Hydroxyl radical

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.