The hydroxyl radical is widely considered the most reactive and damaging of the common reactive oxygen species found in skin. It forms primarily through the Fenton reaction, in which hydrogen peroxide reacts with free transition metal ions, or through the breakdown of other reactive species under UV exposure. Because of its extremely high reactivity and short half-life, measured in nanoseconds, the hydroxyl radical reacts almost instantly with whatever molecule happens to be nearby, including lipids in cell membranes, proteins, and DNA, often initiating chain reactions like lipid peroxidation. Unlike hydrogen peroxide, there is no dedicated enzyme in the body that neutralizes hydroxyl radicals directly, since it reacts too quickly for enzymatic systems to intercept it. The body’s main strategy against hydroxyl radical damage is preventive, meaning it focuses on controlling the upstream molecules (like excess hydrogen peroxide and free iron) that generate it in the first place, along with relying on small molecule antioxidants such as vitamin C and glutathione that can react directly with hydroxyl radicals when they do form.
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| Names | |||
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| IUPAC name
Hydroxyl radical
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Systematic IUPAC name
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Other names
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| Identifiers | |||
3D model (JSmol)
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| ChEBI | |||
| ChemSpider | |||
| 105 | |||
| KEGG | |||
PubChem CID
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CompTox Dashboard (EPA)
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| Properties | |||
| HO | |||
| Molar mass | 17.007 g·mol−1 | ||
| Acidity (pKa) | 11.8 to 11.9 | ||
| Thermochemistry | |||
Std molar
entropy (S⦵298) |
183.71 J K−1 mol−1 | ||
Std enthalpy of
formation (ΔfH⦵298) |
38.99 kJ mol−1 | ||
| Related compounds | |||
Related compounds
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O2H+ OH− O22− | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The hydroxyl radical, denoted as •OH or HO•, is the neutral form of the hydroxide ion (OH–). As a free radical, it is highly reactive and consequently short-lived, making it a pivotal species in radical chemistry.
In nature, hydroxyl radicals are most notably produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. They are also significant in radiation chemistry, where their formation can lead to hydrogen peroxide and oxygen, which in turn can accelerate corrosion and stress corrosion cracking in environments such as nuclear reactor coolant systems. Other important formation pathways include the UV-light dissociation of hydrogen peroxide (H2O2) and the Fenton reaction, where trace amounts of reduced transition metals catalyze the breakdown of peroxide.
In organic synthesis, hydroxyl radicals are most commonly generated by photolysis of 1-Hydroxy-2(1H)-pyridinethione.
The hydroxyl radical is often referred to as the "detergent" of the troposphere because it reacts with many pollutants, often acting as the first step to their removal. It also has an important role in eliminating some greenhouse gases like methane and ozone. The rate of reaction with the hydroxyl radical often determines how long many pollutants last in the atmosphere, if they do not undergo photolysis or are rained out. For instance, methane, which reacts relatively slowly with hydroxyl radicals, has an average lifetime of >5 years and many CFCs have lifetimes of 50+ years. Pollutants, such as larger hydrocarbons, can have very short average lifetimes of less than a few hours.
The first reaction with many volatile organic compounds (VOCs) is the removal of a hydrogen atom, forming water and an alkyl radical (R•):
The alkyl radical will typically react rapidly with oxygen forming a peroxy radical:
The fate of this radical in the troposphere is dependent on factors such as the amount of sunlight, pollution in the atmosphere and the nature of the alkyl radical that formed it (see chapters 12 & 13 in External Links "University Lecture notes on Atmospheric chemistry").
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