Fenton's reagent

Fenton's reagent is a solution of hydrogen peroxide (H2O2) with ferrous iron (typically iron(II) sulfate, FeSO4) as a 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 (perchloroethylene, PCE). It was developed in the 1890s by Henry John Horstman Fenton as an analytical reagent. Fe2+ + H2O2 → Fe3+ + HO• + OH−    (1) Fe3+ + H2O2 → Fe2+ + HOO• + H+    (2) 2 H2O2 → HO• + HOO• + H2O    (net reaction: 1+2) C6H6 + FeSO4 + H2O2 → C6H5OH    (3) Fenton's reagent is a solution of hydrogen peroxide (H2O2) with ferrous iron (typically iron(II) sulfate, FeSO4) as a 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 (perchloroethylene, PCE). It was developed in the 1890s by Henry John Horstman Fenton as an analytical reagent. Iron(II) is oxidized by hydrogen peroxide to iron(III), forming a hydroxyl radical and a hydroxide ion in the process. Iron(III) is then reduced back to iron(II) by another molecule of hydrogen peroxide, forming a hydroperoxyl radical and a proton. The net effect is a disproportionation of hydrogen peroxide to create two different oxygen-radical species, with water (H+ + OH−) as a byproduct. 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 and results in the oxidation of contaminants to primarily carbon dioxide and water. Reaction (1) was suggested by Haber and Weiss in the 1930s as part of what would become the Haber–Weiss reaction. Iron(II) sulfate is typically used as the iron catalyst. The exact mechanisms of the redox cycle are uncertain, and non-OH• oxidizing mechanisms of organic compounds have also been suggested. Therefore, it may be appropriate to broadly discuss Fenton chemistry rather than a specific Fenton reaction. In the electro-Fenton process, hydrogen peroxide is produced in situ from the electrochemical reduction of oxygen. 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. 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. As the Fenton reaction depends on the simultaneous presence in solution of dissolved Fe2+ and Fe3+ ions, its kinetics is influenced by the respective solubilities of both species whose are directly function of the solution pH. As Fe3+ is about 100 times less soluble than Fe2+ in natural waters at near-neutral pH, the ferric ion concentration is the limiting factor for the reaction rate. The reaction only proceeds rapidly under acidic conditions. At high pH, under alkaline conditions, the reaction slows down due to precipitation of Fe(OH)3, lowering the concentration of the Fe3+ species in solution.