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In chemistry free radicals are atomic or molecular species with unpaired electrons or an otherwise open shell configuration. These unpaired electrons are highly reactive[?], so free radicals are likely to take part in strong chemical reactions. Free radicals play an important role in combustion, atmospheric chemistry, polymerization and many other chemical processes.
In written chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows:
Reactions involving free radicals are usually divided into three categories: initiation, propagation, and termination.
The formation of radicals requires covalent bonds to be broken homolytically, a process that requires significant amounts of energy. For example, splitting H2 into 2H· has a ΔH° of +435 kJ/mol, and Cl2 into 2Cl· has a ΔH° of +243 kJ/mol. This is known as the homolytic bond dissociation energy[?], and is usually abbreviated as the symbol DH°. The bond energy between two covalently bonded atoms is affected by the structure of the molecule as a whole, not just the identity of the two atoms, and radicals requiring more energy to form are less stable than those requiring less energy.
Probably the most familiar free-radical reaction for most people is combustion. In order for combustion to occur the relatively strong O=O double bond must be broken to form oxygen free radicals. The flammability of a given material is strongly dependent on the concentration of free radicals that must be obtained before initiation and propagation reactions dominate leading to combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out.
In addition to combustion, many polymerization reactions involve free radicals. As a result many plastics, enamels, and other polymers are formed through free-radical reactions.
In the upper atmosphere free radicals are produced through dissociation of the source molecules, particularly the normally unreactive chlorofluorocarbons by solar ultraviolet radiation or by reactions with other stratospheric constituents. These free radicals then react with ozone in a catalytic chain reaction which destroys the ozone, but regenerates the free radical, allowing it to participate in additional reactions. Such reactions are believed to be the primary cause of depletion of the ozone layer and are the reason why the use of chlorofluorocarbons as refridgerants has been restricted.
A widely-used technique for studying free radicals, and other paramagnetic species, is electron spin resonance[?] spectroscopy (ESR). This is alternately referred to as "electron paramagnetic resonance[?]" (EPR) spectroscopy. It is conceptually related to nuclear magnetic resonance, though electrons resonate with higher-frequency fields at a given fixed magnetic field than do most nuclei.
Free radicals play an important role in a number of biological processes, some of which are necessary for life. However, because of their reactivity, these same free radicals can participate in unwanted side reactions resulting in cell damage. Some forms of cancer are the result of reactions between free radicals and DNA, resulting in cancerous cell mutations. Some of the symptoms of ageing such as atherosclerosis are also attributed to free-radical induced oxidation of many of the chemicals making up the body. In addition free radicals contribute to alcohol-induced liver damage, perhaps more than alcohol itself. Radicals in cigarette smoke have been implicated in inactivation of an antiprotease in the lung, which leads to the development of emphysema.
Because free radicals are necessary for life, the body has a number of mechanisms to minimize free radical induced damage and to repair damage which does occur. Antioxidants play a key role in these defense mechanisms.