What is Fenton’s Reaction?
Fenton’s reaction is a named reaction in which hydrogen peroxide is converted into a hydroxyl free radical via a catalytic process. The hydrogen peroxide reactant is usually formed by the mitochondrial oxidative respiration. It is important to note that the hydroxyl free radical formed during Fenton’s reaction is highly toxic (due to its unstable and reactive nature). This reaction is named after the British chemist Henry John Horstman Fenton.
Fenton’s reagent is a term that is used to denote a solution of hydrogen peroxide that contains the ferrous ion (the Fe2+ cation in which iron exhibits an oxidation state of +2). The ferrous ion works as a catalyst and promotes the oxidation of contaminants and wastewater. It can be noted that Fenton’s reagent is typically prepared by dissolving iron(II) sulfate (FeSO4) in hydrogen peroxide.
Fenton’s reagent can be employed to destroy certain organic compounds such as tetrachloroethylene and trichloroethylene. It can also be noted that Fenton’s reagent was developed as an analytical reagent in the 1890s by Henry Fenton.
Breakdown of Fenton’s Reaction
Fenton’s reaction begins with the oxidation of the ferrous ion (Fe2+ cation) to the ferric ion (Fe3+ cation) in the presence of hydrogen peroxide, which acts as an oxidizing agent. This results in the formation of a hydroxide ion and a hydroxyl free radical as byproducts. The chemical equation for this reaction is provided below.
Fe2+ + H2O2 → Fe3+ + OH– + HO•
Now, in the next step of Fenton’s reaction, the ferric ion is reduced back into the ferrous ion in the presence of another hydrogen peroxide molecule. This results in the formation of a hydroperoxyl free radical and a proton as the byproducts. Thus, the ferrous ion catalyst is regenerated. The chemical equation for this step of Fenton’s reaction is provided below.
Fe3+ + H2O2 → HOO• + Fe2+ + H+
Therefore, two different oxygen free radicals are produced when hydrogen peroxide molecules undergo disproportionation in Fenton’s reaction. It can be noted that hydroxide ions and protons are also formed as byproducts, which combine to form water. The chemical equation for the entire reaction can be written as follows.
2H2O2 → HOO• + HO• + H2O
Thus, the presence of ferric ions in the hydrogen peroxide solution facilitates the disproportionation of the H2O2 molecules, resulting in the formation of highly toxic free radical species such as the hydroxyl free radical. The free radicals that are formed during Fenton’s reaction usually go on to participate in secondary reactions (since the hydroxyl free radical is a very powerful, non-selective oxidizing agent). When organic compounds are exposed to Fenton’s reagent, they undergo rapid oxidization in a highly exothermic reaction. The contaminants are usually oxidized to water and carbon dioxide.
How does the pH of the Environment Affect Fenton’s Reaction?
Ferric ions are approximately 100 times less soluble than ferrous ions at neutral pH ranges. The concentration of ferric ions is usually the limiting factor for the reaction rate in Fenton’s reaction. Therefore, the pH of the environment has a large impact on the rate of Fenton’s reaction.
Under acidic conditions, Fenton’s reaction proceeds at a very rapid rate because of the increased solubility of ferric ions in acidic media. However, under alkaline conditions, the reaction rate of Fenton’s reaction slows down. This can be explained by the formation of ferric hydroxide (which precipitates out of the solution). The lowered ferric ion concentration as a consequence of the formation of ferric hydroxide is the reason behind the lowered reaction rate of Fenton’s reaction under alkaline conditions).
Applications of Fenton’s Reaction
- The first stage of Fenton’s reaction (oxidation of ferrous ions with hydrogen peroxide) forms the basis for the Haber-Weiss reaction, which is a named reaction that generates hydroxyl radicals from superoxide and hydrogen peroxide.
- Fenton’s reagent can also be used in organic synthesis reactions. For example, the hydroxylation of arenes via a free radical substitution mechanism can be achieved with the help of Fenton’s reagent.
- Benzene can be converted into phenol by employing Fenton’s reagent.
- Fenton’s reaction can also be used for the oxidation of barbituric acid into alloxan.
- Another important application of Fenton’s reaction is in the coupling reactions of alkanes.
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Frequently Asked Questions – FAQs
What is Fenton’s reagent?
Fenton’s reagent is a solution of hydrogen peroxide and the ferrous ion. Generally, Fenton’s reagent is a solution of ferrous sulfate in hydrogen peroxide. In this reagent, the ferrous ion acts as a catalyst and promotes the formation of free radicals.
What are the free radical species formed during Fenton’s reaction?
In Fenton’s reaction, hydroxyl free radicals and hydroperoxyl free radicals are formed from the reactions between ferrous ions and ferric ions with hydrogen peroxide.
How does pH affect the rate of Fenton’s reaction?
The pH affects the solubility of Fe3+ ions, which is the limiting factor for Fenton’s reaction. Since ferric ions are more soluble in acidic media, the reaction rate of Fenton’s reaction increases in acidic media. Under basic or alkaline media, the reaction rate slows down due to the formation of ferric hydroxide.
What is the electro-Fenton process?
The electro-Fenton process involves the production of hydrogen peroxide in situ as a result of the electrochemical reduction of oxygen.
What are the applications of Fenton’s reagent?
Fenton’s reagent can be used in the conversion of benzene into phenol. This reagent can also be used for the conversion of barbituric acid into alloxan. Fenton’s reagent is also useful in the hydroxylation of arenes.