Fenton cations

The reverse reaction (that is, the oxidation of a vinyl radical by Fe to the corresponding vinyl cation) may be involved in the reaction of the dimethyl ester of acetylenedicarboxyUc acid 261 with Fenton s reagent [Fe —H2O2, (217)] (216). When 261 was treated with Fe —H2O2 and the reaction mixture was extracted with ether, a small amount of furan 262 was isolated. A possible mechanism (216) for its formation may be addition of hydroxyl radical to the triple bond of 261, followed by addition of the intermediate vinyl radical to a second molecule of 261 and oxidation of the resulting radical with Fe to the corresponding vinyl cation, followed by cyclization to 262, as shown in Scheme XX. 

Oxidation of thiophene with Fenton-like reagents produces 2-hydroxythiophene of which the 2(570 One isomer is the most stable (Eq. 1) <96JCR(S)242>. In contrast, methyltrioxorhenium (Vn) catalyzed hydrogen peroxide oxidation of thiophene and its derivatives forms first the sulfoxide and ultimately the sulfone derivatives <96107211>. Anodic oxidation of aminated dibenzothiophene produces stable radical cation salts <96BSF597>. Reduction of dihalothiophene at carbon cathodes produces the first example of an electrochemical halogen dance reaction (Eq. 2) <96JOC8074>. 

It was first assumed that the oxidant was H2O2. However, since traces of Fe-cations are present, a Fenton s type oxidation pathway, based on OH radicals, is more likely taking place. 

We discuss here a combined process including detemplation and Fe incorporation by ion-exchange in the zeolite framework [147]. To achieve this, oxidants to decompose the organic template and Fe-cations for exchange are needed. Both requirements are in harmony with Fenton chemistry. The OH radicals can oxidize the template and the Fe-cations be exchanged simultaneously. 

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) ... 

Under oxygen in the absence of water, toluene will transfer an electron to the positive hole, concurrently with electron transfer from the conduction band to oxygen, to give a toluene radical cation. On the other hand, in the presence of water, both toluene and water will transfer an electron to the positive holes. The resulting toluene radical cation may subsequently lose a proton affording a benzyl radical, which will be oxidized with oxygen or the superoxide anion to benzyl alcohol and benzaldehyde, as proposed for the reactions of Fenton s reagent with toluene (57). 

Reaction (76) has been believed to ruled out in a kinetic analysis of the Fenton reaction by EPR spin trapping (Mizuta et al. 1997) [a caveat is the observation that in water spin traps may be oxidized to the OH-adduct via the spin trap radical cation by strong oxidants (Eberson and Persson 1997 von Sonntag et al. 2004)], but reaction (77), already suggested earlier (Rush and Koppenol 1987), was required to account for their data. 

The degradation of 2-deoxyribose by Fenton s reagent has been conducted in acidic, neutral, and alkaline media, and in the presence and absence of hydroxyl-radical scavengers. It seems that both the substrate and the scavengers interact with the metal ions.110 Traces of Fe(II) accelerate the oxidation of carbohydrates by H202, but larger quantities of such a cation has a retarding elfect.111... 

However, increases in the photocatalytic activity have been reported for Ti02 doped by lanthanides, tin, and iron (III) (55). It may be questioned whether these increases could, in fact, arise from the photo-Fenton reaction between the cations located at the surface and the hydrogen peroxide formed in situ, or even possibly because of a partial dissolution of these cations in the case of aqueous-phase reactions. 

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. 

This section is devoted to discuss the main results obtained for the treatment of organics in wastewaters by electro-Fenton using a divided cell usually with a cationic membrane (e.g., of Nation) to separate the anolyte and catholyte. Although the anode reactions do not affect the degradation of pollutants under these conditions, it is interesting to know the characteristics of their homogeneous oxidation with OH when this radical is uniquely formed in the medium from Fenton s reaction (19.12). 

It is possible to generate ferryl species by peroxide treatment of ferrous iron ions [18,249], The two-electron oxidation of ferrous (as opposed to ferric) iron does not require the formation of a cation radical, although subsequent reactions may generate hydroxyl radicals. These reactions therefore provide an alternative mechanism to the Fenton reaction for free radical damage associated with low-molecular-weight iron species. In the absence of a protective protein environment, however, such low-molecular-weight ferryl species are unstable and difficult to detect and therefore their existence is controversial [see the review by Koppenol in this volume (Chapter 1)]. 

Although superoxide ion reacts with reduced transition metal complexes (Fe , Mn Co Fe (EDTA), Fe (TPP)) bi62 to give oxidized products, for simple solvated cations the reaction sequence is a Tewis acid catalyzed disproportionation (equation 155) with subsequent Fenton... 

On the basis of this study, Sudoh et al. [78] proceeded to generate hydrogen peroxide in an acidic solution (1 M H2SO4) for use in Fenton s reagent oxidation of wastewater streams (Fig. 20). The peroxide, produced at the cathode in an alkaline KOH electrolyte, was transferred by electrodialysis to a central chamber, separated by anion exchange membrane (ACLE-5P, Tokuyama Soda, Japan) on the cathode side and a cation exchange membrane (CM-2, Tokuyama Soda, Japan) on the anode side. At a current of 4 A, 2.2 kmol/m of H2O2 was found to accumulate in the cen-... 

During the following 15 years, only small advances were achieved in increasing catalyst efficiencies. Independently, Fenton [9a] at Union Oil and Nozaki [9b] at Shell Development Company (USA) discovered several related palladium chlorides, palladium cyanides, and zero-valent palladium complexes as catalysts. Sen and co-workers [10] reported that cationic bis(triphenyl-phosphine)-palladium tetrafluoroborate complexes in aprotic solvents such as dichloromethane, produced ethylene/carbon monoxide copolymers under very mild conditions. The reaction rates were, however, very low, as were the molecular weights. 

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