Reaction autoxidation

Although some of these stabilizers are added specifically to react with evolved hydrogen chloride, when the primary function of the stabilizer is to repair defect sites or dismpt autoxidation reactions, the degree that these stabilizers react with hydrogen chloride can actually detract from their primary function necessitating the use of higher levels of stabilizers ia the PVC formulation. 

Autoxidation reaction of alkenes with singlet oxygen... 

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. 

R. F. Vasil ev and coworkers 14> suggested that within the well-known general scheme of radical chain autoxidation reactions ... 

In a series of chemiluminescent autoxidation reactions the following quantum yields were measured 38) ... 

Beside the phosphorescence of the carbonyl compounds produced in autoxidation reactions, there is some additional luminescence by singlet oxygen 14,43) it js sometimes difficult to differentiate between emission and the longer-wavelength part of the ketone phosphorescence 38>. 

Recent detailed studies on autoxidation reactions have been published for tetralin 44-46) cumene and ethylbenzene 46,47) methyl oleate 48,49) and benzaldehyde 50h... 

The hypothesis of Kellogg 38> described above, that autoxidation reactions display low quantum yields in spite of high yields of excited products, due to oxygen quenching in the solvent cage, is criticized by J. Beutel 13) who very thoroughly investigated the chemiluminescent autoxidation of dimedone (1.1. dimethyl 3.5 cyclohexandione). Here the recombination of dimedone peroxy radicals should be the excitation step ... 

The fact that the kinetic chain length of dimedone autoxidation is very low appears to indicate structural effects in autoxidation reactions. These may account for some of the discrepancies found in autoxidation chemiluminescence studies of different types of compounds. 

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

In the preceding paragraph peroxides were described as key intermediates in autoxidation chemiluminescence. In most cases hydroperoxides were involved. The majority are well-defined compounds (e.g. cumene hydroperoxide), but autoxidation reactions are rather complex and peroxides are only one, though very important type of compound involved. 

Kudlich M, Hetheridge MJ, Knackmuss HJ, Stolz A (1999) Autoxidation reactions of different aromatic o-aminohydroxynaphthalenes that are formed during the anaerobic reduction of sulfonated azo dyes. Env Sci Technol 33 869-901... 

METAL ION CATALYZED AUTOXIDATION REACTIONS KINETICS AND MECHANISMS... 

Metal ions play an important role as catalysts in many autoxidation reactions and have been considered instrumental in regulating natural as well as industrial processes. In these reactive systems, in particular when the reactions occur under environmental or in vivo biochemical conditions, the metal ions are involved in complicated interactions with the substrate(s) and dioxygen, and the properties of the actual matrix as well as the transport processes also have a pronounced impact on the overall reactions. In most cases, handling and analyzing such a complexity is beyond the capacity of currently available experimental, computational and theoretical methods, and researchers in this field are obliged to use simplified sub-systems to mimic the complex phenomena. When the simplified conditions are properly chosen, these studies provide surprisingly accurate predictions for the real systems. In this paper we review the results obtained in kinetic and mechanistic studies on the model systems, but we do not discuss their broad biological or environmental implications. 

A brief overview on why most of the autoxidation reactions develop complicated kinetic patterns is given in Section II. A preliminary survey of the literature revealed that the majority of autoxidation studies were published on a small number of substrates such as L-ascor-bic acid, catechols, cysteine and sulfite ions. The results for each of these substrates will be discussed in a separate section. Results on other metal ion mediated autoxidation reactions are collected in Section VII. In recent years, non-linear kinetic features were discovered in some systems containing dioxygen. These reactions form the basis of a new exciting domain of autoxidation chemistry and will be covered in Section VIII. 

The common element of Schemes 1-3 is that they each postulate direct interaction between the metal center and dioxygen. Although it is not stated explicitly, Eqs. (3) and (11) most likely proceed via an inner-sphere mechanism. Thus, the metal-dioxygen interaction implies spin pairing between the reactants when the metal ion is paramagnetic. As a consequence, the formation of the M-O2 type intermediates circumvents the restriction posed by the triplet to singlet transition which seems to be the major kinetic barrier of autoxidation reactions (5). 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Autoxidation reactions of L-ascorbic acid (H2A) have been the subject of intensive studies for decades. It was shown that some of the most... 

The kinetic consequence of the non-participating ligand was also noticed in the autoxidation reactions catalyzed by Ru(III) ion, Ru(EDTA) (1 1) and Ru(IMDA) (1 1) (EDTA = ethylenediaminetetraace-tate, IMDA = iminodiacetate) (24,25). Each reaction was found to be first order in ascorbic acid and the catalysts and, owing to the protolytic equilibrium between HA /H2A, an inverse concentration dependence was confirmed for [H+]. Only the oxygen dependencies were different as the Ru(III)-catalyzed reaction was half-order in [02], whereas the rates of the Ru(III)-chelate-catalyzed reactions were independent of [02]. In the latter cases, the rate constants were in good agreement with those... 

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