Fenton's reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants or waste waters. Fenton's reagent can be used to destroy organic compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE).
Ferrous Iron(II) is oxidized by hydrogen peroxide to ferric iron(III), a hydroxyl radical, and a hydroxyl anion. Iron(III) is then reduced back to iron(II), a superoxide radical, and a proton by the same hydrogen peroxide. The net effect is a disproportionation of hydrogen peroxide to create two different oxygen-radical species, with water (H+ + OH–) as a byproduct.
- (1) Fe2+ + H2O2 +H+ → Fe3+ + HO• + H2O
- (2) Fe3+ + H2O2 → Fe2+ + HOO• + H+
The free radicals generated by this process then engage in secondary reactions. For example, the hydroxyl is a powerful, non-selective oxidant. Oxidation of an organic compound by Fenton's reagent is rapid and exothermic (heat-producing) and results in the oxidation of contaminants to primarily carbon dioxide and water.
Reaction (1) was suggested by Haber and Weiss in the 1930s. Iron(II) sulfate is a typical iron compound used as the catalyst. The exact mechanisms of the redox cycle are not certain, and also non-OH• oxidizing mechanisms of organic compounds have been suggested, therefore, it may be appropriate to broadly discuss "Fenton chemistry" rather than a specific "Fenton reaction".
Fenton's reagent is also used in organic synthesis for the hydroxylation of arenes in a radical substitution reaction such as the classical conversion of benzene into phenol. (3) C6H6 + FeSO4 + H2O2 → C6H5OH
A recent hydroxylation example involves the oxidation of barbituric acid to alloxane. Another application of the reagent in organic synthesis is in coupling reactions of alkanes. As an example tert-butanol is dimerized with Fenton's reagent and sulfuric acid to 2,5-dimethyl-2,5-hexanediol.
The Fenton reaction has importance in biology because it involves the creation of free radicals by chemicals that are present in vivo. Transition-metal ions such as iron and copper donate or accept free electrons via intracellular reactions and help in creating free radicals. Although most intracellular iron is in ferrous (+2 ion) form, superoxide ions can convert it to the ferric (+3) form to take part in Fenton reaction. Since superoxide ions and transition metals act in a synergistic manner in the creation of free radical damage, Iron supplementation must not be done in patients with any active infections or in general any diseases.
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