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.
Fe2+ + H2O2 → Fe3+ + HO• + OH−
Fe3+ + H2O2 → Fe2+ + HOO• + H+
2 H2O2 → HO• + HOO• + H2O
(net reaction: 1+2)
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.
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.
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.
Effect of pH on the reaction rate
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 as a direct-function of the solution's pH. As Fe3+ is about 100 times less soluble than Fe2+ in natural water 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.
The Fenton reaction has different implications in biology because it involves the formation of free radicals by chemical species naturally present in the cell under in vivo conditions. Transition-metal ions such as iron and copper can donate or accept free electrons via intracellular reactions and so contribute to the formation, or at the contrary to the scavenging, of free radicals. In oxygen respirating organisms, most of the intracellular iron is in the ferric (Fe3+) state and must therefore be reduced to the ferrous (Fe2+) form to take part in the Fenton reaction. Superoxide ions and transition metals act in a synergistic way in the appearance of free radical damages. Therefore, although the clinical significance is still unclear, it is one of the viable reasons to avoid iron supplementation in patients with active infections, whereas other reasons include iron-mediated infections. The formation of oxidative species in the brain is maintained by the abundance of Fe2+ ions in the striatum nervous structure via the Fenton reaction.
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