Fenton's reagent is a solution of hydrogen peroxide (H2O2) and an iron catalyst (typically iron(II) sulfate, FeSO4).
Reactions
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
hydrogen atom. The net effect is a disproportionation of hydrogen peroxide to create two different oxygen-radical species, with water (H
+ + OH
−) as a byproduct.
The generated by this process engage in secondary reactions. For example, the hydroxyl is a powerful, non-selective oxidant.
Reaction () was suggested by Fritz Haber and Weiss in the 1930s as part of what would become the Haber–Weiss reaction.
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. 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.
Fenton's reagent is also used in organic synthesis for the hydroxylation of in a radical substitution reaction such as the classical conversion of benzene into phenol.
An example hydroxylation reaction involves the oxidation of barbituric acid to alloxane.
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
Free radical (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.
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 H
2O
2 is also affected, resulting in its self-decomposition.
Higher pH also decreased the redox potential 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.
>| +Impacts of operation pH on reaction rate
! rowspan="2" | Low pH | Formation of Fe(H2O)62+ complex, hence reducing Fe2+ for radical generation |
| Scavenging of •OH by excess H+ |
|
| Self-decomposition of H2O2 due to decreased stability at high pH |
| Precipitation of Fe(OH)3 species in solution |
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 conditions.
Applications
Fenton's reagent is used as a sewage treatment agent.
Fenton's reagent can be used in different chemical processes that supply Hydroxy group ion or oxidize certain compounds:
-
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
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 (LmMn+) 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.
See also
-
Carbon monoxide-releasing molecules
Further reading
External links
