Minimizing Autoxidation

Autoxidation of unsaturated fatty acids is well known. Modifications of the extraction and separation procedures were made to investigate the possibility that these oxygenated fatty acids (OFAs) arise as artefacts. When exposure to light and air were minimized, no changes were noted in TLC and HPLC. 

Ideally samples from living organisms should be extracted without any delay to prevent autoxidative or enzymic deterioration of their lipid constituents. If this is not feasable the sample should be frozen immediately and stored at — 20 °C in a glass container under nitrogen. Often lipids will be extracted into an organic solvent and during this and subsequent steps in the analytical procedure minimal exposure of the lipids to air, light and heat is very important to prevent oxidation or destruction of the lipids. 

An interesting effect of Ca2+ ions on the autoxidation of hydroxy-1,4-naphthoquinone, which may have a bactericidal function in sea urchins, has been reported. Autoxidation resulted in the detection of a semiquinone, which was accompanied by only minimal oxygen uptake. The presence of Ca2+ resulted in a massive enhancement in the rate of 02 uptake. Potentiometric titrations revealed that Ca2+ ions, by forming complexes with the compound, lowered the pKa values of its phenolic groups, thereby generating the oxidation-prone phenolate anions. Autoxidation was also facilitated by the spin stabilisation effect of the Ca2+ ions on the naphthosemiquinone.121 This effect may be of importance in mammalian cells, where quinones induce elevations in the free Ca2+ concentration. 

Antioxidants are additives that minimize autoxidation reactions. They function as hydrogen atom donors that stop the chain oxidation process of the olefins. The two different types of antioxidants used in gasoline are phenylenediamines (PDA) and hindered phenols (such as BHT). 

In the cyclohexane oxidation route cyclohexane is oxidized with air at 125-126°C and 8-15 bar in the liquid phase using Co or Mn naphthanates as the catalyst. This affords a mixture of cyclohexanol and cyclohexanone via a classical free radical autoxidation mechanism. Cyclohexane conversion is limited to 10-12% in order to minimize by-product formation via further oxidation. The selectivity to cyclohexanol/cyclohexanone is 80-85%. 

A process with potential practical applicability is the hydrogenation of edible oils. Reduction of multiply unsaturated triglycerides with hydrogen over Ni-based catalysts is frequently used to gain autoxidative stability of edible oils. According to the Polanyi-Horiuti mechanism, multiple 1,2 or 1,4 diadsorption of the fatty acid tail with exclusively c/s-configuration around the double bonds causes cis-trans isomerisation, whilst the number of double bonds is being reduced. The trans-fatty acid chains have adverse effects on the human metabolism and must be minimized. 

Watkins, J.A. Kawanishi, S. Caughey, W.S. Autoxidation reactions of hemoglobin a free from other red cell components a minimal mechanism. Biochem. Biophys. Res. Commun. 1985,132 (2), 742-748. 

TBHP vide supra). The autoxidation of EB is performed at 120-160 °C and 1- bar. MBA and acetophenone (ACP) are formed as by-products via the facile termination of the secondary 1-methylbenzylperoxy radicals. In order to minimize by-product formation by further oxidation of MBA and ACP, the autoxidation is carried out to only low conversions (< 12 %). This solution (ca. 10 %) of EBHP in EB is used in the epoxidation step, i.e., EB is the solvent for the latter step. A high propene/EBHP molar ratio is used and reaction conditions are similar to those of the TBHP process vide supra). The PO selectivity is reported to be 90 % at 92 % EBHP conversion [30] but in practice it may be higher. For comparison the heterogeneous Ti /SiOa catalyst in fixed-bed operation reportedly gives 93-94 % PO selectivity at 96 % EBHP conversion [11]. The products are separated by distillation and MBA is dehydrated to styrene in the vapor phase over a Ti02 catalyst. 

If these product POMs can oxidize the desired substrate directly without simultaneous generation of oxy radicals and consequent minimally selective autoxidation, an attractive 02-based POM-catalyzed chemistry is realized. 

Because linolenic acid is much more subject to autoxidation than is linoleic acid, nutritionists and industrial laboratories attempted to eliminate linolenic acid from food formulations to minimize rancidity during storage. Autoxidation was the great enemy for designers of stable foods, and a lifetime of effort was required to breed linolenic acid out of soybeans sufficiently to make soybean oil a more stable and convenient component of industrial products intended for human consumption. 

Exposure of polyolefins to UV radiation in the presence of air leads to autoxidative reactions with the incorporation of oxygenated groups into the polymer. The increase in UV absorbance (measured at 310 mn) of irradiated polypropylene is reported to be too low to measure for wavelengths of irradiation in the range of 260-600 nm. Even at 260 nm, only a minimal increase was observed [121], For polypropylene containing flame retardants, either DBDE (decabromodiphenyl oxide) or TBA (tetrabromobisphenol A) at a 2% level, the maximum efficiency of chain scission was observed at X. = 280 and 260 nm, respectively. 

The temperature had to be kept below 230°C to prevent decomposition of the starch, and exposure of the master batch to air had to be minimized to avoid water absorption. Direct addition of starch and autoxidant withont the master batch step can also be used as this requires some specific equipment, it is only practical for large volumes (42). It was claimed that under appropriate conditions, the disintegration time of a buried carrier bag, containing an Ecostar additive to reach 6% starch, will be reduced from himdreds of years to 3-6 years (38). However there is no evidence of a compliance of such materials with the norms of biodegradability and compostability already in place at the international level. Moreover, the destabilization of polyethylene indneed by the pro-oxidants may significantly affect its in-use performance as a function of time. 

It is commonly desirable to minimize autoxidation (as when it is desired to improve ageing resistance) and in principle there are two main methods of achieving this by chemical means. 

A vitamin may participate in synthesizing cellular catalysts and in degrading major substrates. Assumingi that the turnover numbers—the wear and tear—of the vitamin as well as the enzyme affinities are about the same for both activities, then a major substrate reaction should demand much more folic enzyme than would the syntheses of quantitatively minor cell constituents. Substrates dissimilated anaerobically would be especially favorable for study when a Ci unit is concerned autoxidative destruction of folic enzymes is minimal. Clostridial fermentations, as will be seen, have furthered folic studies remarkably. 

The autoxidation of acyl lipids is also influenced by the moisture content of food. The reaction rate is high for both dehydrated and water-containing food, but is minimal at a water activity (aw) of 0.3 (Fig. 0.4). The following hypotheses are discussed to explain these differences The high reaction rate in dehydrated food is due to metal... 

Therefore, stabilizers always have to be added when processing PVC. They delay dehydrochlorination by binding HCI and eliminating initiation points, suppress autoxidation, and minimize mechano-chemical fragmentation by an internal lubricating action [20],... 

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