:Fenton's 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.{{cite journal |doi=10.1021/acs.chemrev.0c00977 |title=Biomedicine Meets Fenton Chemistry |date=2021 |last1=Tang |first1=Zhongmin |last2=Zhao |first2=Peiran |last3=Wang |first3=Han |last4=Liu |first4=Yanyan |last5=Bu |first5=Wenbo |journal=Chemical Reviews |volume=121 |issue=4 |pages=1981–2019 |pmid=33492935 |s2cid=231712587 }}

{{NumBlk|:| Fe²⁺ + H₂O₂ → Fe³⁺ + HO^• + OH⁻|{{EquationRef|1}}}}

{{NumBlk|:| Fe³⁺ + H₂O₂ → Fe²⁺ + HOO^• + H⁺|{{EquationRef|2}}}}

{{NumBlk|:| 2 H₂O₂ → HO^• + HOO^• + H₂O|{{EquationRef|net reaction: 1+2}}}}

The free radicals generated by this process engage in secondary reactions. For example, the hydroxyl is a powerful, non-selective oxidant.{{cite book |doi=10.1016/b978-0-12-821011-6.00011-6 |chapter=Fenton- and ozone-based AOP processes for industrial effluent treatment |title=Advanced Oxidation Processes for Effluent Treatment Plants |year=2021 |last1=Cai |first1=Q.Q. |last2=Jothinathan |first2=L. |last3=Deng |first3=S.H. |last4=Ong |first4=S.L. |last5=Ng |first5=H.Y. |last6=Hu |first6=J.Y. |pages=199–254 |isbn=978-0-12-821011-6 |s2cid=224976088 }} 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 ({{EquationNote|1}}) was suggested by Haber and Weiss in the 1930s as part of what would become the Haber–Weiss reaction.{{cite journal |last1=Haber |first1=F. |last2=Weiss |first2=J. | year = 1932 | title = Über die katalyse des hydroperoxydes | trans-title= On the catalysis of hydroperoxides | journal = Naturwissenschaften | doi = 10.1007/BF01504715|volume = 20|issue = 51|pages = 948–950|bibcode = 1932NW.....20..948H|s2cid=40200383 }}

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.{{citation needed|date=October 2013}} 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.{{cite journal |first1=Juan |last1=Casado |first2=Jordi |last2=Fornaguera |first3=Maria I. |last3=Galan |title=Mineralization of aromatics in water by sunlight-assisted electro-Fenton technology in a pilot reactor |journal=Environmental Science and Technology |volume=39 |issue=6 |pages=1843–1847 |date=January 2005 |pmid= 15819245|doi= 10.1021/es0498787|bibcode=2005EnST...39.1843C}}

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.

{{NumBlk|:| C₆H₆ + FeSO₄ + H₂O₂ → C₆H₅OH + (byproducts)|{{EquationRef|3}}}}

An example hydroxylation reaction involves the oxidation of barbituric acid to alloxane.{{cite journal |last1=Brömme |first1=H. J. |last2=Mörke |first2=W. |last3=Peschke |first3=E. |title=Transformation of barbituric acid into alloxan by hydroxyl radicals: interaction with melatonin and with other hydroxyl radical scavengers |journal=Journal of Pineal Research |volume=33 |issue=4 |pages=239–247 |date=November 2002 |pmid=12390507 |doi= 10.1034/j.1600-079X.2002.02936.x|s2cid=30242100 }} 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.{{OrgSynth | collvol = 5 | collvolpages = 1026 | prep = cv5p1026 | title = α,α,α′,α′-Tetramethyltetramethylene glycol | first= E. L. |last=Jenner | year = 1973}} Fenton's reagent is also widely used in the field of environmental science for water purification and soil remediation. Various hazardous wastewater were reported to be effectively degraded through Fenton's reagent.{{cite journal |last1=Cai |first1=Q. Q. |last2=Lee |first2=B. C. Y. |last3=Ong |first3=S. L. |last4=Hu |first4=J. Y. |title=Fluidized-bed Fenton technologies for recalcitrant industrial wastewater treatment–Recent advances, challenges and perspective |journal=Water Research |date=15 February 2021 |volume=190 |pages=116692 |doi=10.1016/j.watres.2020.116692 |pmid=33279748 |bibcode=2021WatRe.19016692C |s2cid=227523802 }}

pH affects the reaction rate due to a variety of reasons. At a low pH, complexation of {{chem2|Fe(2+)}} also occurs, leading to lower availability of {{chem2|Fe(2+)}} to form reactive oxidative species (OH^•).{{cite journal |last1=Xu |first1=Xiang-Rong |last2=Li |first2=Xiao-Yan |last3=Li |first3=Xiang-Zhong |last4=Li |first4=Hua-Bin |title=Degradation of melatonin by UV, UV/H₂O₂, Fe²⁺/H₂O₂ and UV/Fe²⁺/H₂O₂ processes |journal=Separation and Purification Technology |date=5 August 2009 |volume=68 |issue=2 |pages=261–266 |doi=10.1016/j.seppur.2009.05.013 }} Lower pH also results in the scavenging of ^•OH by excess {{chem2|H+}},{{cite journal |last1=Tang |first1=W. Z. |last2=Huang |first2=C. P. |title=2,4-Dichlorophenol Oxidation Kinetics by Fenton's Reagent |journal=Environmental Technology |date=1 December 1996 |volume=17 |issue=12 |pages=1371–1378 |doi=10.1080/09593330.1996.9618465 |bibcode=1996EnvTe..17.1371T }} hence reducing its reaction rate. Whereas at high pH, the reaction slows down due to precipitation of Fe(OH)₃, lowering the concentration of the {{chem2|Fe(3+)}} species in solution. Solubility of iron species is directly governed by the solution's pH. {{chem2|Fe(3+)}} is about 100 times less soluble than {{chem2|Fe(2+)}} in natural water at near-neutral pH, the ferric ion concentration is the limiting factor for the reaction rate. Under high pH conditions, the stability of the H₂O₂ is also affected, resulting in its self-decomposition.{{cite journal |last1=Szpyrkowicz |first1=Lidia |last2=Juzzolino |first2=Claudia |last3=Kaul |first3=Santosh N |title=A Comparative study on oxidation of disperse dyes by electrochemical process, ozone, hypochlorite and fenton reagent |journal=Water Research |date=1 June 2001 |volume=35 |issue=9 |pages=2129–2136 |doi=10.1016/s0043-1354(00)00487-5 |pmid=11358291 |bibcode=2001WatRe..35.2129S }} Higher pH also decreased the redox potential of ^•OH thereby reducing its effectiveness.{{cite journal |last1=Velichkova |first1=Filipa |last2=Delmas |first2=Henri |last3=Julcour |first3=Carine |last4=Koumanova |first4=Bogdana |title=Heterogeneous fenton and photo-fenton oxidation for paracetamol removal using iron containing ZSM-5 zeolite as catalyst |journal=AIChE Journal |date=2017 |volume=63 |issue=2 |pages=669–679 |doi=10.1002/aic.15369 |bibcode=2017AIChE..63..669V |url=http://oatao.univ-toulouse.fr/20832/1/Velichkova_20832.pdf }} pH plays a crucial role in the formation of free radicals and hence the reaction performance. Thus ongoing research has been done to optimize pH and amongst other parameters for greater reaction rates.{{cite journal |last1=Cai |first1=Qinqing |last2=Lee |first2=Brandon Chuan Yee |last3=Ong |first3=Say Leong |last4=Hu |first4=Jiangyong |title=Application of a Multiobjective Artificial Neural Network (ANN) in Industrial Reverse Osmosis Concentrate Treatment with a Fluidized Bed Fenton Process: Performance Prediction and Process Optimization |journal=ACS ES&T Water |date=9 April 2021 |volume=1 |issue=4 |pages=847–858 |doi=10.1021/acsestwater.0c00192 |s2cid=234110033 }}

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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.{{cite journal |last1=Matavos-Aramyan |first1=S. |last2=Moussavi |first2=M. |last3=Matavos-Aramyan |first3=H. |last4=Roozkhosh |first4=S. |title=Cryptosporidium-contaminated water disinfection by a novel Fenton process |journal=Free Radical Biology and Medicine |volume=106 |date=2017 |pages=158–167 |doi=10.1016/j.freeradbiomed.2017.02.030 |pmid=28212822 |s2cid=3918519 }} 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. Superoxide ions and transition metals act in a synergistic way in the appearance of free radical damages.{{cite book|author=Robbins|author2=Cotran|title=Pathologic basis of disease|edition=7th|year=2008|publisher=Elsevier|isbn=978-0-8089-2302-2|page=16}} 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.{{cite journal |last1=Lapointe |first1=Marc |title=Iron supplementation in the intensive care unit: when, how much, and by what route? |journal=Critical Care |date=14 June 2004 |volume=8 |issue=2 |pages=S37-41 |doi=10.1186/cc2825 |pmid=15196322 |pmc=3226152 |doi-access=free }}

{{See also|Advanced oxidation process}}

Fenton's reagent is used as a sewage treatment agent.{{Cite journal |last1=Chen |first1=Yan-Jhang |last2=Fan |first2=Tang-Yu |last3=Wang |first3=Li-Pang |last4=Cheng |first4=Ta-Wui |last5=Chen |first5=Shiao-Shing |last6=Yuan |first6=Min-Hao |last7=Cheng |first7=Shikun |date=2020-02-18 |title=Application of Fenton Method for the Removal of Organic Matter in Sewage Sludge at Room Temperature |journal=Sustainability |volume=12 |issue=4 |pages=1518 |doi=10.3390/su12041518 |issn=2071-1050|doi-access=free |bibcode=2020Sust...12.1518C }}

Fenton's reagent can be used in different chemical processes that supply hydroxyl ion or oxidize certain compounds:{{Citation needed|date=May 2024}}

• The first stage of Fenton's reaction (oxidation of Fe³⁺ with hydrogen peroxide) is used in Haber–Weiss reaction
• Fenton's reagent can be used in organic synthesis reactions: e.g. hydroxylation of arenes via a free radical substitution
• Conversion of benzene into phenol by using Fenton's reagent
• Oxidation of barbituric acid into alloxan.
• Coupling reactions of alkanes

Mixtures of {{chem2|Fe(2+)}} and {{chem2|H2O2}} are called Fenton reagent. If {{chem2|Fe(2+)}} is replaced by {{chem2|Fe(3+)}}, it is called Fenton-like reagent.

Numerous transition metal ions and their complexes in their lower oxidation states (L_mMⁿ⁺) were found to have the oxidative features of the Fenton reagent, and, therefore, the mixtures of these metal compounds with {{chem2|H2O2}} were named "Fenton-like" reagents.{{cite journal|title=The Fenton reagents|journal=S Goldstein et al. Free Radic Biol Med. |date=October 1993|pmid=8225025 |last1=Goldstein |first1=S. |last2=Meyerstein |first2=D. |last3=Czapski |first3=G. |volume=15 |issue=4 |pages=435–445 |doi=10.1016/0891-5849(93)90043-t }}

• Carbon monoxide-releasing molecules

{{Reflist}}

• {{cite journal

| title = The Fenton reagents

|last1=Goldstein |first1=Sara |last2=Meyerstein |first2=Dan |last3=Czapski |first3=Gidon | journal = Free Radical Biology and Medicine

| volume = 15

| issue = 4

| pages = 435–445

| year = 1993

| doi = 10.1016/0891-5849(93)90043-T

| pmid = 8225025

}}

• {{cite journal|last=Barbusiński |first=K. |date=2009 |url=http://tchie.uni.opole.pl/freeECE/S_16_3/Barbusinski_16(3).pdf |title=Fenton Reaction – Controversy concerning the chemistry |journal=Ecological Chemistry and Engineering |volume=16 |number=3 |pages=347–358}}

• [http://www.h2o2.com/applications/industrialwastewater/fentonsreagent.html Reference Library Peroxide Applications]
• Companies that use Fenton's Reagent for chemical remediation: [http://www.orinrt.com ORIN]

{{DEFAULTSORT:Fenton's Reagent}}

Category:Analytical reagents

Category:Catalysis

Category:Environmental chemistry

Category:Free radical reactions

Category:Free radicals

Category:Iron compounds

Category:Name reactions

Category:Oxidizing agents

Category:Peroxides