of O2, is removed when Fe(II) is added. This suppresses the step and doubles the rate. Compare Sec. 2.2.1(b). The value of k can be assessed as 1.3 x 10 M s Ref. 176. The behavior of pentacyanoruthenium complexes has been compared with the iron analogs. Substitution in M"(CN)5L" with both M = Fe and Ru is dissociative, with decreased lability for tbe Ru(II) species. Table 8.10. 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

The direct oxidation, shown by Reaction 2, is regarded by some authors (40, 41, 42) as an important process which initiates new chains in the autoxidation. Within the observed range of concentrations, the reaction ratio causing the deactivation of antioxidant and/or initiation was not large enough with any of the three antioxidants to cause inversion of activity. The results of this study confirm that by a suitable substitution of the pyrocatechol nucleus it is possible to influence the rate of Reaction 1 and to suppress the side reactions of the inhibitor as well. 

Sulfoxides form complexes with hydroperoxides and can inhibit a partially oxidized substrate, but their inhibiting activity is destroyed by the simultaneous addition of an acidic substance such as stearic acid. The "activity of sulfur compounds cannot be wholly accounted for by their peroxide-decomposing action, and although they suppress peroxide-initiated autoxidation, they do not suppress oxidations initiated by free radical sources such as azobisisobutyronitrile (10). 

Another improvement is the use of a Ru/TEMPO catalyst combination for the selective aerobic oxidations of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively (Fig. 1.22) [72]. The method is effective (>99% selectivity) with a broad range of primary and secondary aliphatic, allylic and benzylic alcohols. The overoxidation of aldehydes to the corresponding carboxylic acids is suppressed by the TEMPO which acts as a radical scavenger in preventing autoxidation. 

Rate and equilibrium constants for O2 and CO binding to Fe hemes have been measured in various solvents and at various temperatures. Dioxygen binding to Fe porphyrins can be studied at low temperatures by a variety of spectroscopic techniques, where reactions that lead to irreversible autoxidation are suppressed. However, at higher temperatures (above 0°C) fast Kinetic... 

Peroxidase catalyzed the oxidative polymerization of fluorinated phenols to give fluorine-containing polymers.46 During the polymerization, elimination of fluorine atom partly took place to give the polymer with a complicated structure. Antioxidant effects of the enzymatically synthesized polyphenols were evaluated.47 The autoxidation of tetralin was significantly suppressed in the presence of the polyphenols. 

This simple kinetic treatment of inhibited autoxidation provides for a useful semiquan-titative explanation of what is meant by antioxidant and antioxidant activity under known and controlled Rj. The ability of a known amount of potential antioxidant to suppress the oxygen uptake depends on the value of the absolute rate constant for inhibition,... 

The idea that the processes, which occur during the combustion of a fuel in an engine cylinder, take place by a chain reaction mechanism comparable to that associated with the liquid phase autoxidation of benzal-dehyde, has led to the proposal that knock suppressors act by destroying the chains and reducing the rate at which the flame front accelerates. It is known that surfaces or solid bodies suppress chain reactions, in fact one of the criteria for a chain reaction in gaseous combustion processes is the decrease in rate of reaction caused by the increase in surface exposed to the gases. However, the mode by which the chains are broken or... 

An important redox equilibrium exists between anthrahydroquinone A and tetra (Eq. (14.9)). Because tetra is more readily reduced than the parent anthraquinone, this equilibrium lies almost entirely to the right. Thus, if tetra formation is not suppressed, eventually all the hydroquinone species present in the oxidation process will consist of the reduced form of tetra. Because the analogous tetra hydroquinone is oxidized more slowly, the autoxidation step will slow down significantly. 

Two very curious but heretofore unexplained phenomena relate to the interaction of reduced polyoxometalates with O2 and are addressed by our experiments described right below. The first involves the lack of oxygenated products seen in the photochemical functionalization of heptacyclotetradecane (HCTD) in the presence of O2 by Christina Prosser-McCartha. The second involves two observations by Neumann and Levin, namely, the suppression of tetralin autoxidation and the clean conversion of a-terpinene to p-cymene in the presence of O2 without formation of oxygenated products. 

The response of polyurethanes to thermally activated autoxidation depends upon polymer structure. In general, polyurethane degradation by this mechanism is suppressed by the addition of antioxidant to the polymer. Ultraviolet initiated autoxidation is suppressed by a suitable screen (e.g. carbon black, titanium dioxide) or a combination of antioxidant and ultraviolet absorber. Irganox 1010 and Tinuvin P (Ciba-Geigy) are particularly suitable antioxidant and ultraviolet absorbers, respectively, for polyurethanes. Polyurethane structures with enhanced resistance to ultraviolet initiated autoxidation may be possible. 

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