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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:
This is derived from Lewis dot notation.
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. 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 ozoneOzone (O) is a molecule consisting of three oxygen atoms. At standard temperature and pressure Ozone is a blue gas. Ozone forms a dark blue liquid, below -112 °C, and a dark blue solid, below -193 °C. Ozone is a powerful oxidizing agent, and is unstable, in a catalytic chain reactionA chain reaction is a sequence of reactions: A causes B, which causes C, etc. In particular, one of the agents necessary to the reaction may itself be produced by the reaction, thus causing additional reactions. Examples: The neutron- fission chain reacti 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 layerThe ozone layer is that part of the Earth's stratosphere which contains ozone. The total quantity of ozone in the ozone layer is not very large; if just the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters 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 nitroxide s, (general formula R2NO·).
A widely-used technique for studying free radicals, and other paramagnetic species, is electron spin resonanceElectron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) is a spectroscopic technique which detects species that have unpaired electrons, generally meaning that it must be a free radical, if it is an organic molecule, or that it has transiti spectroscopySpectroscopy is the study of spectra, ie. the dependence of a physical measure to frequency. Spectroscopy is often used in physical and analytical chemistry for the identification of substances, through the spectrum emitted or absorbed. A device for recor (ESR). This is alternately referred to as " electron paramagnetic resonance " (EPR) spectroscopy. It is conceptually related to nuclear magnetic resonanceNuclear magnetic resonance NMR is a physical phenomenon involving the interaction of atomic nuclei placed in an external magnetic field with an applied electromagnetic field oscillating at a particular frequency. Magnetic conditions within the material ar, though electrons resonate with higher-frequency fields at a given fixed magnetic field than do most nuclei.
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