Formed in autoxidation

The excited carbonyl compounds formed in autoxidation reactions are the primary source of chemiluminescence. However, it was reported... 

Considerable effort has been expended in recent years on the odorous compounds formed in autoxidized dairy products. Although some of the early identification studies lack present-day sophisticated methodology, may be incomplete, and do not differentiate between their isomeric forms of the various compounds, their contribution to the knowledge of the products of autoxidation in dairy products is invaluable. 

Table 2-23 Hydroperoxides and Aldehydes (with Single Oxygen Function) That May Be Formed in Autoxidation of Some Unsaturated Fatty Acids...

Oxidative polymers are formed in autoxidation when the free radicals terminate each other as under autoxidation. When a triacylglycerol molecule breaks down during autoxidation, the partial triacylglycerol molecules are not removed in the deodorization process and can react with each other, forming dimers, trimers, or polymers. 

Superoxide anion, O2 One-electron reduction products of O2, produced by phagocytes formed in autoxidation reactions generated by oxidases (heme protein). 

Since metal dioxygen complexes are usually diamagnetic, it was of interest to determine whether the allylic hydroperoxide formed in autoxidation reactions catalyzed by [RhCl(Ph3P)3] arose via an ene addition pathway, equation (275), or via conventional radical pathways. 

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. 

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. 

Common impurities found in aldehydes are the corresponding alcohols, aldols and water from selfcondensation, and the corresponding acids formed by autoxidation. Acids can be removed by shaking with aqueous 10% sodium bicarbonate solution. The organic liquid is then washed with water. It is dried with anhydrous sodium sulfate or magnesium sulfate and then fractionally distilled. Water soluble aldehydes must be dissolved in a suitable solvent such as diethyl ether before being washed in this way. Further purification can be effected via the bisulfite derivative (see pp. 57 and 59) or the Schiff base formed with aniline or benzidine. Solid aldehydes can be dissolved in diethyl ether and purified as above. Alternatively, they can be steam distilled, then sublimed and crystallised from toluene or petroleum ether. 

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

Kim, S.J., Cleavage products formed through autoxidation of zeta-carotene in liposomal suspension. Food Sci. Biotech., 13, 202, 2004. 

Zhang, H. et al., A novel cleavage product formed by autoxidation of lycopene induces apoptosis in HL-60 cells.. Free Radic. Biol. Med., 35, 1653, 2003. 

Since alkoxy radicals are known precursors to chain scission in autoxidation (24), the "hot" alkoxy radicals formed as shown should undergo facile chain scission or fragmentation. The chain scission is illustrated for the alkoxy radical derived from either the ethylene or propylene monomer unit in EPM ... 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

These energy-transfer processes are especially interesting in those chemiluminescence reactions where the primary electronically excited product is formed in its triplet state (autoxidation reactions, radical-ion recombination reactions see Sections III and VIII), although some reactions have been reported to involve direct emission from the excited triplet state 14>. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

A detailed mechanism is proposed for this recombination process. On the basis of the experimental results obtained, Beutel (for details see 13>) comes to the conclusion that in the case of dimedone autoxidation the triplet triketone D = O cannot be efficiently quenched by ground-state triplet oxygen formed in the decomposition of a Russell tetroxide which in this case should have the formula. 

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... 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

It was suggested that initiation proceeds via the same dimer that was proposed in aprotic media (34). Furthermore, the reverse step of this reaction was considered to be very slow. The Cu02 intermediate is formed in a reversible step in this model, which is in agreement with the results reported for autoxidation of Cu(T) (17). It should be added that Shtamm et al. assumed that this reaction is irreversible (40). 

Earlier studies demonstrated a rich variety of oxidation states, geometries and compositions of the intermediates and products formed in the autoxidation reactions of cysteine (RSH). Owing to the complexity of these systems, only a limited number of detailed kinetic papers were published on this subject and, not surprisingly, some of the results are... 

The effect of non-participating ligands on the copper catalyzed autoxidation of cysteine was studied in the presence of glycylglycine-phosphate and catecholamines, (2-R-)H2C, (epinephrine, R = CH(OH)-CH2-NHCH3 norepinephrine, R = CH(OH)-CH2-NH2 dopamine, R = CH2-CH2-NH2 dopa, R = CH2-CH(COOH)-NH2) by Hanaki and co-workers (68,69). Typically, these reactions followed Michaelis-Menten kinetics and the autoxidation rate displayed a bell-shaped curve as a function of pH. The catecholamines had no kinetic effects under anaerobic conditions, but catalyzed the autoxidation of cysteine in the following order of efficiency epinephrine = norepinephrine > dopamine > dopa. The concentration and pH dependencies of the reaction rate were interpreted by assuming that the redox active species is the [L Cun(RS-)] ternary complex which is formed in a very fast reaction between CunL and cysteine. Thus, the autoxidation occurs at maximum rate when the conditions are optimal for the formation of this species. At relatively low pH, the ternary complex does not form in sufficient concentration. 

Blood protein binding of IQ was found in rats dosed intragastrally with the labelled compound. The same adducts, though in much higher yields, were found when purified rat serum albumin was exposed either to /V-hydroxy-IQ or incubated with parent IQ in the presence of a microsomal system. A tripeptide was isolated which contained lV-(cystein-S -yl)TQ-,S-oxide (sulfinamide) that easily liberated IQ on acidification. Pretreatment of albumin with p-chloromercuri ben zoale reduced covalent binding drastically61. The authors concluded that the reactant most likely to yield this structure is 2-nitroso-3-methylimidazo[4,5-/]quinoline, which is probably formed by autoxidation of /V-hydroxy-IQ. 

For the same reasons the forms RED with relatively positive potentials seem to be stable to air in a pure state or a pure solution. In the presence of some acid, however, the colour of the radical cation SEM develops rapidly. The acid traps the highly nucleophilic Of formed in the course of autoxidation. Thus SEM and especially OX are no loiter removed from the redox-equilibrium since anions of low nucleo-philicity are provided by the added acid. 

Crystal structure analysis of the major isomer (43) formed on autoxidation of 3a-hydroxy-4,4,14a-trimethyl-19-nor-10a-pregna-5,16-diene-l 1,20-dione (44) followed by reduction of the intermediate hydroperoxide has confirmed the 17a configuration which had been assigned to the hydroxy-group. The conformation of the 17-acetyl group in both isomers has been studied in different solvents. A second compound, formed in addition to (45) in the rearrangement of... 

Peracids form as transient species from the oxidation of benzaldehyde during autoxidation. For convenience we have chosen m-chloroperbenzoic acid (MCPBA) as our oxidant since this would be similar to the peracid formed from the very important intermediate 4-carboxybenzaldehyde formed during the oxidation of p-xylene (2). MCPBA would be formed in very low concentrations during oxidation hence we normally study the reaction of MCPBA with an excess of catalyst components i.e. MCPBA < pseudo first order conditions). The sequence of reactions that occurs when MCPBA is reacted with Co(II), Mn(II), and HBr has been previously discussed by Jones (9) in the presence of 5% water in acetic acid. We have repeated much of this work in 10% HjO/HOAc solutions and in general agree with his findings when one accounts for differences in temperatures, concentrations, and water concentrations. 

By using the differential form of the copolymer composition equation (26, 28) the products of oxidation of mixtures at low conversions permit comparison of rates of chain propagation in autoxidations of various compounds, essentially free from effects of chain initiation, chain termination, and over-all rates. 

The observed half life at 100°C. of 23 hours for a dilute solution of hydroperoxide in benzene indicates that significant decomposition may occur in the autoxidation of butene, depending on reaction conditions. No reliable evaluation can be made because of the known complications introduced on hydroperoxide decomposition by the effect of the solvent, the hydroperoxide concentration (2), the presence of oxygen (12), and the possibility of a strong acceleration in rate in the presence of oxidizing olefin, observed in at least one system (8). However, using the data reported by Bateman for a benzene solvent at 100 °C. in the presence of air (2), l-butene-3-hydroperoxide decomposes 13 times faster than cyclohexene hydroperoxide, a product which may be formed in extremely high yield by the oxidation of cyclohexene. 

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