Oxidation free radical chain

Free-radical chain inhibitors are of considerable economic importance. The term antioxidant is commonly appUed to inhibitors that retard the free-radical chain oxidations, termed autoxidations, that can cause relatively rapid deterioration of many commercial materials derived from organic molecules, including foodstuffs, petroleum products, and plastics. The chain mechanism for autoxidation of hydrocarbons is ... 

Free-radical chain oxidation of organic molecules by molecular oxygen is often referred to as autoxidation (see Section 12.2.1). The general mechanism is outlined below. 

Biological antioxidants such as a-tocopherol (65, vitamin E) serve to inhibit free radical chain oxidation, and the mechanisms of their reactions have attracted close attention. The chain-breaking reaction of such phenols with peroxyl radicals is by hydrogen transfer (equation 101). 

All free radical chain oxidations are inevitably co-oxidations of the starting hy-drocarbon(s) and products. Most acetic acid produced in an oxidation generally survives in the product. At higher hydrocarbon conversions, however, increasing amounts of acetic acid are converted to waste products. 

It has been long known that free-radical chain oxidation of hydrocarbons is followed by chemiluminescence (CL). In the oxidation of hydrocarbons with a single oxidizable group, excited-state generation comes about from the disproportionation of peroxy radicals (ROO ). This termination step of the oxidation chain, which proceeds through the tetroxide intermediate, yields three products, namely, ketone (R.h=0), alcohol (ROH) and molecular oxygen (Eq. 1). 

Because the luminol detection system is also sensitive to NO2, chemical amplification methods have been attempted to further decrease detection limits for PANs below the pptv range for trace-level measurements. With this approach, the PANs are thermally decomposed to NO2 in the presence of large amounts of NO (6 ppm) and CO (8%). Thermal decomposition of the PANs yields peroxy radicals which initiate a free-radical chain oxidation of NO to NO2, producing several NO2 molecules (approximately 180 (20) for each PAN decomposed. This technique has been used as a gas chromatography detector to achieve ultratrace detection limits without sample preconcentration. The detector exhibits a slightly nonlinear response relative to conventional BCD, attributed to the nonlinear response of the luminol reaction in the presence of NO at 6 ppm. 

In very general terms, the Co-Br-catalyzed oxidation is a particular case of the free radical chain oxidation, common for all liquid phase oxidations of hydrocarbons [8-10]. The free radical chain oxidation occurs with four types of free radicals alkyl, alkoxy, alkylperoxy, and acylperoxy radicals [11, 12]. Other key active intermediates are hydroperoxides and peracids [11,12]. The nomenclature and structures are displayed in Figure 4.2. 

Low-molecular-weight HALS, a hydroperoxide inhibitor, has been shown to considerably reduce photooxidation of PBT [107]. It can be expected that the combination of a UV absorber with HALS will be particularly effective. There is little data on stabihzation of thermoplastic polyester elastomers, but the stabilizers used to protect polymers that degrade by free-radical chain oxidations should also be efficient for these types. Thus, UV absorbers of the benzotriazole and benzophenone types may be used for their screening capabihties, and synergistic effects can be expected with the addition of HALS to reduce the rate of chain oxidations. 

Conventional free radical chain oxidation (referred to as autoxidation) involves chain initiation, propagation, and termination steps [15-17]. Thepossibility of hydrogen atom abstraction by radical species in arenes without alkyl side chains is small because of the strong sp C—H bonds (the dissociation energies of the sp Ar—H bonds and sp ArCR —H are 112 and ca. 90kcal/mol, respectively [18]). Phenols readily react by direct abstraction of H from the hydroxyl gronp to form phenoxyl radical ArO, and this pathway is implicated in their antioxidant properties [16, 19]. Numerous literature addresses structure-reactivity relationships in the chemistry of phenols and phenoxyl radicals (see Refs. 1, 16, 19 and references therein). 

SCHEME 14.2 Main steps in free radical chain oxidation of phenols. 

Based on this already classical free-radical chain oxidation mechanism, it is obvious that it is possible to inhibit this process by introduction of the substances that, from one hand concur with RH for the binding with hydroxyl radical in the reaction (6.8), and on other hand concur with RH in the reaction (6.10), which does not form the macroradical R or lead to transformation of the hydroperoxide ROOH without formation of the radical OH. It means that the reaction (6.11) cannot go through the radical mechanism. For an efficient antioxidant the following equations must be followed ... 

In recent years, the studies in the field of homogeneous catalytic oxidation of hydrocarbons with molecular oxygen were developed in two directions, namely, the free-radical chain oxidation catalyzed by transition metal complexes and the catalysis by metal complexes that mimic enzymes. Low yields of oxidation products in relation to the consumed hydrocarbon (RH) caused by the fast catalyst deactivation are the main obstacle to the use of the majority of biomimetic systems on the industrial scale. 

The effects of the absorbed UV light on the polymers have been interpreted as free-radical chain oxidations which finally lead to the cleavage of the polymer backbone, to unsaturations, crosslinking, and the formation of small molecule fragments. However, at least two important photochemical non-radical reactions are also known (see Sections 11.2.2.2 and III. 1.3). 

Xu, L., Korade, Z., and Porter, N.A. (2010) Oxysterols from free radical chain oxidation of 7-dehydrocholesterol Product and mechanistic studies./. Am. Chem. Soc., 132, 2222-2232. 

The kinetics of the coherent synchronous reactions of HP decomposition and oxidation of pyridine derivatives has been reported. Regioselective oxidation of the pyridine derivatives were studied and conditions have been optimized for the production of 4-vinylpyridine, 4-vinylpyridine oxide, 2,2 -dipyridyl, and pyridine. A probable synchronized reaction mechanism has been suggested for the decomposition of HP and the free radical chain oxidation of pyridine derivatives. It is suggested that the hydroperoxy radical (H02-radical) plays a key role in this reaction mechanism. The activation energy has been calculated for the elementary steps of the dehydrogenation of 4-ethylpyridine. Oxidation of formamidine disulfide with HP in acidic medium results in the formation... 

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