Fenton processes, production radicals

The fact that reaction (12) is much slower than reaction (8), implies that Fe is faster depleted from the solution. As a result, Fenton process is halted because the redox chain cannot be supported itself. In addition, it is accepted that (Pignatello 1992 Boye et al. 2003) the hydroperoxyl radical (HO2 ) has a much lower oxidant power than OH. In the presence of organics, Fenton chemistry is even more complex because hydroxyl radical, both iron cations and the oxidation products enter into a series of consecutive and parallel reactions. An example of the complexity of these reactions is discussed elsewhere (Gozzo 2001) but a brief description is given here. The initial step for an organic substrate (R-H) oxidation starts with the interaction of itself with OH, according to (Walling and Kato 1971) ... 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

In the presence of PhSeSePh and excess hydrocarbon substrate the Fenton process [Fe(PA)2/HOOH] produces carbon radicals (RO, which are trapped by PhSeSePh to give PhSe-R products (Table 5-2 and Scheme 5-1). The distribution of PhSe-R isomers appears to reflect the isomer abundance for the R- radicals from the Fenton cycle [Eqs. (5-10) and (5-11)]. For n-hexane and 2-Me-butane the R-SePh isomer distribution (Table 5-2) indicates that the relative reaction probabilities of HO- with a C-H bond in — CH3, CH2, and CH groups are 0.074, 0.44, and 1.00 (the respective C-H bond energies are 100, 95, and 93 kcal), which are in accord with the relative values for aqueous HO- (0.10/0.48/1.00).i3 Thus, PhSeSePh provides the means to trap first-formed carbon radicals and thereby give insight to the mechanism of their generation. 

Advanced oxidation processes (AOP) can be classified using their manner of OH radical production. The most known AOP in water treatment is the combination of ozone with H2O2. Other AOP include the UV-induced AOP (UV/H2O2, UV/ozone, UV/Ti02), Fenton and Photo-Fenton processes, sonolytically induced AOP and those AOP which create OH radicals directly from water by homolysis (gamma radiolysis/electron beam injection, VUV-irradiation). In the following chapters, each technology is discussed with respect to MTBE elimination. 

In 1876, Henry J.H. Fenton publicly announced that the use of a mixture of H2O2 and Fe " (thereafter so-called Fenton s reagent) allowed the destruction of an organic compound, namely, tartaric acid [1], Such discovery triggered an intense research to elucidate the mechanistic fundamentals and propose different variants and applications of the Fenton process. The possible formation of Fe(IV) as an active Fenton intermediate, as well as the modeling of the real structure of the iron aqua complexes, is still the subject of discussion [2, 3]. However, at present, it is quite well established that the classical Fenton s reaction (1) involves the production of highly oxidative hydroxyl radicals ( OH) in the bulk as the main reactive species, and its optimum pH value is 2.8-3.0 [1] ... 

Humans Hydrogen peroxide has been used as an enema or as a cleaning agent for endoscopes and may cause mucosal damage when applied to the surface of the gut wall. Hydrogen peroxide enteritis can mimic an acute ulcerative, ischaemic or pseudomembranous colitis, and ranges from a reversible, clinically inapparent process to an acute, toxic fulminant colitis associated with perforation and death (Bilotta and Waye, 1989). It is conceivable that anecdotal reports of exacerbation of IBD by iron supplementation (Kawai et al. 1992) are mediated by hydroxyl radical production by the Fenton reaction. 

Lipid peroxidation is probably the most studied oxidative process in biological systems. At present, Medline cites about 30,000 publications on lipid peroxidation, but the total number of studies must be much more because Medline does not include publications before 1970. Most of the earlier studies are in vitro studies, in which lipid peroxidation is carried out in lipid suspensions, cellular organelles (mitochondria and microsomes), or cells and initiated by simple chemical free radical-produced systems (the Fenton reaction, ferrous ions + ascorbate, carbon tetrachloride, etc). In these in vitro experiments reaction products (mainly, malon-dialdehyde (MDA), lipid hydroperoxides, and diene conjugates) were analyzed by physicochemical methods (optical spectroscopy and later on, HPLC and EPR spectroscopies). These studies gave the important information concerning the mechanism of lipid peroxidation, the structures of reaction products, etc. 

Reaction 22 has an RC>2H/Fe2+ mole ratio of 160 but has about the same yield as an uncontaminated reaction, 17 and 21. Reaction 19 has an RC>2H/Fe2+ mole ratio of 13.3 but again shows no change in yield from that of an uncontaminated reaction. Sample 20 has a 1.35 mole ratio of RO2H to Fe2+ and shows a sharp decrease in reaction yield and grafting. Here, the approximately 1 1 mole ratio of peroxide to iron should produce a high concentration of hydroxide radicals and extensive polymerization if these radicals are part of the polymerization process. Instead of a high yield, however, the reaction yield was less than one third of that obtained in the absence of iron. Therefore, a Fenton s initiation mechanism for this reaction is inconsistent with the data and probably does not occur. Elemental analysis data of Table VIII showed that product composition is proximate to, but not equal to, reaction mixture composition. 

The hexaaqua ion, [Cr(OH2)6]3+, is also the final product in the reduction process (7, 12). The complete process using Fe(II) as a reducing agent is shown in the reaction sequence (8)-(ll), where a Fenton-type (15) mechanism is proposed (7,16), with [(H20)5CrIV0]2+ behaving as the chromium equivalent of the -OH radical (16). 

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