Radical (chemistry)
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In chemistry, Radicals (often referred to as free radicals) are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions. Radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes, including human physiology. For example, superoxide and nitric oxide regulate many biological process, such as controlling vascular tone. "Radical" and "Free Radical" are frequently used interchangeably, however a radical may be trapped within a solvent cage or be otherwise bound. Historically, "Radical" was used to refer to a collection of atoms that remain unchanged over the course of a reaction, however this usage is, today, uncommon. The first organic free radical (the triphenylmethyl radical) was identified by Moses Gomberg in 1900.
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Depicting radicals in chemical reactions
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:
- Cl2 + hν → 2 Cl·
Radical reaction mechanisms use single-headed arrows to depict the movement of single electrons:
The homolytic cleavage of the breaking bond is drawn with a 'fish-hook' arrow to contrast the usual movement of two electrons depicted by a standard curly arrow. It should be noted that the second electron of the breaking bond moves also to pair up with the attacking radical electron; this is not explicitly indicated in this case.
The chemistry of radicals
Terminology
In chemistry free radicals take part in radical addition and radical substitution as reactive intermediates. 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: 2Cl·→ Cl2
Formation
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. Homolytic bond cleavage most often happens between two atoms of similar electronegativity. In organic chemistry this is often the O-O bond in peroxide species or O-N bonds.
However, propagation is a very exothermic reaction. Note that all species are electrically neutral although radical ions do exist.
Persistence and stability
Image:VitE.gif Long lived radicals can be placed into two categories
- Stable Radicals
- Radicals can be long lived if they occur in a conjugated π system, such as the radical derived from α-tocopherol (vitamin E)
- Persistent Radicals
- Persistent radical compounds are those whose longevity is due to steric crowding around the radical center and makes it physically difficult for the radical to react with another molecule. Examples of these include Gomberg's radical (triphenylmethyl), Fremy's salt (Potassium nitrosodisulfonate, (KSO3)2NO·)and nitroxides, (general formula R2NO·) such as TEMPO. The longest-lived free radical is melanin, which may persist for millions of years.
- diradicals are molecules containing two radical centers. Multiple radical centers can exist in a molecule.
Combustion
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.
Tetraethyl lead was added to gasoline, because it very easily breaks up into radicals, which consume other free radicals in the gasoline-air mixture. This prevents the combustion from initiating.
Polymerization
In addition to combustion, many polymerization reactions involve free radicals. As a result many plastics, enamels, and other polymers are formed through radical polymerization.
Recent advances in radical polymerization methods known as Living Radical Polymerization such as:
- Reversible Addition-Fragmentation chain Transfer (RAFT)
- Atom Transfer Radical Polymerization (ATRP)
- Nitroxide Mediated Polymerization (NMP)
These methods produce polymers with a much narrower distribution of molecular weights.
Atmospheric radicals
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 refrigerants has been restricted.
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.
Free radicals may also be involved in Parkinson's disease, senile and drug-induced deafness, schizophrenia, and Alzheimer's. The classic free-radical syndrome, the iron-storage disease hemochromatosis, is typically-associated with a constellation of free-radical-related symptoms including movement disorder, psychosis, skin pigmentary melanin abnormalities, deafness, arthritis, and diabetes. The free radical theory of aging proposes that free radicals underly the aging process itself.
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 and polyphenol antioxidants. 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.
An overview of the role of free radicals in biology and of the use of electron spin resonance in their detection may be found in a recent book: *Rhodes C.J.: Toxicology of the Human Environment - the critical role of free radicals, Taylor and Francis, London (2000).
Diagnostics
Radical diagnostic techniques include:
- Electron Spin Resonance
- 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.
- Nuclear magnetic resonance using a phenomenon called CIDNP
- Chemical Labelling
- Chemical labelling by quenching with free radicals, e.g. with NO or DPPH, followed by spectroscopic methods like X-ray photoelectron spectroscopy (XPS) or absorption spectroscopy, respectively.
See also
External links
- Free Radicals, Types, Sources and Damaging Reactions
- Free Radicals and Human Disease
- Electron-transfer Factors in Psychosis and Dyskinesia--early review articlear:جذر حر
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