Fenton-like reaction

As shown above, addition of H202 into a solution of the complex caused a prompt color change into blue to give hydroxyl radical, which was detected with a spin-trapping method. The reaction was found therefore to be a Fenton-like reaction. 

Nitrophenols are persistent pollutants and commonly found in industrial wastewaters. Degradation of nitrophenols to less dangerous materials or mineralization is difficult by natural process involving biodegradation or oxidation. Nitrophenols are commonly found in degradation of pesticides such as parathion and nitrofen. Kiwi et al. (1994) showed efficient photo-and dark oxidation via Fenton-like reactions on 2 and 4-nitrophenols. Photolysis of acidic solutions of H2Oz give OH radicals as primary photoproducts (Baxendale and Wilson, 1957) as follows ... 

CuZn-SOD is inactivated by H2O2 through the following mechanism H2O2 first reduces the enzyme-Cu2+ to the Cu+ form (reaction 10.15), then the enzyme-Cu+ reacts with H2O2 to form a strong oxidant at the active site by a Fenton-like reaction (reaction 10.16). The oxidant destroys the Cu-ligated His-63.84 ... 

Tsou TC, Yang JL. 1996. Formation of reactive oxygen species and DNA strand breakage during interaction of chromium(IU) and hydrogen peroxide in vitro Evidence for a chromium(III)-mediated Fenton-like reaction. Chem Biol Interact 102 133-153. 

Fenton-like reactions play an important role in a variety of catalytic and biological processes. In biology these reactions are believed to be the main source of reactive oxygen species (ROS) in the cell causing a variety of diseases, eg cancer, arteriosclerosis, essential hypertension, Alzheimer s disease, amyloidosis, osteoarthritis [9],... 

Iron ions cleave LOOHs in a Fenton like reaction to LO radicals (Scheme 14). 

Scheme 14 Generation of LO radicals by a Fenton like reaction.

Murphy AP, Boegli WJ, Price MK, et al. 1989. A fenton-like reaction to neutralize formaldehyde waste solutions. Environ Sci Technol 23 166-169. 

The Fenton process consists of the reaction of ferrous ion (Fe ) with H2O2 generating OH radicals, hydroxid ions and ferric ions (Fe ). The ferric ion itself can further react with H2O2 in a Fenton-like reaction creating protons, HO2 radicals and ferrous ion, the reaction rate and the reactivity of the HO2 radical being smaller than that of the OH radical. In photo-assisted Fenton processes OH radicals and ferrous ions are created directly from aqueous Fe " complexes (Fe(OH) ) and additional Fenton reactions can subsequently take place [107]. 

Recent results in the field of catalytic oxidations with hydrogen peroxide are reviewed. Most effective catalysts fall into three categories metallorganic compounds, phase-transfer catalysts, redox zeolites. Metalloporphyrins and Pt-phosphine complexes are representative of first category. Mo and W polyoxome-talates and related systems, in association with phase transfer agents, belong to the second one. Titanium silicalite (TS-1) is the most studied redox zeolite. The oxidation of nitrogen and sulphur compounds and Fenton-like reactions are not reviewed. 

Under some conditions, the active oxidant is Fe(IV)=0 and not the hydroxyl radical (101). Reactions entailing other low-valent transition-metal complexes that react like iron are called Fenton-like reactions (100,102,103). Fenton-like reactions were studied with Cu (aq) and its complexes, such as [Cu(D (phenanthroUnelal (102). The results of these studies indicate that transient complexes, Cu(I)-H202, are formed. However, OH is not formed directly as proposed in reaction (47) (103). 

The study of Fenton-like reactions is significant for biological systems. Csapski et al. studied the Fenton reaction in the presence of [Cu(I)(phenanthroline)2] and DNA 102). They showed that the Cu(I) complex binds to DNA, the double helical structure of which was degraded by either the OH radical or [Cu(III)(phenanthroline)2], which is formed via the oxidation of [Cu(IXphenanthroline)2] by H2O2. 

This shift in mechanism is due to the fact that reaction (79) is considerably slower than reaction (80) (115,141). These reactions have naturally to be considered also in the Cu(I)-catalyzed Fenton-like reactions where R radicals are formed. These processes are of special importance in biological systems in which copper complexes are known to induce oxidative stress (142-147). The Cu(I) species are formed in biological systems by reduction of Cu(II) by ascorbate, thiols, etc. 

The reaction is postulated to proceed through a hydroperoxide intermediate, [Cu2(L-H)(OOH)]. Experimental studies into Fenton and Fenton-like reactions point out that the detailed mechanism involves the formation of a low-valent cation.However, while [(H20)5Cr " 02H ] decomposes directly to [Cr(H20)6] " and OH in the presence of >0.1 M EtOH, 2-PrOH, and 2-BuOH, [(H20), Cu 02H ] reacts directly with the alcohols. In the absence of organic substrate, the latter complex does yield [Cu(H20) ] and OH. 

The Mechanism of Fenton-like Reactions and Their Importance for Biological Systems. A Biologist s View... 

In case the trioxane moiety is cleaved unsymmetrically, the resulting hydroperoxide fragments in a Fenton-like reaction, into energy-rich OH radicals and carbon radicals. 

Superoxide radicals are not able to cross cell membranes in contrast to hydrogen peroxide [47]. The hydrogen peroxide formed can then react with the Fe(II)species present in the red blood cell by a Fenton-like reaction, thereby forming hydroxyl radicals [47] (HO ) ... 

The discussed observations led to study the redox properties of the FQ. Indeed, the presence of H2O2 in the digestive vacuole of the parasite, estimated to be 15 mM [47,54] locally (this value is subject of controversy), is particularly interesting because of the presence of Fe(II) cation in the structure of FQ. Indeed, as for the Fe(II)/Fe(III) couple, it has been shown that under oxidative conditions, close to those of the digestive vacuole, FQ is capable of generating Fenton-like reactions leading to the production of hydroxyl radicals [55] ... 

---