Hydroxyl_radical

Hydroxyl radical

Hydroxyl in chemistry describes a molecule consisting of an oxygen atom and a hydrogen atom joined by a covalent bond. The neutral form is known as a hydroxyl radical and the singly-charged hydroxyl anion is called hydroxide. When its oxygen atom is covalently bonded within a larger molecule, the hydroxyl group either is, or otherwise is part of, a functional group (HO– or –OH) .

Hydroxyl group

The term hydroxyl group is used to describe the functional group –OH when it is a substituent in an organic compound. Organic molecules containing a hydroxyl group, such that the hydroxyl has a negative inductive effect, are known as alcohols (the simplest of which have the formula C_nH_2n+1–OH).

Hydroxyl radical

The hydroxyl radical, OH, is the neutral form of the hydroxide ion. Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydro-peroxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H₂O₂ (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

In organic synthesis hydroxyl radicals are most commonly generated by photolysis of 1-Hydroxy-2(1H)-pyridinethione.

Atmospheric importance

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. The first reaction with many volatile organic compounds (VOCs) is the removal of a hydrogen atom forming water and an alkyl radical (R·).

OH + RH → H₂O + R·

The alkyl radical will typically react rapidly with oxygen forming a peroxy radical.

R· + O₂ → RO₂

The fate of this radical in the troposphere is dependent on factors such as the amount of sunlight (light from the sun), pollution in the atmosphere and the nature of the alkyl radical that formed it.

Biological significance

The hydroxyl radical has a very short in vivo half-life of approx. 10^-9 s and a high reactivity. This makes it a very dangerous compound to the organism. Unlike superoxide, which can be detoxified by superoxide dismutase, the hydroxyl radical cannot be eliminated by an enzymatic reaction, as this would require its diffusion to the enzyme's active site. As diffusion is slower than the half-life of the molecule, it will react with any oxidizable compound in its vicinity. It can damage virtually all types of macromolecules: carbohydrates, nucleic acids (mutations), lipids (lipid peroxidation) and amino acids (e.g. conversion of Phe to m-Tyrosine and o-Tyrosine). The only means to protect important cellular structures is the use of antioxidants such as glutathione and of effective repair systems.