Fenton reaction

Fe2+ + H2O2-->Fe3+ + OH· + OH−. This is the iron-salt-dependent decomposition of dihydrogen peroxide, generating the highly reactive hydroxyl radical, possibly via an oxoiron(IV) intermediate. Addition of a reducing agent, such as ascorbate, leads to a cycle which increases the damage to biological molecules.
See also: Haber–Weiss reaction
PAC, 1997, 69, 1251. 'Glossary of terms used in bioinorganic chemistry (IUPAC Recommendations 1997)' on page 1274 (https://doi.org/10.1351/pac199769061251)