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Fenton’s Reaction is defined as the reaction in which Hydrogen Peroxide is converted into a hydroxyl free radical through a catalytic process. Due to its unstable and reactive nature, Hydroxyl free radical is extremely toxic. It is formed during Oxidative respiration in Mitochondria. This Fenton’s Reaction is named after the British Chemist, Henry John Horstman Fenton.
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Key Takeaways: Fenton’s reaction, Fenton’s reagent, Hydrogen peroxide, Oxidation, Exothermic reaction
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Fenton’s Reagent
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Initially, in 1890, this Fenton’s reagent was developed as an analytical reagent by Henry Fenton. Fenton’s Reagent refers to a solution of hydrogen peroxide that contains the Ferrous ion ( Fe2+, oxidative state of iron is +2). It is prepared by dissolving Iron (II) sulfate ( FeSO4) in hydrogen peroxide ( H2O2). The Ferrous ion of Fenton’s Reagent works as a catalyst and enhances the oxidation capacity of wastewater and contaminants. This Reagent can be used to destroy organic compounds like Trichloroethylene and Tetrachloroethylene by undergoing rapid oxidation in a highly exothermic reaction.
Breakdown of Fenton’s Reaction
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Majorly three steps are involved in the Breakdown of Fenton’s Reaction:
First Step
It begins with the oxidation of the ferrous ion ( Fe2+ ) to the Ferric ion ( Fe3+) in the presence of hydrogen peroxide ( H2O2). In this Fenton’s Reaction, Hydrogen Peroxide acts as an Oxidizing agent. This reaction leads to the formation of a Hydroxyl free radical and a hydroxide ion as byproducts. The required chemical equation for this reaction will be:
Fe2+ + H2O2 → Fe3+ + OH– + HO•
Second Step
The Formed ferric ion (Fe3+) is reduced back into the Ferrous ion ( Fe2+) in the presence of another Hydrogen Peroxide molecule and this results in the formation of a proton and a hydroperoxyl free radical as the byproducts. Hence, now the Ferrous ion catalyst is regenerated. The required chemical equation for this reaction will be:
Fe3+ + H2O2 → HOO• + Fe2+ + H+
Third Step
From these steps, when the hydrogen peroxide molecule undergoes disproportionation in the Fenton’s reaction, it will produce two different oxygen-free radicals. Along with this, protons and hydroxide are also formed as a byproduct and which can combine to form water. The required chemical equation for this reaction will be
2H2O2 → HOO• + HO• + H2O
Thus, in the end, the disproportionation of the H2O2 molecule occurs due to the presence of ferric ions in the hydrogen peroxide solution which results in the formation of highly toxic free radical species like the hydroxyl free radical. These free radicals usually participate in secondary reactions.
Environment’s pH effect on Fenton’s Reaction
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The pH of the environment creates a large impact on the rate of Fenton’s reaction because of the concentration of ferric ions which behaves as a limiting factor. At the neutral pH range, Ferric ions ( Fe3+)are less soluble than Ferrous ions (Fe2+).
As Ferric ions have increased solubility in the Acidic medium, this Fenton’s reaction proceeds at a rapid rate under acidic conditions. On the other hand, under the alkaline condition, the reaction rate slows down due to the formation of ferric hydroxide which precipitates out of the solution.
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Application of Fenton’s Reaction
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Given below are the applications of Fenton’s reagent
- In the Breakdown of Fenton’s Reagent, the oxidation of ferrous ions with hydrogen peroxide forms the basis of the Haber-Weiss reaction.
- Benzene can be transformed to phenol using Fenton's reagent..
- It is used in the coupling reaction of Alkanes.
- It can be used for the oxidation of Barbituric acid into Alloxan.
- This reagent can be used in Organic Synthesis reactions like for the hydroxylation of Arenes through the free radical substitution mechanism.
Also read: Molar Heat Capacity
Things to Remember
- Fenton’s Reaction refers to the chemical reaction using the catalytic process for the conversion of Hydrogen Peroxide to Hydroxyl free radical molecule.
- The breakdown of Fenton’s Reaction involves the oxidation of ferrous ion to Ferric ion and later the reduction of Ferric ion to Ferrous ion.
- Fenton’s Reagent is a solution of Iron (II) sulfate ( FeSO4) in hydrogen peroxide.
- This Reagent is used in different reactions like in Organic Synthesis, Coupling Reaction, and Oxidation reactions.
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Sample Questions
Ques. What is Fenton’s Reaction? (3 marks)
Ans. Fenton’s Reaction is named after Henry John Horstman Fenton, a British Chemist. This reaction involves Hydrogen Peroxide being converted into a hydroxyl free radical through a catalytic process. In most cases, the hydrogen peroxide reactant is produced by mitochondrial oxidative respiration. It's important to note that the hydroxyl free radical produced during Fenton's reaction is extremely dangerous (due to its unstable and reactive nature).
Ques. WHat steps are involved in the Breakdown of Fenton’s Reaction? (3 marks)
Ans. Steps involved in the Breakdown of Fenton’s Reaction:
- The first step begins with the oxidation of the ferrous ion (Fe2+ cation) to the ferric ion (Fe3+ cation) in the presence of hydrogen peroxide, which functions as an oxidising agent.
- The ferric ion is reduced back to the ferrous ion in the presence of another hydrogen peroxide molecule, resulting in the formation of a hydroperoxyl free radical and a proton as byproducts. As a result, the ferrous ion catalyst is regenerated.
- In Fenton's reaction, two distinct oxygen free radicals are produced when hydrogen peroxide molecules are disproportionated.
Ques. What is Fenton’s Reagent? Who developed it? (3 marks)
Ans. A solution of hydrogen peroxide containing the ferrous ion (the Fe2+ cation in which iron has an oxidation state of +2) is referred to as Fenton's reagent. The ferrous ion acts as a catalyst, allowing contaminants and wastewater to be oxidised. Fenton's reagent is made by dissolving iron(II) sulphate (FeSO4) in hydrogen peroxide.
Certain organic molecules, such as tetrachloroethylene and trichloroethylene, can be destroyed using Fenton's reagent. It's also important to note that Henry Fenton developed Fenton's reagent as an analytical reagent in the 1890s.
Ques. What is the application of Fenton’s Reagent? (3 marks)
Ans. Some of the applications are:
- Organic synthesis reactions can also benefit from Fenton's reagent. The use of Fenton's reagent, for example, can be used to hydroxylate arenes via a free radical substitution mechanism.
- Fenton's reagent can be used to convert benzene to phenol.
- Barbituric acid can also be converted to alloxan using Fenton's reaction.
- The coupling reactions of alkanes are another major use of Fenton's reaction.
Ques. What is the effect of Alkaline pH on Fenton’s Reaction and why so? (3 marks)
Ans. At neutral pH, ferric ions are approximately 100 times less soluble than ferrous ions. In Fenton's reaction, the concentration of ferric ions is generally the rate limiting factor. As a result, the pH of the surrounding environment has a significant influence on the rate of Fenton's reaction. Under Alkaline conditions in the Environment, the rate of Fenton’s reaction slows down due to the formation of Ferric Hydroxide as a precipitation of the solution.
Ques. What is the Electro-Fenton Process? Name a few compounds which can be destroyed using Fenton’s Reagent? (3 marks)
Ans. The electro-Fenton process produces hydrogen peroxide in situ as a result of electrochemical oxygen reduction.
Some of the organic compounds which can be destroyed using Fenton’s Reagent include Trichloroethylene and Tetrachloroethylene by undergoing rapid oxidation in a highly exothermic reaction.
Ques. What is Haber-Weiss's reaction? (3 marks)
Ans. Haber- Weiss reaction refers to a reaction between superoxide and hydrogen peroxide to form Hydroxyl radicals. The first stage of Fenton’s reaction plays an important role in the Haber-Weiss Reaction.
The reduction of ferric ion to ferrous is the first step in the catalytic cycle:
Fe3+ + •O2− → Fe2+ + O2
The second step is the Fenton reaction:
Fe2+ + H2O2 → Fe3+ + OH− + •OH
Net reaction:
O2- + H2O2 → •OH + HO- + O2
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