Primary oxidation products

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

A number of methods are available for following the oxidative behaviour of food samples. The consumption of oxygen and the ESR detection of radicals, either directly or indirectly by spin trapping, can be used to follow the initial steps during oxidation (Andersen and Skibsted, 2002). The formation of primary oxidation products, such as hydroperoxides and conjugated dienes, and secondary oxidation products (carbohydrides, carbonyl compounds and acids) in the case of lipid oxidation, can be quantified by several standard chemical and physical analytical methods (Armstrong, 1998 Horwitz, 2000). 

Alcohol abuse is a major clinical problem in many countries and has been the subject of investigation for many years by those interested in determining the molecular basis of ethanol-induced liver dam e (see Lieber, 1990). These intensive and extended efforts have revealed much about the metabolism of ethanol in the liver and about the toxicity of its primary oxidative product, acetaldehyde. They have not, however, folly elucidated the molecular mechanisms that lead to the typical features of alcoholic liver injury steatosis, necrosis and eventually cirrhosis. 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

The source of chemiluminescence in the oxidation of luminol was explored by Merenyi and co-workers in detail (153). The oxidation of luminol yields aminophthalate as a final product and the reaction proceeds via a series of electron transfer steps. The primary oxidation product is the luminol radical which is transformed into either diazaquinone or the a-hydroxide-hydroperoxide intermediate (a-HHP). The latter oxidation step occurs between the deprotonated form of the luminol radical and O -. The chemiluminescence is due to the decomposition of the mono-anionic form of a-HHP into the final products ... 

C8. Cori, O., and Lipmann, F., The primary oxidation product of enzymatic glucose-6-phosphate oxidation. ]. Biol. Chem. 194, 417-425 (1952). 

The heterogeneous and the homogeneous photoredox reactions lead to formation of H2O2 through reaction of the primary oxidation product, e.g. CO, with oxygen. 

Alternative primary oxidation products are possible, e.g. structures containing the CH2OH group. 

Furthermore, a base-catalyzed transformation by OH from the reaction medium between glycerate and hydroxypyruvate aldehyde (or hydroxypyruvic acid) could be excluded, while hydroxyacetone and glyceraldehyde interconversion was possible (Scheme 11.11). The existence of two major routes, of which hydroxyacetone and glyceric aldehyde are the primary oxidation products and glycolic and oxalic acid are the end-members, respectively, is now firmly established. Clearly, rapid oxidation of glyceraldehydes favors glyceric acid rather than hydroxyacetone formation. 

Antipyrine metabolism in vivo has also been demonstrated. Following extraction of media from uptake and elimination studies, TLC analysis revealed the parent compound and 4-hydroxy-antipyrine. Based upon the amount of radioactivity recovered, the metabolite may account for up to 4% of the total. This hy-droxylated metabolite is the primary oxidation product in animals studied to date (23). Further characterization of the extracts using high-pressure liquid chromatography will be done in the future. 

The UV absoibance is used for the preliminary control of the degree of decomposition. The GC/MS and HPLC analysis are used to identily intermediate and final products formed during ozonation. It was found that reaction of ozone with phenol at pH 9, in addition to catechol (C) and hydroquinone (HQ) are likely primary oxidation products, p-benzoquinone (PBQ) and o-benzoquinone (OBQ), the others are more oxidized species, and CO and water the final oxidation products. The detected degradation products are shown in Scheme 24.1. 

The products of thermal oxidation of polyethylene films can be characterized by C FTNMR furthermore, using the spin-lattice relaxation technique, quantitative estimates can be made of the oxidized functional groups. Observation of the development progress of the various functional groups leads to the postulation of hydroperoxides as the primary oxidation products, which undergo further transformations to the other derivatives in a complex scheme . 

Of the organohahdes, only the iodides are prone to oxidation by dioxirane for example, iodobenzene is oxidized by DMD to a mixture of iodosobenzene and iodylbenzene. In contrast, alkyl iodides afford labile primary oxidation products, which eliminate the oxidized iodine functionality to result in aUcenes (equation 23). In such a dioxirane oxidation, the subsequent in-situ reaction of the alkene affords the corresponding epoxides . 

The rates of production are reported as turnover rates based on the number of V metal atoms as titrated by the adsorption of oxygen (77). These rates represent the number of product molecules produced per site per unit time and thus are measures of the actual product yield (conversion x selectivity) of the catalytic site. Conversions were less than 10% so that conclusions are derived for primary oxidation products. 

It is generally agreed that alkenyl hydroperoxides are primary products in the liquid-phase oxidation of olefins. Kamneva and Panfilova (8) believe the dimeric and trimeric dialkyl peroxides they obtained from the oxidation of cyclohexene at 35° to 40° to be secondary products resulting from cyclohexene hydroperoxide. But Van Sickle and co-workers (20) report that, The abstraction/addition ratio is nearly independent of temperature in oxidation of isobutylene and cycloheptene and of solvent changes in oxidations of cyclopentene, tetramethylethylene, and cyclooctene. They interpret these results to support a branching mechanism which gives rise to alkenyl hydroperoxide and polymeric dialkyl peroxide, both as primary oxidation products. This interpretation has been well accepted (7, 13). Brill s (4) and our results show that acyclic alkenyl hydroperoxides decompose extensively at temperatures above 100°C. to complicate the reaction kinetics and mechanistic interpretations. A simplified reaction scheme is outlined below. 

Although Equation C often adequately describes the kinetics of hydrocarbon oxidation inhibition by phenols, more complicated kinetics are common (12), particularly for hydrocarbons which yield hydroperoxide as the primary oxidation product. The complications arise mainly from reversibility of Reaction 7 but can also result from a chain-transfer reaction ... 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

On anodic oxidation of 3,6-diisobutylpiperazine-2,5-dione in acetonitrile, a compound was obtained, which was suggested to be 1,6-diisopropyl-3,8-dimethyl-5//, 10//-diimidazo[ 1,5-n 1, 5 -d]pyrazine-5,10-dione (44), formed by 1,3-cycloaddition of a primary oxidation product to the solvent.96 Another heterocyclic synthesis by intermolecular coupling of 2,4,5-tri-tert-butylphenol with acetonitrile has been reported.97... 

One can speculate on the nature of the material that contributed to each burst of nucleation and the growth of the initial nuclei. The early nucleation did not occur under the same conditions without SO , so it is probable that it results from the primary oxidation product of that species, namely, H2S04. The second nucleation burst is probably the same material that condensed without the initial S02, that is, the condensible hydrocarbons that result from the 1-octene photooxidation. Because the initial SO concentration was much smaller than that of the hydrocarbon, much of the growth of the early nuclei is likely due to hydrocarbon condensation, that is, condensation of species that did not nucleate until much later in the first experiment. An examination of the quantity of aerosol produced in the two experiments supports this interpretation. As shown in Figure 8, particle formation occurs before significant hydrocarbon reaction in the SO -containing experiment. Once the hydrocarbon reaction begins in earnest, the aerosol yield increases by an amount that is comparable to that in the S02-free experiment. Two... 

Measuring the content of primary oxidation products is limited due to the transitory nature of peroxides. Yet, their presence may indicate a potential for later formation of sensorially objectionable compounds. The peroxide content increases only when the rate of peroxide formation exceeds that of its destruction. In cases where peroxide breakdown is as fast as or faster than peroxide formation, monitoring lipid peroxides is not a good indicator of oxidation. This can occur in frying oils and sometimes in meat products, particularly in cooked meats where iron is very active and peroxide breakdown is quite rapid. Because the acceptability of an oil or lipid-containing food product depends on the degree to which oxidation has progressed, the simultaneous detection of primary and secondary lipid oxidation products helps to better characterize lipid quality. It is... 

Fig. 45 Reversed-phase HPLC of autoxidized trilinolenin (peroxide value = 236.4 meq/kg). Nova-Pak C18 cartridge column (Waters, Milford, MA) (3.9 X 150 mm, 60 A, 4 yam), mobile phase acetonitrile/ dichloromethane/methanol (80 10 10). Ultraviolet (UV) detector (235 nm) and evaporative light-scattering detector (ELSD). Primary oxidation products, double peak at 3.6 min secondary oxidation products elute before primary oxidation products.

Fig. 46 Reversed-phase HPLC of autoxidized triolein (peroxide value = 149.7 meq/kg). See Fig. 45 for abbreviations and chromatographic conditions. Primary oxidation products, peak at 18.6 min, secondary oxidation products elute before primary oxidation products.

Glycols undergo oxidation with H202 and titanium silicates, but it is also possible that some of the reactions observed proceed as noncatalytic reactions once the primary oxidation products are formed. Ethylene glycol is oxidized to glycolic acid ... 

The oxidation of a diol with active M11O2 produces the selective oxidation of an allylic alcohol as the major reaction pathway, with a 10-20% of product arising from oxidation of both alcohols and 5% of a product resulting from an intramolecular attack of an alcohol on the enone being the primary oxidation product. 

The other pathway leading to the formation ofoxogroups in the coordination sphere of the metal atom is provided by uncontrolled oxidation of the basic alkoxides such as alkali, alkaline earth metal, and quite probably the rare earth metal ones by oxygen dissolved in solvents and present in the atmosphere. The primary oxidation products are peroxides and hydroperoxides — M(OOR)n and M(OOH)n, whose decomposition gives water among the other... 

Similar stoichiometric reactions can be conducted with other organic substrates. Beside mechanistic importance, such reactions are a convenient way for estimating the potential of a-oxygen oxidation. For that, various organic substrates were tested for their room temperature interaction with a-oxygen to identify the primary oxidation products extracted from the surface. Substrates included alkanes, cycloalkanes, alkenes and aromatics [121,122]. Analysis of products showed that in all cases selective formation of hydroxylated compounds took place. 

Flosdorf and Chambers (1933) reported that metal sulfides were oxidized in the presence of audible sound (1 to 15 kHz) while investigating the bactericidal action of audible sound however, Schmitt et al. (1929) were the first researchers to observe the rapid oxidation of dissolved H2S gas to colloidal sulfur during sonication at 750 kHz with a 250-W power source. They reported that an increase in the total pressure of the system (P02) led to higher oxidation rates up to a limiting critical pressure. This critical pressure depended on the amount of dissolved H2S gas and the intensity of irradiation. The primary oxidation product was found to be elemental sulfur. The overall reaction was thought to proceed via reactions of HS with OH radicals, HO radicals, or H202. 

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