Stable organic and inorganic molecules feature covalent bonds formed by the sharing of two electrons: one from each atom. Heat or radiation, typically in the ultraviolet band, will cause such a bond to cleave with one electron being kept by each half of the broken molecule.
This type of bond breaking is called homolytic cleavage, homolytic fission or homolysis.
These free radicals are extremely short lived and are best described as chemical intermediates, as it is almost impossible to isolate them.
Such reactions typically occur in gaseous systems or in nonpolar solvents in which ions are not readily formed. Examples of nonpolar solvents include carbon tetrachloride and hexane.
Free radicals form a crucial part in the creation of the vast range of organic chemicals that are used in laboratories and industrial plants worldwide. The vast majority of the organic compounds that are used come from naturally occurring oil. Crude oil consists of a range of saturated hydrocarbons called alkanes. These alkanes are separated using a heat process called fractional distillation. The lightest compounds, such as propane, butane and octane, are used to fuel automobiles and airplanes, while the heavier compounds are used as bitumen and tar.
Alkanes are not used much as chemical "building blocks" because they only react with oxygen to form heat, water and carbon dioxide. Free radical processes can be used to generate far more useful chemicals.
Free radical chain reactions convert alkanes into more chemically useful haloalkanes. These reactions typically occur by heating a mixture of an alkane with a halogen, such as chlorine to form a gas. The chain reaction is initiated by subjecting the gas to ultraviolet light, which breaks up some of the diatomic chlorine molecules into chlorine radicals. These radicals will then react with the alkane to form a chloroalkane. This second stage, called propagation, will generate further radicals, but ultimately the process will terminate once all of the free radicals have paired up to form stable compounds. Haloalkanes are more reactive than alkanes and can be used to create other organic compounds.
Unfortunately, free radical reactions have had a serious effect on the Earth's ozone layer. This layer blocks out most of the sun's harmful ultraviolet radiation, but the abundance of this type of radiation in this layer has led to considerable damage, thanks to the presence of chlorofluorocarbons, or CFCs.
CFCs were once widely used as aerosol propellants and refrigerants. At sea level they are very stable and nonflammable, so appeared to be ideal for these roles. In the mid-1980s atmospheric scientists detected a depletion of the ozone layer over the south pole. A similar hole was soon discovered over the north pole. Further investigation found this depletion was because of the presence of CFCs, which, in the presence of ultraviolet light, break up to form free radicals. These radicals then destroy the nearby ozone. This discovery led to the phasing out of CFCs and, fortunately, the ozone layer appears to be slowly recovering.