Mechanism of degradation reaction

It is planned to describe more extensively the mechanism of degradation reactions in another volume of this series and it is hoped that it will be possible to give then more details on the current work of Flory and Simha. 

CM-chitosan is a kind of carboxymethylated derivative of chitosan, which is prepared by the chemical reactions of chitosan and monochloroacetic acid. It contains -COOH and -NH2 groups in the molecular structure. Being a soluble chitosan derivative in both acidic and basic physiological media, it might be a better candidate than chitosan for studying the mechanism of degradation reaction of chitosan derivatives. 

The mechanisms of degradation reactions depend closely on the molecular structure of the polymer considered. The monomer units can be subject to attack by all sorts of reactive species, but only those resulting from the exposure to a natural environment are considered here. Thus, only hydrolytic, oxidizing, photochemical, and thermal degradations and any combination among them will be described below. 

Section 8 deals with reactions which occur at gas—solid and solid—solid interfaces, other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reaction at the liquid—solid interface (and corrosion), involving electrochemical processes outside the coverage of this series, are not considered. With respect to chemical processes at gas-solid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in to the consideration of the kinetics and mechanism of catalytic reactions. 

The mechanisms of degradation and the mode of action of the various PVC stabilisers have both been widely studied. Often at least one aspect of their operation is some sort of reaction with the first trace of hydrogen chloride evolved. This removes what would otherwise act as the catalyst for further dehydrochlorination, and hence significantly retards the degradation process. In addition, many stabilisers are themselves capable of reacting across any double bonds formed, thereby reversing the process that causes discoloration and embrittlement. 

The initial electrochemical and biological oxidation with xanthine oxidase are essentially identical. However, electrochemically 2,8-dioxyadenine the final product in the presence of xanthine oxidase is much more readily oxidizable than adenine 59) so that considerable further oxidation occurs. To the authors knowledge, 2,8-dioxyadenine is not a major metabolite of adenine in man or other higher organisms. Accordingly, it is likely that other enzymes accomplish further degradation of 2,8-dioxyadenine. The relationship between the products so formed and the mechanism of the reaction to the related electrochemical processes has yet to be studied. 

The introduction of ionol in oxidized PIB decreases the peroxyl radical concentration by 94 times and rate of degradation via the reaction P02 + P02 by 942 = 8836 times, but the observed rate of macromolecules degradation decreases by seven times only. This means the appearance of another mechanism of degradation in the presence of the antioxidant. The kinetic study of PIB degradation in the process of initiated oxidation confirmed this hypothesis. The PIB degradation via the reaction P02 + P02 leads to the proportionality vs [InH]-2 v2. Experiments on PIB oxidation with different antioxidants (phenols, amines, and aminophenols) and variation of concentrations of an initiator, as well as inhibitor, proved the following equation ... 

Fig. 6.25. Simplified mechanism of two degradation reactions between peptides and reducing sugars occurring in solids, a) Maillard reaction between a side-chain amino (or amido) group showing the formation of an imine (Reaction a), followed by tautomerization to an enol (Reaction b) and ultimately to a ketone (Reaction c). Reaction c is known as the Amadori rearrangement (modified from [8]). b) Postulated mechanism of the reaction between a reducing sugar and a C-terminal serine. The postulated nucleophilic addition yields an hemiacetal (Reaction a) and is followed by cyclization (intramolecular condensation Reaction b). Two subsequent hydrolytic steps (Reactions c and d) yield a serine-sugar conjugate and the des-Ser-peptide...

This brief description of oxidant formation in polluted air is based on our current understanding of the chemistry involved. It is evident fix>m an examination of the detailed mechanism that many of the important reactions have not been well studied. For example, the sequences of degradation reactions for the hydrocarbons are only poorly understood. As a result of these uncertainties, it is not possible to make accurate predictions of photochemical oxidant concentrations. However, with another 5 yr of progress similar to the last 5, it should be possible to construct chemical models that will permit ozone predictions accurate to within... 

Proteins are subject to a variety of chemical modification/degradation reactions, viz. deamidation, isomerization, hydrolysis, disulfide scrambling, beta-elimination, and oxidation. The principal hydrolytic mechanisms of degradation include peptide bond... 

Less than 10% of the reports on pyridine electrochemistry deal with anodic reactions. The mechanisms of these reactions are rarely known and, as a result, yields or current efficiencies have not always been optimized. Many of the anodic reactions were studied in beaker cells, which are simply not good models for modern flow cells moreover, uncontrolled power supplies were often used. Consequently, anode overpolarization caused ring degradation in many cases. 

At low temperatures, D-glucose and D-fructose in the presence of ferrous sulfate are converted into D-uru/u770-hexos-2-ulose (36), which can be degraded by further oxidation to glycolic acid, glyoxylic acid, and D-erythronic acid. The nature of the products formed under various conditions and the mechanism of the reaction have been described (see Ref. 1, p. 1133). In dilute solution, in the presence of ferrous sulfate at low temperature, compound 36 gave D-ura mo-2-hexulosonic acid (37) and D-ery//zro-hexo-2,3-diulosonic acid (38). In concentrated solutions, formaldehyde was also found. The formation of these products at low temperature was ascribed to the series of reactions in Scheme 19. 

Once the mechanism of a reaction is established, kinetic studies constitute the next step to determine the rate of pollutant degradation. Kinetic laws must be deduced experimentally following well-established rules [125], For the degradation of a compound M through O3/H2O2/UV oxidation, the rate of M disappearance is given by Eq. (70) ... 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

The aim of all studies of polymer degradation is to understand the mechanism of the reaction in detail to be able to develop a suitable stabilizing system and to predict the lifetime of the product under conditions of use. 

---