Mechanisms of oxidation

Polymers oxidation is an in chain radical reaction involving abstractable hydrogens of polymer, and oxygen which is dissolved in polymer 

Hydroperoxides (POOH) decomposition. These species, of which origin will be explained in the following, contain a weak 0-0 bond [ 16] which is easily cleaved. Hydroperoxides decompose thermally [17]  

HO° radicals are extremely reactive and can abstract a hydrogen atom HO° -I- PH P° -I- H  

Alkoxy radicals PO° are rmstable and react by P scission process for example to give carbonyls, chain scissions or alcohols  

It is thus licit to write kinetically equivalent reactions [18]  

The earliest serious studies of polymer oxidation were by Bolland and his coworkers [8 to 11 ]. They proposed that this begins with a chain auto-oxidation mechanism. 

The highest radical concentrations would be ROO. Usually the second step. Formula 4.6 is rate determining. The termination usually involved the reactions 

In the absence of antioxidants, Reaction 4.7a would be predominant. At low O2 pressure or at high rates of initiation where oxygen diffusion is important, Reactions 4.7b and 4.7c become important. 

The auto-oxidation step described above is common among hydrocarbons. Examples include 

Auto-oxidation can also result in chain degradation of crosslinked polymers depending upon the polymer. 

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The mechanism of oxidative dyeing involves a complex system of consecutive, competing, and autocatalytic reactions in which the final color depends on the efficiency with which the various couplers compete with one another for the available diimine. In addition, hydrolysis, oxidation, or polymerization of diimine may take place. Therefore, the color of a mixture caimot readily be predicted and involves trial and error. Though oxidation dyes produce fast colors, some off-shade fading does occur, particularly the development of a red tinge by the slow transformation of the blue indamine dye to a red phenazine dye. 

In oiological systems, the most frequent mechanism of oxidation is the remov of hydrogen, and conversely, the addition of hydrogen is the common method of reduc tion. Nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP) are two coenzymes that assist in oxidation and reduction. These cofactors can shuttle between biochemical reac tions so that one drives another, or their oxidation can be coupled to the formation of ATP. However, stepwise release or consumption of energy requires driving forces and losses at each step such that overall efficiency suffers. 

The use of antioxidants has already been generally described in Chapter 7. The mechanism of oxidation and the effect of antioxidants are altered by the sulphide cross-links and other structures present in the vulcanisate. There are indeed grounds for arguing that a correct choice of curing system is more important than the decision whether or not to incorporate an antioxidant. 

The mechanism of oxidation with chromic acid has been the object of much study. The pioneering efforts of Westheimer and his group have resulted in the postulation of the mechanism shown in the reaction sequence below... 

The mechanism of oxidation has not been studied in detail. In aqueous media it may be assumed that hypobromous acid is formed and that this is the true oxidizing agent. Indeed, Deno" has shown that the rate of oxidation of alcohols by aqueous bromine closely parallels the HOBr concentration and that the alkyl hypohalide is a most likely intermediate. Rate studies and... 

The mechanisms of oxidation using bismuthate, periodate or lead tetraacetate, while still not completely understood, are probably similar, involving some type of cyclic ester formation as the first step. ... 

The mechanism of oxidation (16 —> 17) is not well understood. Both tetrahydroquinoline (19) and A-alkylaniline (20) have been isolated from reaction... 

Probably the only feature common to the mechanism of oxidation of the two groups is that, because of crack or pore formation in the continuous oxide, the rate of transport of oxygen in a molecular form has increased to the point where a phase-boundary reaction has assumed rate control. In... 

The mechanisms of oxide dissolution and scale removal have been widely studied in recent years. This work has been thoroughly reviewed by Frenier and Growcock who concluded, in agreement with others", that oxide removal from the surface of steel occurs predominantly by a process of reductive dissolution, rather than by chemical dissolution, which is slow in mineral acids. 

DeLuca, M., and Dempsey, M. E. (1970). Mechanism of oxidation in firefly luminescence. Biochem. Biophys. Res. Commun. 40 117-122. 

DeLuca, M., et al. (1971). Mechanism of oxidative carbon dioxide production during Renilla reniformis bioluminescence. Proc. Natl. Acad. Sci. USA 68 1658-1660. 

In the luminescence systems that require a peroxide or an active oxygen species in addition to molecular oxygen (the scaleworm, the tube worm Chaetopterus, the clam Pholas, the squid Symplecto-teuthis), their in vitro luminescence reactions reported are much slower and inefficient compared to their bright in vivo luminescence. The true, intrinsic activation factor in their in vivo luminescence should be determined, and the detailed mechanisms of oxidation should be elucidated. 

The mechanism of oxidation of the basal plane and that of the cross-section are entirely different (Fig. 10). Oxygen content on the cross-section rises with oxidation, while that on the basal plane drops from about 10 to 1 percent. This may correlate with the decrease in the ratio of edge planes to basal planes due to selective burning of the edge planes. 

There are few studies in the literature on the kinetics and mechanism of oxidation over base metal oxides. Blumenthal and Nobe studied the oxidation of CO over copper oxide on alumina between 122 and 164°C. They reported that the kinetics is first order with respect to CO concentration, and the activation energy is 20 kcal/mole (77). Gravelle and Teichner studied CO oxidation on nickel oxide, and found that the kinetics is also first order with respect to CO concentration (78). They suggested that the mechanism of reaction is by the Eley-Rideal mechanism... 

In contrast to the extensive work of the pure thermal degradation of polymers, less fundamental chemical information is available on the mechanism of oxidative degradation of polymeric materials. As another point of... 

It is concluded [634] that, so far, rate measurements have not been particularly successful in the elucidation of mechanisms of oxide dissociations and that the resolution of apparent outstanding difficulties requires further work. There is evidence that reactions yielding molecular oxygen only involve initial interaction of ions within the lattice of the reactant and kinetic indications are that such reactions are not readily reversed. For those reactions in which the products contain at least some atomic oxygen, magnitudes of E, estimated from the somewhat limited quantity of data available, are generally smaller than the dissociation enthalpies. Decompositions of these oxides are not, therefore, single-step processes and the mechanisms are probably more complicated than has sometimes been supposed. 

The Mechanism of Oxidation of Organic Fuels Gileadi, E. Piersma, B. 4... 

Taube, H. (1968). Mechanisms of oxidation-reduction reactions. /. Chem. Educ. 45,453-461. 

The mechanism of oxidation with acid dichromate has been intensely studied. 

The mechanism of oxidation probably involves in most cases the initial formation of a glycol (15-35) or cyclic ester,and then further oxidation as in 19-7. In line with the electrophilic attack on the alkene, triple-bonds are more resistant to oxidation than double bonds. Terminal triple-bond compounds can be cleaved to carboxylic acids (RC=CHRCOOH) with thallium(III) nitrate or with [bis(trifluoroacetoxy)iodo]pentafluorobenzene, that is, C6F5l(OCOCF3)2, among other reagents. 

When the oxidizing agent is a peroxide, the mechanism of oxidation to the sulfoxide is similar to that of 19-27. ... 

In addition to the foregoing, three further examples in this list (numbers 5-7) deserve consideration. These are (5) interaction of endocrine disrupters with the estrogen receptor, (6) the action of uncouplers of oxidative phosphorylation, and (7) mechanisms of oxidative stress. Until now only the first is well represented by biomarker assays that have been employed in ecotoxicology. 

These conclusions were supported by the results obtained in a study of the reactions of various types of acetylenes with TTN (94). Hydration of the C=C bond was found to occur to a very minor extent, if at all, with almost all of the compounds studied, and the nature of the products formed was dependent on the structure of the acetylene and the solvent employed. Oxidation of diarylacetylenes with two equivalents of TTN in either aqueous acidic glyme or methanol as solvent resulted in smooth high yield conversion into the corresponding benzils (Scheme 23). The mechanism of this oxidation in aqueous medium most probably involves oxythallation of the acetylene, ketonization of the initially formed adduct (XXXV) to give the monoalkylthallium(III) derivative (XXXVI), and conversion of this intermediate into a benzoin (XXXVII) by a Type 1 process. Oxidation of (XXXVII) to the benzil (XXXVIII) by the second equivalent of reagent would then proceed in exactly the same manner as described for the oxidation of chalcones, deoxybenzoins, and benzoins to benzils by TTN. The mechanism of oxidation in methanol solution is somewhat more complex and has not yet been fully elucidated. 

Figure 11-4. Mechanism of oxidation and reduction of nicotinamide coenzymes. There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AHj. One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H ) and is transferred to the 4 position, where it may be attached in either the A or the B position according to the specificity determined by the particular dehydrogenase catalyzing the reaction. The remaining hydrogen of the hydrogen pair removed from the substrate remains free as a hydrogen ion.

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