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Title: Hydroperoxyl  
Author: World Heritage Encyclopedia
Language: English
Subject: Superoxide, Hydrogen peroxide, Lipid peroxidation, Tropospheric ozone, Free radicals
Collection: Free Radicals, Hydrogen Compounds, Oxides, Superoxides
Publisher: World Heritage Encyclopedia


Ball-and-stick model of the hydroperoxyl radical
Preferred IUPAC name
Systematic IUPAC name
Other names
Peroxyl radical
ChemSpider  Y
Jmol-3D images Image
Molar mass 33.01 g·mol−1
Acidity (pKa) 4.88[1]
Basicity (pKb) 9.12 (basicity of superoxide ion)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

The hydroperoxyl radical, also known as the perhydroxyl radical, is the protonated form of superoxide with the chemical formula HO2.


  • Formation 1
  • Reactivity 2
  • Effect on environment 3
  • References 4


Hydroperoxyl is formed through the transfer of a hydrogen atom to molecular oxygen, an oxygen atom to a hydroxyl radical or a proton to a superoxide anion.[2]


The superoxide anion, O2, and the hydroperoxyl radical are in equilibrium in aqueous solution:

O2 + H2O HO2 + OH

The protonation/deprotonation equilibrium exhibits a pKa of 4.88;[3] consequently, about 0.3% of any superoxide present in the cytosol of a typical cell is in the protonated form.

Unlike O2, which predominantly acts as a reductant, HO2 can act as an oxidant in a number of biologically important reactions, such as the abstraction of hydrogen atoms from tocopherol and polyunstaturated fatty acids in the lipid bilayer. As such, it may be an important initiator of lipid peroxidation.

Because dielectric constant has a strong effect on pKa, and the dielectric constant of air is quite low, superoxide produced (photochemically) in the atmosphere is almost exclusively present as HO2. As HO2 is quite reactive, it acts as a "cleanser" of the atmosphere by degrading certain organic pollutants. As such, the chemistry of HO2 is of considerable geochemical importance.

Effect on environment

Hydroperoxyl is responsible for the destruction of ozone in the stratosphere, and it is formed as a result of the oxidation of hydrocarbons in the troposphere.[2]


  1. ^ Radicals in Aqueous Solution2/O2Reactivity of HO. J Phys Chem Ref Data, 1985. 14(4): p. 1041-1091
  2. ^ a b "Hydroperoxyl radical". Glossary of Meteorology. American Meteorological Society. 25 April 2012. Retrieved 22 August 2013. 
  3. ^ Radicals in Aqueous Solution2/O2Reactivity of HO. J Phys Chem Ref Data, 1985. 14(4): p. 1041-1091
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