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In chemistry, free radicals are uncharged 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 chemical reactions. Free radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry , biochemistry, 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:
- H2 + hν → 2 H·
This is derived from Lewis dot notation.
Reactions involving free radicals are usually divided into three categories: initiation, propagation, and termination.
- Initiation reactions are those which result in a net increase in the number of free radicals. They may involve the formation of free radicals from stable species as in Reaction 1 above or they may involve reactions of free radicals with stable species to form more free radicals.
- Propagation reactions are those reactions involving free radicals in which the total number of free radicals remains the same.
- Termination reactions are those reactions resulting in a net decrease in the number of free radicals. Typically two free radicals combine to form a more stable species, for example: 2H· → H2
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. This shows that free radicals are difficult to form.
However, the extreme reactivity of the radical comes from their very low activation energy; when a free radical reacts with something, it will produce another free radical, e.g., H· + 1/2O2 → OH· (hydroxyl). Note that all species are electrically neutral; 1 electron to 1 proton for hydrogen, 6 electrons to 6 protons for oxygen, and 7 electrons for 7 protons for the hydroxyl radical. Very little force is required to get the atoms close enough to react → low activation energy → extremely fast reaction rate and kinetics. The hydroxyl radical is free to undergo another reaction, and the lone electron may be swapped around many times before it joins with another radical in termination (see polymerization).
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. It is noteworthy that oxygen is actually a diradical with two unpaired electrons in the outer orbitals. Reactivity is limited because these electrons have parallel spins. However, this barrier is overcome by enzymes in the body (respiration) and by energy (heat). 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 this is why the use of chlorofluorocarbons as refridgerants has been restricted.
Relatively stable, persistent free radical compounds include Fremys salt (Potassium nitrosodisulfonate, (KSO3)2NO·)and nitroxides , (general formula R2NO·).
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 in Biology
Free radicals play an important role in a number of biological processes, some of which are necessary for life, such as the intracellular killing of bacteria by neutrophil granulocytes. Free radicals have also been implicated in certain cell signalling processes. The two most important oxygen-centered free radicals are superoxide and hydroxyl radical. They are derived from molecular oxygen under reducing conditions. However, because of their reactivity, these same free radicals can participate in unwanted side reactions resulting in cell damage. Many forms of cancer are thought to be the result of reactions between free radicals and DNA, resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy. Some of the symptoms of aging 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 alpha 1-antitrypsin in the lung. This process promotes 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, such as the enzymes superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase . In addition, antioxidants play a key role in these defense mechanisms. These are often the three vitamins, vitamin A, vitamin C and vitamin E. Further, there is good evidence bilirubin and uric acid can act as antioxidants to help neutralize certain free radicals. Bilirubin comes from the breakdown of red blood cells' contents, while uric acid is a breakdown product of purines. Too much bilirubin, though, can lead to jaundice which could eventually damage the central nervous system, while too much uric acid causes gout.
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