Fenton reactions

Fenton Fe " catalyzed oxidation of tartaric acid Fenton reaction) Fenton (1894)... 

Reaction (16) is a Fenton-type reaction (Fenton, 1894 Croft et al., 1992). There is still controversy as to whether the reaction of Cu+ and H2O2 leads to the formation of (HO) or even a Cu + species (Bielski and Cabelli, 1991). As for iron-catalysed reactions, Fenton chemistry probably involves (oxygen)iron(2-i-) [FeO] + intermediates, which are strong oxidants as well. The reaction of Fe +(chelate) -1- H2O2 could yield (O2), but is reported to be rather slow (Halliwell and Gutteridge, 1989, 1992 Chiueh et al., 1992). In vivo formation of (HO) is determined by measurement of hy-droxylated salicylic acid (Van Steveninck et al.,... 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

The uv—hydrogen peroxide system has advantages over the iron—hydrogen peroxide (Fenton s reagent) procedures, eg, the reaction is not limited to an acid pH range and the iron catalyst and resulting sludges are eliminated. However, the system to date is not effective for dye wastewaters because of absorption of uv by colored effluent. 

Heterocyclic compounds have in most cases been hydroxylated by modified forms of Fenton s reagent. For instance, EDTA or pyrophosphate have been added to the system to complex the ferrous ions. It has been shown in the reactions of bcnzenoid compounds, however, that addition of complexing agents does not affect the distribution of isomers obtained by Fenton s reagent,and therefore the hydroxyl radical must still be the hydroxylating species. 

It is notable that flavone and its derivatives are substituted predominantly in the phenyl group at positions 3 and 4 whereas reaction with Fenton s reagent gives mainly the S-hydroxy isomer." " Similarly, comparison of the distribution of products from coumarin obtained with Udenfriend s system with that given by Fenton s reagent re-... 

Combinations of H2O2 and Fe(II) salts, generally known as Fenton reagents, have for many years been used in organic and polymer chemistry, Medalia and Kolthoff [77] summarized the principal features of this reaction. 

The Tf- CF system is preferred over Fenton s reagent because Ti4 is a less powerful oxidizing agent than Fc5+ and the above mentioned pathway and other side reactions are therefore of less consequence.252 Much of the discussion on redox initiation in Section 3.3.2.6.1 is also relevant to hydrogen peroxide. 

Much work has been done on the mechanism of the reaction with Fenton s reagent, and it is known that free aryl radicals (formed by a process such as HO- + ArH AR- + H2O) are not intermediates. The mechanism is essentially that outlined on page 898, with HO- as the attacking species, formed by... 

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. 

Scheme XX. Possible Mechanism of Reaction of Fenton s Reagent with Dimethyl Acetyledicarboxylate.

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

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>. 

Floyd, R.A. Zs-Nagy (1984). Formation of long lived hydroxyl free radical adducts of proline and hydroxyproline in a Fenton reaction. Biochimica Bio-physica Acta, 790, 94-7. 

Fenton chemistry comprises reactions of H2O2 in the presence of iron species to generate reactive species such as the hydroxyl radical OH. These radicals ( = 2.73 V) lead to a more eflident oxidation chemistry than H2O2 itself (E° = 1.80 V). 

The reaction was reported by Henry J. Fenton [140]. This reaction is appHed in the treatment of hazardous organic wastes. A search for Fenton reaction in the website ISI Web of Knowledge [141] throws up thousands of scientific papers due to the exponential growth in its use over the years. It has been reviewed in various papers [142-145]. Below, the reaction pathway in the absence of an organic compound is given ... 

---