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Inorganic — radical generation (redox)

Fenton reaction: Fe²⁺ + H₂O₂

How the Fenton reaction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.

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In short. A 3-step mechanism. Typical reagents: Fe²⁺ (e.g. FeSO₄), H₂O₂. Conditions: Aqueous, pH ≈ 3, 25 °C. Bonds formed: Fe³⁺; HO⁻; HO•; bonds broken: O–O (σ, homolysis). Stereochemistry: None (non-selective radical). Rate and selectivity: k ≈ 63 M⁻¹ s⁻¹; Fe³⁺ + H₂O₂ regenerates Fe²⁺ slowly.

At a glance

ReagentsFe²⁺ (e.g. FeSO₄), H₂O₂
Solvent / conditionsAqueous, pH ≈ 3, 25 °C
Bonds formedFe³⁺; HO⁻; HO•
Bonds brokenO–O (σ, homolysis)
StereochemistryNone (non-selective radical)
Rate / selectivityk ≈ 63 M⁻¹ s⁻¹; Fe³⁺ + H₂O₂ regenerates Fe²⁺ slowly

Step 1: One-electron reduction of H₂O₂

+++Fe²⁺OHOHFe³⁺OHOH1 · ONE-ELECTRON REDUCTION OF H₂O₂iron(II)REDUCTANThydrogen peroxideH₂O₂OXIDANTiron(III)PRODUCThydroxideHO⁻BYPRODUCThydroxyl radicalHORADICALFe²⁺ gives one e⁻ into σ*(O–O): the O–O bond splits into HO⁻ and HO•.

Fe²⁺ transfers a single electron to H₂O₂, cleaving the weak O–O bond homolytically and generating a hydroxide ion plus a highly reactive hydroxyl radical (•OH) — one of the most oxidizing species in aqueous chemistry.

Step 2: Inner-sphere transfer

e⁻ transfer++Fe²⁺OHOH‡Fe³⁺OHOH2 · INNER-SPHERE TRANSFERFe²⁺···H₂O₂ complexFe²⁺·H₂O₂TRANSITION STATEFe³⁺PRODUCTHO⁻BYPRODUCTHO•RADICALH₂O₂ binds Fe²⁺ first; oxidation state +2 → +3 as the O–O bond breaks.

In the process, iron is oxidized from Fe²⁺ to Fe³⁺ — a single-electron inner-sphere transfer, with Fe²⁺ coordinating H₂O₂ directly before donating the electron, unleashing a radical chain.

Step 3: Hydroxyl radical attacks organics

++OHHδ⁺OH₂3 · HYDROXYL RADICAL ATTACKS ORGANICShydroxyl radicalHORADICALorganic substrate (R–H)C₂H₆SUBSTRATEwaterH₂OBYPRODUCTcarbon radical R•C₂H₅RADICALHO• abstracts H (BDE of O–H in H₂O ≈ 119 kcal/mol): unselective.

The hydroxyl radical abstracts hydrogen atoms or adds to organic substrates indiscriminately, initiating oxidative radical chemistry — the basis of Fenton's reagent's use in water treatment and oxidative organic degradation.

Abbreviations and terms

R
R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
M
Metal. A generic metal centre.
H2
Hydrogen gas. Reductant for catalytic hydrogenation.
H2O2
Hydrogen peroxide. Oxidant; its anion HOO⁻ is an excellent nucleophile.
σ*
Sigma antibonding orbital. The empty orbital an incoming nucleophile attacks in SN2 (the back lobe of C–LG).

How it compares with related reactions

Fenton reactionInner-sphere electron transferJacobsen-Katsuki epoxidation
ReagentsFe²⁺ (e.g. FeSO₄), H₂O₂Cr²⁺(aq) + [Co(NH₃)₅Cl]²⁺Chiral Mn(III)–salen (2–10 mol%), NaOCl (bleach) or mCPBA, alkene
ConditionsAqueous, pH ≈ 3, 25 °CAqueous acid, 25 °CCH₂Cl₂, 0 °C, optional pyridine N-oxide
StereochemistryNone (non-selective radical)Chloride transferred from Co to Cr (Taube)High ee for cis-alkenes; salen pocket selects the face
Rate / selectivityk ≈ 63 M⁻¹ s⁻¹; Fe³⁺ + H₂O₂ regenerates Fe²⁺ slowlyk ≈ 6×10⁵ M⁻¹s⁻¹; bridging ligand neededOxygen transfer to alkene is rate-limiting

Test yourself

Try to answer each question before opening it.

Which reagents and conditions are typical for the Fenton reaction?

Fe²⁺ (e.g. FeSO₄), H₂O₂. Conditions: Aqueous, pH ≈ 3, 25 °C.

Which bonds are formed and which are broken?

Formed: Fe³⁺; HO⁻; HO•. Broken: O–O (σ, homolysis).

What is the stereochemical outcome?

None (non-selective radical)

What controls the rate or the selectivity?

k ≈ 63 M⁻¹ s⁻¹; Fe³⁺ + H₂O₂ regenerates Fe²⁺ slowly

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