Fenton’s reagents

Fenton s reagent. To a solution of tartaric acid or a tartrate add 1 drop of freshly prepared ferrous sulphate solution, i drop of hydrogen peroxide solution and then excess of NaOH solution an intense violet coloration is produced, due to the ferric salt of dihydroxyfumaric acid, HOOC C(OH) C(OH)COOH. 

No coloration with Fenton s reagent cf. tartaric acid). 

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). 

Oxidation. Coumarin is not readily oxidized by chromic acid but, by action of the Fenton s reagent, it is converted into 7-hydroxycoumarin (umbeUiferone) [93-35-6] (28). 

Oxidation. Oxidation is one of the main chemical methods to treat and decompose dyes in wastewater. The oxidation agents used ate chlorine, bleach, ozone, hydrogen peroxide, Fenton s reagent, and potassium permanganate. 

One method of generating hydroxyl radicals is by a dding a soluble iron salt to an acid solution of hydrogen peroxide (Fenton s reagent) (176—180), ie ... 

Several papers and patents describe treating dye-containing wastewaters with Fenton s reagent with and without other methods such as coagulation, incineration, biodegradation, etc (130,181—187). 

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. 

The yield of hydroxylated products is always very low, and there are usually a number of by-products. For instance, side chains of aromatic nuclei are easily attacked, as shown by the formation of 5-hydroxymethyluracil from thymine. Breslow and Lukens measured both the amount of 3-hydroxyquinoline formed and the quinoline consumed during hydroxylation with Fenton s reagent and EDTA in the presence of several adducts (Table XII). 

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-... 

The distributions of phenolic isomers in hydroxylations in the animal body arc often similar to those obtained by Fenton s reagent. For example, the hydroxylation of coumarin by the rabbit gives the six hydroxycoumarins in amounts decreasing in the order 3- >7->6- >8- >4- /—5-, whereas Fenton s reagent gives mainly the 3-, 5-, and 7-derivatives with traces of the 6- and 8-derivatives. It may, however, be misleading to draw conclusions about the nature of... 

Fenton s reagent, 243-246 Ferrous ions (Fe2+), 218-225, 243-246 Fireflies See also Lampyridae eggs, 2... 

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... 

Protonated nitrogen heterocycles can be carbalkoxylated" by treatment with esters of a-keto acids and Fenton s reagent. Pyridine is carbalkoxylated at C-2 and C-4, for example. The attack is by "COOR radicals generated from the esters ... 

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.

Reduction of vinyl radicals by to the corresponding anion also has been observed (216). When purified acetylene is bubbled through Fenton s reagent, acetaldehyde is formed as a product, presumably via the following mechanism ... 

The preparation was performed on a commercial microcrystalline beta zeolite. The zeolite was treated with the Fenton s reagent and less than 0.3 wt% of carbon remained after the treatment. The porosity was fully developed as revealed by the pore-size distribution. Elemental analysis combined with TPR did confirm the high degree of Fe-exchange (98%) on the Bronsted sites. 

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