Fenton's Reagent

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).

It was developed in the 1890s by Henry John Horstman Fenton as an analytical reagent.

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 peroxide radical and a proton by the same hydrogen peroxide (disproportionation).The hydroxyl free radical generated by Fenton’s reagent 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.

(1) Fe2+ + H₂O₂ → Fe3+ + OH· + OH−

(2) Fe3+ + H₂O₂ → Fe2+ + OOH· + H+

Reaction (1) was suggested by Haber and Weiss in the 1930s. In the net reaction the presence of iron is truly catalytic and two molecules of hydrogen peroxide are converted into two hydroxyl radicals and water. The generated radicals then engage in secondary reactions. Iron(II) sulfate is a typical iron compound in Fenton's reagent. The exact mechanisms are debated (also non-OH· oxidizing mechanisms of organic compounds have been suggested) and, therefore, it may be appropriate to broadly discuss 'Fenton chemistry' rather than a 'Fenton reaction'.

In the electro-Fenton process, hydrogen peroxide is produced in the required amount 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.

(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.