What Is the Hydroxyl Radical?

Water is an electrically neutral molecule that can be formed through the combining of hydrogen ions and hydroxide ions, as in acid-base reactions. Written in chemists' shorthand, the reaction is H⁺ & OH^- → H₂O. Here, the lone hydrogen atom is actually an ion, carrying no negative electrons, and so bearing a positive charge, while the hydrogen-oxygen combine possesses two free electrons, giving it a negative charge. It is possible to produce another form of the hydrogen-oxygen combine, which carries only one free electron, not two. This is the electrically-neutral, yet chemically very reactive hydroxyl radical.

Oxygen is strongly electronegative, whereas hydrogen is electropositive. In addition, the hydrogen atom is quite small in comparison with the oxygen atom. It is the "electron-loving" oxygen atom, rather than the hydrogen atom, that carries the free electron. A population of free radicals decreases in number with time through attrition, as, for example, when two hydroxyl radicals unite to form one molecule of hydrogen peroxide. Conversely, the readily cleaved peroxides — especially organic peroxides with large appendages that tend to stretch those bonds, such as di-tert-butyl peroxide or benzoyl peroxide — are used as sources of free radicals in laboratory and commercial syntheses.

The hydroxyl radical is important, not only in the laboratory and in commerce, but in medicine, and in our atmosphere as well. Troposphere nitrogen oxide pollutants from motor vehicles and factories decompose to release excited atoms of oxygen, •O. These individual atoms, not to be confused with oxygen molecules, O₂, combine with moisture in the air, converting into hydroxyl radicals, •OH. There are other sources of the hydroxyl radical, more commonly seen in other situations, including the reaction of ozone with doubly-bonded organic compounds called alkenes. In most instances occurring in nature, hydroxyl radicals are not considered desirable; such is the case in the field of health and medicine.

This is because the hydroxyl radical is a small and highly mobile, water-soluble form of free radical. In the human body free radicals are usually considered undesirable, and may be associated with illness and with aging. One enzymatic system of concern is the respiratory system, in which free radicals attack delicate body tissues. Atmospheric particulates — notably transition-metal-ions — have been recognized as hydroxyl radical formers in moist environments such as lung tissue, and lead to pulmonary disorders, including cancer. The hydroxyl radical has also been associated with chemical attack upon strands of DNA.