Fenton's reagent is a solution of

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

(H₂O₂) and an iron catalyst (typically

iron(II) sulfate Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of sulfate) denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7), but several values for ...

, FeSO₄). It is used to

oxidize Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

contaminants Contamination is the presence of a constituent, impurity, or some other undesirable element that renders something unsuitable, unfit or harmful for the physical body, natural environment, workplace, etc. Types of contamination Within the scie ...

waste water Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...

as part of an advanced oxidation process. Fenton's reagent can be used to destroy

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s such as

trichloroethylene Trichloroethylene (TCE) is an organochloride with the formula C2HCl3, commonly used as an industrial metal-degreasing solvent. It is a clear, colourless, non-flammable, volatile liquid with a chloroform-like pleasant mild smell and sweet taste.

and

tetrachloroethylene Tetrachloroethylene, also known as perchloroethylene or under the systematic name tetrachloroethene, and abbreviations such as perc (or PERC), and PCE, is a chlorocarbon with the formula . It is a non-flammable, stable, colorless and heavy liqu ...

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

Reactions

Iron(II) In chemistry, iron(II) refers to the element iron in its +2 oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II) chloride (). The adjective ''ferr ...

is oxidized by hydrogen peroxide to

iron(III) In chemistry, iron(III) or ''ferric'' refers to the element iron in its +3 oxidation state. ''Ferric chloride'' is an alternative name for iron(III) chloride (). The adjective ''ferrous'' is used instead for iron(II) salts, containing the catio ...

, forming a

hydroxyl radical The hydroxyl radical, •HO, is the neutral form of the hydroxide ion (HO–). Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are pr ...

and a

hydroxide ion Hydroxide is a polyatomic ion, diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually Self-ionization ...

in the process. Iron(III) is then reduced back to iron(II) by another molecule of hydrogen peroxide, forming a

hydroperoxyl The hydroperoxyl radical, also known as the hydrogen superoxide, is the protonated form of superoxide with the chemical formula HO2, also written HOO•. This species plays an important role in the atmosphere and as a reactive oxygen species ...

radical and a

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

. The net effect is a

disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation state. The reverse of disproportionatio ...

of hydrogen peroxide to create two different oxygen-radical species, with water (H⁺ + OH⁻) as a byproduct. The

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...

s generated by this process 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 In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...

and results in the oxidation of contaminants to primarily carbon dioxide and water. Reaction () was suggested by

Haber Haber is a surname of German origin. The meaning in old German is "oat". The cereal is now in German called . Notable people with this surname include: * Alan Haber, American student activist * Alessandro Haber, Italian actor, director, and singer ...

and Weiss in the 1930s as part of what would become the Haber–Weiss reaction.

Iron(II) sulfate Iron(II) sulfate or ferrous sulfate (British English: sulphate instead of sulfate) denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7), but several values for ...

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 Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typica ...

of oxygen. Fenton's reagent is also used in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

for the

hydroxylation In chemistry, hydroxylation refers to the installation of a hydroxyl group () into an organic compound. Hydroxylations generate alcohols and phenols, which are very common functional groups. Hydroxylation confers some degree of water-solubility ...

arene Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

s in a radical substitution reaction such as the classical conversion of

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

into

phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...

. An example hydroxylation reaction involves the

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

barbituric acid Barbituric acid or malonylurea or 6-hydroxyuracil is an organic compound based on a pyrimidine heterocyclic skeleton. It is an odorless powder soluble in water. Barbituric acid is the parent compound of barbiturate drugs, although barbituric acid i ...

to alloxane. Another application of the reagent in organic synthesis is in

coupling reaction In organic chemistry, a coupling reaction is a type of reaction in which two reactant molecules are bonded together. Such reactions often require the aid of a metal catalyst. In one important reaction type, a main group organometallic compound o ...

s of alkanes. As an example ''tert''-butanol is dimerized with Fenton's reagent and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

to 2,5-dimethyl-2,5-hexanediol. Fenton's reagent is also widely used in the field of environmental science for

water purification Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for hu ...

and soil remediation. Various hazardous wastewater were reported to be effectively degraded through Fenton's reagent.

Effect of pH on formation of free radicals

pH affects the reaction rate due to a variety of reasons. At a low pH, complexation of also occurs, leading to lower availability of to form reactive oxidative species (OH^•). Lower pH also results in the scavenging of ^•OH by excess , 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 species in solution.

Solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

of iron species is directly governed by the solution's pH. is about 100 times less soluble than 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. Higher pH also decreased the

redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

of ^•OH thereby reducing its effectiveness. 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. :

Biomedical implications

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 Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...

'' conditions. Transition-metal ions such as

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

and

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

can donate or accept free electrons via intracellular reactions and so contribute to the formation, or at the contrary to the scavenging, of

Superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

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.

Applications

Fenton's reagent is used as a sewage treatment agent. Fenton's reagent can be used in different chemical processes that supply

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

ion or oxidize certain compounds: * The first stage of Fenton's reaction (oxidation of Fe³⁺ with

) 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

into

by using Fenton's reagent * Oxidation of barbituric acid into alloxan. * Coupling reactions of alkanes

Fenton-like reagent

Mixtures of and are called Fenton reagent. If is replaced by , 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 were named "Fenton-like" reagents.

References

External links

Reference Library Peroxide Applications
* Companies that use Fenton's Reagent for chemical remediation
ORIN
{{DEFAULTSORT:Fenton's Reagent Analytical reagents Catalysis Environmental chemistry Free radical reactions Free radicals Iron compounds Name reactions Oxidizing agents Peroxides