Autooxidation mechanism

Carbon black is the best UV screening compound and provides long-term protection. Carbon black not only screens out UV but also inhibits photooxidation through a complex series of autooxidative mechanisms. Not only is the particle size of carbon black important (the best performance is in the range of 15-25 nm), but also the chemical composition of its surface. It was proven experimentally that the best results were obtained when Channel Black was used. Channel Black is no longer manufactured by the channel process but by... 

SCHEME 3.1 Simplified autooxidation mechanism of polyolefins considering the chemiluminescence pathway of reaction. 

Hayon, R., Treinin, A., and Wilf, J. (1972) Electronic spectra, photochemistry and autooxidation mechanism of the sulfite-bisulfite-pyrosulfite systems, J. Am. Chem. Soc. 94, 47-57. 

This work demonstrates that PEUU degradation is a surface phenomenon resulting from a classical autooxidation mechanism. By modelling the depth of the surface degraded layer with a diffusion-reaction model, it was shown that PEUU degradation was controlled by diffusion of oxygen into the polymer. 25 refs. 

Scheme 18.1 General proposed autooxidation mechanism for polymers (R = polymer chain, H = most labile hydrogen).

Polyethers are susceptible to oxidation and usually are stabilized with an antioxidant. The oxidatiye attack of polyoxyethylene proceeds by an autooxidation mechanism that inyolyes intrachain hydroperoxide formation, which decomposes and causes chain... 

There are two basic methods for making polymer materials photochemically degradable (2,3). One method is to chemically incorporate a clu omophore into the polymer chains. Although numerous chromophores have been evaluated, the most commercially successful chromophore is the carbonyl group 2,3,S). Absorption of UV radiation leads to degradation by the Norrish Type I and II processes or by an atom abstraction process (Scheme 1), all of which are typical photoreactions of the carbonyl chromophore. Note diat once radicals are introduced into the system, chain degradation can occur by the autooxidation mechanism (Scheme 2). 

Since the autooxidative mechanism varies with the nature of the metal ion, it is sometimes recommended to use drier mixtures rather than individual driers. For example, cobalt is an effective catalyst for hydroperoxide formation, whereas manganese favours the breakdown of oxygenated... 

Tertiary peroxyl radicals also produce chemiluminescence although with lower efficiencies. For example, the intensity from cumene autooxidation, where the peroxyl radical is tertiary, is a factor of 10 less than that from ethylbenzene (132). The chemiluminescent mechanism for cumene may be the same as for secondary hydrocarbons because methylperoxy radical combination is involved in the termination step. The primary methylperoxyl radical terminates according to the chemiluminescent reaction just shown for (36), ie, R = H. 

Graham, D.G. Tiffany, S.M. Bell, W.R., Jr. and Gutknecht, W.F. Autooxidation versus covalent binding of quinones as the mechanism of toxicity of dopamine, 6-hydroxydopamine and related compounds toward C1300 neuroblastoma cells in vitro. Mol Pharmacol 14 644-653, 1978. 

There are still some non-explained observations. For example, syndiotactic PP was reported [45,46] as being more stable than isotactic polymer. At 140°C, the maximum chemiluminescence intensity was achieved after 2,835 min for syndiotactic PP, while isotactic polymer attained the maximum after only 45 min. Atactic PP was reported to be more stable than the isotactic polymer [46]. An explanation has been offered that the structure of isotactic PP is much more favourable for autooxidation, which proceeds easier via a back-biting mechanism where peroxyl radicals abstract adjacent tertiary hydrogens on the same polymer chain. 

The reactions depicted in Fig. 32 are most often carried out at low temperatures. The incursion of a thermal process at elevated temperatures has occasionally been observed. In some cases the thermal oxygenation products are identical to the photochemical products and in other cases are different. For example, when 2,3-dimethyl-2-butene/02 NaY is warmed above — 20 °C a reaction was observed which led to pinacolone (3,3-dimethyl-2-butanone) as the major product.98,110 Pin-acolone is not formed in the photochemical reaction at the same temperature. On the other hand, identical products were observed in the thermal and photochemical intrazeolite oxygenations of cyclohexane.114,133 135 These intrazeolite thermal processes occur at temperatures well below that necessary to induce a classical autooxidation process in solution. Consequently, the strong electrostatic stabilization of oxygen CT complexes may also play a role in the thermal oxygenations. Indeed, the increase in reactivity of the thermal oxygenation of cyclohexane with increasing intrazeolite electrostatic field led to the conclusion that initiation of both the thermal and photochemically activated processes occur by the same CT mechanism.134 Identical kinetic isotope effects (kH/kD — 5.5+0.2) for the thermal and photochemical processes appears to support this conclusion.133... 

Further research in the library and discussion with other chemists in the company leads you to a new mechanism autooxidative degradation. Fatty chemicals are known to undergo such degradation with the formation of a series of compounds, some of which are volatile and potentially explosive (Table 21.1). These reactions would occur more readily at elevated temperatures and in the presence of trace metals, such as iron, cobalt, and nickel. 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Information on the formation and decomposition of iron(III)-sulfur(IV) complexes, in the presence and absence of dioxygen, is vital to the understanding of iron-catalyzed autooxidation of sulfur(IV). The kinetics and mechanism of formation of mono-, bis-, and tris-sulfito-iron(III) from Fe " aq/FeOH aq, have been established, together with estimates of their stability... 

No amount of sterilization wiU prevent or even slow autooxidation, and there are only two defenses removal of O2 and addition of inhibitors. Oxygen barriers in food packaging are a major topic in the engineering of polymer films. The barrier properties of various polymers are very important in food applications, and many of these are multilayer polymers that have a thin layer of an impermeable polymer (such as polyacrylonitrile and ionic polymers) on a cheaper but O2-permeable polymer such as a polyolefin, which gives mechanical strength to the fikn. 

Procarbazine (Matulane) may autooxidize spontaneously, and during this reaction hydrogen peroxide and hydroxyl free radicals are generated. These highly reactive products may degrade DNA and serve as one mechanism of procarbazine-induced cytotoxicity. Cell toxicity also may be the result of a transmethylation reaction that can occur between the A-methyl group of procarbazine and the N7 position of guanine. 

Brandt, C., I. Fabian, and R. van Eldik, Kinetics and Mechanism of the Iron(III)-Catalyzed Autooxidation of SulfuKIV) Oxides in Aqueous Solution—Evidence for the Redox Cycling of Iron in the Presence of Oxygen and Modeling of the Overall Reaction Mechanism, Inorg. Chem., 33, 687-701 (1994). 

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