Oxidation s. a. Autoxidation

Oxidation (s. a. Autoxidation, Decarboxylation, oxidative. Hydrogen abstraction, Isomerization, oxidative,... 

Oxidation (s. a. Ammoxidation, Autoxidation, Pairing, oxidative, Ring closure, —, Solvolysis, —) ON OS OC —, promoted by steric strain 19 923... 

Poly(hydrosilane)s are stable compounds and can be manipulated in the air only for a short period since they are oxygen sensitive. In order to study the oxidation products, a xylene solution of poly(phenylhydrosilane)(Mw = 2340, Mw/Mn = 1.72) was refluxed (140 °C) for 12 h in a system exposed to the air [15]. Only minor changes were observed by GPC analysis whereas FTIR showed characteristic absorptions due to siloxane-type structures on the polymer backbone. A detailed NMR analysis, based on H NMR, Si INEPT and H- Si HMQC spectroscopies, indicated that the oxidized material contains the units 7-10 shown in Scheme 8.2. In particular, units 7,8 and 9+10 were present in relative percentages of 27%, 54% and 19%, respectively, which mean that more than 70% of the catenated silicons were altered. It has also been reported that silyl hydroperoxides and peroxides are not found as products in the autoxidation of poly(phenylhy-drosilane) [16]. 

N-Heterocyclics. The reaction of primary amines with the carbonyl products derived from lipid oxidation is a major pathway in lipid-protein interactions. Formation of Schiff s base intermediates followed by cyclization and rearrangement can yield imines, pyridines and pyrroles (5,15,30,31). For example, 2-pentylpyridine may result from the reaction of ammonia with 2,4-decadienal, one of the principle aldehydes from the autoxidation of linoleate (5). 

In fats and oils, the process of oxidation is similar to that oxidation of any other unsaturated organic material and requires an initiation process, in order to generate free radicals from the substrate. As antioxidants inhibit oxidation or autoxidation process, the mechanism(s) involved need(s) to be discussed. Figure 1 explains the relationship of antioxidant activity and oxidation of a lipid as examined by a typical evaluation method. 

As a contribution to a subproject of CMD of EUROTRAC-2, scientists from ICHF studied experimentally the effect of a-pinene and cis-verbenol on the rate of S(IV) oxidation catalyzed by Fe, as well as the ozone-affected autoxidation of aqueous SO2 in the presence of calcium. The investigations were aimed at clarifying the mechanism of the aqueous phase oxidation of SO(IV) simultaneously by O3 and O2, calcium acting as condensation nuclei. Inhibition of the S(IV) autoxidation in the atmosphere by secondary terpenic compounds was also studied... 

Ingold, K.U., Bowry, V.W., Stocker, R., and Walling, C., Autoxidation of lipids and antioxidation by alpha-tocopherol and ubiquinol in homogeneous solution and in aqueous dispersions of lipids unrecognized consequences of lipid particle size as exemplified by oxidation of human low density lipoprotein, Proc. Natl. Acad. Sci. U.S.A. 90 (1), 45-49, 1993. 

The Fe(III)/S(IV) reaction has long been of interest because of its importance in the catalytic autoxidation of S(IV). The latter reaction is known to have a complex chain mechanism, and the production of SOr radicals has been considered to be the essential chain-initiating step. It is also widely believed that the direct oxidation of S(IV) by Fe(III) is the source of SO -. There is little agreement among the various papers published on the direct reaction of Fe(III) with S(IV) with regard to its mechanism, and much of this disagreement can be traced to the potential for Fe(III) to bind several S(IV) ligands under the typical conditions of excess S(IV). 

The kinetic results reported by Jameson and Blackburn (11,12) for the copper catalyzed autoxidation of ascorbic acid are substantially different from those of Taqui Khan and Martell (6). The former could not reproduce the spontaneous oxidation in the absence of added catalysts when they used extremely pure reagents. These results imply that ascorbic acid is inert toward oxidation by dioxygen and earlier reports on spontaneous oxidation are artifacts due to catalytic impurities. In support of these considerations, it is worthwhile noting that trace amounts of transition metal ions, in particular Cu(II), may cause irreproducibilities in experimental work with ascorbic acid (13). While this problem can be eliminated by masking the metal ion(s), the masking agent needs to be selected carefully since it could become involved in side reactions in a given system. 

Fig. 4. Absorbance-time traces for the iron(III) catalyzed autoxidation of sulfur(IV) oxides (a) [02] =0m (b) [02] = 7.5xlO 4M. Experimental conditions [Fe(III)] = 5.0 x 1(T6 M [S(IV)] = 5.0 x 1(T3 M ionic strength = 0.5 M T= 25 °C pH = 2.5 A = 390 nm absorbance scale is in V (10 V = 1 absorbance unit). Reprinted with permission from Brandt, C. Fabian, I. van Eldik, R. Inorg. Chem. 1994, 33, 687. Copyright (2002) American Chemical Society.

Recent studies demonstrated that the composition of the reaction mixture, and in particular the pH have significant effects on the kinetics of iron(III)-catalyzed autoxidation of sulfur(IV) oxides. When the reaction was triggered at pH 6.1, the typical pH profile as a function of time exhibited a distinct induction period after which the pH sharply decreased (98).The S-shaped kinetic traces were interpreted by assuming that the buffer capacity of the HSO3 / SO3- system efficiently reduces the acidifying effect of the oxidation process. The activity of the... 

The presence of ascorbic acid as a co-substrate enhanced the rate of the Ru(EDTA)-catalyzed autoxidation in the order cyclohexane < cyclohexanol < cyclohexene (148). The reactions were always first-order in [H2A]. It was concluded that these reactions occur via a Ru(EDTA)(H2A)(S)(02) adduct, in which ascorbic acid promotes the cleavage of the 02 unit and, as a consequence, O-transfer to the substrate. While the model seems to be consistent with the experimental observations, it leaves open some very intriguing questions. According to earlier results from the same laboratory (24,25), the Ru(EDTA) catalyzed autoxidation of ascorbic acid occurs at a comparable or even a faster rate than the reactions listed in Table III. It follows, that the interference from this side reaction should not be neglected in the detailed kinetic model, in particular because ascorbic acid may be completely consumed before the oxidation of the other substrate takes place. 

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. 

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. 

Products of the LOX pathway or compounds formed by autoxidation of fatty acids (Scheme 7.2) are also important for leek aroma [31, 163]. Volatile compounds of the LOX pathway are not pronounced in the aroma profile of freshly cut leeks owing to a high content of thiosulfinates and thiopropanal-S-oxide [30]. In processed leeks that have been stored for a long time (frozen storage), however, these aliphatic aldehydes and alcohols have a greater impact on the aroma profile owing to volatilisation and transformations of sulfur compounds [31, 165]. The most important volatiles produced from fatty acids and perceived by GC-O of raw or cooked leeks are pentanal, hexanal, decanal and l-octen-3-ol (Table 7.5) [31, 35, 148, 163, 164]. 

---