Fenton conditions

As preliminary examples, AIBN was reported to give a 48 hr degradation of tetrazepam very similar to that obtained in 6 months accelerated degradation of tablets (28). Similarly, the water-soluble initiator 2,2 -azobis (2-amidinopropane) dihydrochloride was shown to accelerate the natural degradation of thymidine. In contrast, systems producing the more reactive hydroxy radicals (e.g., Fenton conditions) give mainly nonnatural degradation products (29). 

Perfluoroalkyl radicals have also been generated from perfluoroalkyl iodides by use of Fenton conditions in DMSO, with the final step being abstraction of iodine from RFI by methyl radical [61]. 

Fenton conditions, which result in eventual abstraction of I from RFI by methyl radicals (see Sect. 4.1.5), can be used effectively to provide perfluoroalkylation of arenes [61]. 

R. W. Kane and J. D. Timpa, A high-performance liquid chromatography study of D-cellobiose degradation under Fenton conditions, J. Carbohydr. Chem., 11 (1992) 779-797. 

Fig. 19.4 Concentration of accumulated H202 vs. time during the electrolysis of 100 cm3 of a 0.05 M Na2S04 + H2S04 solution of pH 3.0 at 25°C in the Pt/02 cell of Fig. 19.3. (a, b, e) Without Pe2+ (anodic oxidation with electrogenerated H202), (c) with 1 mM Fe2+ (electro-Fenton conditions), (d) in the presence of 1 mM Fe2+ and under 6-W UVA irradiation (photoelectro-Fenton conditions). Applied current (a) 450, (b, c, d) 300,(e) 100 mA (Brillas et al. 2000)...

Figure 11.6 Free radical chemistry under Fenton conditions in the presence of Chitosan-containing hydrogels.

Perez M, Torrades F, Garcia Hortal JA, Domenech X, Peral J (2002) Removal of organic contaminants in paper pulp treatment effluents under Fenton and photo-Fenton conditions. Appl Catal B Environ 36 63-74... 

LaFeOs perovskites were also used as nanocatalysts for the heterogeneous Fenton-like reactions of phenol and methyl tert-butyl ether as model contaminants and H2O2 as oxidant [47]. The collected experimental results suggested that the performances of the AFeOs perovskite catalysts are influenced by the intrinsic properties of the perovskite (crystal structure, A site cation, or structural impurities) and processes that occur under the Fenton conditions. 

Fenton conditions (for investigations Metal 7 days maximum Fe(II)S04 or Fe(II)Cl2... 

The situation can be (hfiferent for fluorinated compounds owing to their high stability against HO. On the basis of the rate constant of HO reactions, it is proposed that hydroxyl radicals can readily react with H O to form peroxyl radicals (selfquenching) before it can react with the polymer under Fenton conditions (Liu et al. 

Water loss in operating an HDR faciUty may result from either increased storage within the body of the reservoir or diffusion into the rock body beyond the periphery of the reservoir (38). When a reservoir is created, the joints which are opened immediately fill with water. Micropores or microcracks may fill much more slowly, however. Figure 11 shows water consumption during an extended pressurization experiment at the HDR faciUty operated by the Los Alamos National Laboratory at Fenton Hill, New Mexico. As the microcracks within the reservoir become saturated, the water consumption at a set pressure declines. It does not go to zero because diffusion at the reservoir boundary can never be completely elirninated. Of course, if a reservoir joint should intersect a natural open fault, water losses may be high under any conditions. 

The discussion above refers to the classical dark conditions where the chemical activation is achieved thermally. Fenton requires a moderate thermal activation, resulting in a reaction temperature ranging from 25 to 90 °C. The oxidizing capacity of the Fenton reaction can be increased by UV or UV-vis Hght irradiation [160, 161]. The increase is interpreted by means of the photoreduction ability of Fe ... 

Iron or copper complexes will catalyse Fenton chemistry only if two conditions are met simultaneously, namely that the ferric complex can be reduced and that the ferrous complex has an oxidation potential such that it can transfer an electron to H2O2. However, we must also add that this reasoning supposes that we are under standard conditions and at equilibrium, which is rarely the case for biological systems. A simple example will illustrate the problem whereas under standard conditions reaction (2) has a redox potential of —330 mV (at an O2 concentration of 1 atmosphere), in vivo with [O2] = 3.5 x 10 5 M and [O2 ] = 10 11 M the redox potential is +230 mV (Pierre and Fontecave, 1999). 

For a long time one question remained unanswered the efficiency of the Fenton reaction as the in vivo producer of hydroxyl radicals due to the low rate of Reaction (2) (the rate constant is equal to 42.11 mol 1 s 1 [18]). It is known that under in vitro conditions the rate of Fenton reaction can be sharply enhanced by chelators such as EDTA, but for a long time no effective in vivo chelators have been found. From this point of view new findings obtained by Chen and Schopfer [19] who found that peroxidases catalyze hydroxyl radical formation in plants deserve consideration. These authors showed that horseradish peroxidase (HRP) compound III is a catalyst of the Fenton reaction and that this compound is one to two orders of magnitude more active than Fe EDTA. 

Several studies suggest that LA and DHLA form complexes with metals (Mn2+, Cu2+, Zn2+, Cd2+, and Fe2+/Fe3+) [215-218]. However, in detailed study of the interaction of LA and DHLA with iron ions no formation of iron LA complexes was found [217]. As vicinal dithiol, DHLA must undoubtedly form metal complexes. However, the high prooxidant activity of DHLA makes these complexes, especially with transition metals, highly unstable. Indeed, it was found that the Fe2+-DHLA complex is formed only under anerobic conditions and it is rapidly converted into Fe3+ DHLA complex, which in turn decomposed into Fe2+ and LA [217]. Because of this, the Fe3+/DHLA system may initiate the formation of hydroxyl radicals in the presence of hydrogen peroxide through the Fenton reaction. Lodge et al. [218] proposed that the formation of Cu2+ DHLA complex suppressed LDL oxidation. However, these authors also found that this complex is unstable and may be prooxidative due to the intracomplex reduction of Cu2+ ion. 

In agreement with literature results (45,46), independent experiments confirmed that the addition of an aliquot of a CuCl acetonitrile solution to an H202 solution induced the immediate decay of H202 at pH 9.0 (44). Most likely, the OH radical produced is involved in fast oxidation of the H2C-R, SQ-R - or even the Q-R forms of the substrate and the stoichiometry shown in Eq. (54) is not valid anymore. The formation of free radicals was excluded under acidic conditions (36) implying that the Fenton-type decomposition of H202 may gain significance only in alkaline solution. 

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