The reaction of a halogen with an alkane in the presence of ultraviolet (UV) light or heat leads to the formation ofa haloalkane (alkyl halide).An example is the chlorination of methane.

Halogenation mechanism.In the methane molecule, the carbon‐hydrogen bonds are low‐polarity covalent bonds. The halogen molecule has a nonpolar covalent bond. UV light contains sufficient energy to break the weaker nonpolar chlorine‐chlorine bond (∼58 kcal/mole), but it has insufficient energy to break the stronger carbon‐hydrogen bond (104 kcal/mole). The fracture of the chlorine molecule leads to the formation of two highly reactive chlorine free radicals (chlorine atoms).A free radicalis an atom or group that has a single unshared electron.

The bond that is ruptured is said to have broken in ahomolyticfashion; that is, each of the originally bonded atoms receives one electron. This initial reaction is called theinitiation stepof the mechanism. The chlorine free radicals that form are in a high‐energy state and react quickly to complete their octets and liberate energy. Once the high‐energy chlorine free radicals are formed, the energy source (UV light or heat) can be removed. The energy liberated in the reaction of the free radicals with other atoms is sufficient to keep the reaction running.

When a chlorine free radical approaches a methane molecule, a homolytic fission of a carbon‐hydrogen bond occurs. The chlorine free radical combines with the liberated hydrogen free radical to form hydrogen chloride and a methyl free radical. This is called apropagation step, a step in which both a product and a reactive species, which keeps the reaction going, are formed.

The bond that is ruptured is said to have broken in ahomolyticfashion; that is, each of the originally bonded atoms receives one electron. This initial reaction is called theinitiation stepof the mechanism. The chlorine free radicals that form are in a high‐energy state and react quickly to complete their octets and liberate energy. Once the high‐energy chlorine free radicals are formed, the energy source (UV light or heat) can be removed. The energy liberated in the reaction of the free radicals with other atoms is sufficient to keep the reaction running.

When a chlorine free radical approaches a methane molecule, a homolytic fission of a carbon‐hydrogen bond occurs. The chlorine free radical combines with the liberated hydrogen free radical to form hydrogen chloride and a methyl free radical. This is called apropagation step, a step in which both a product and a reactive species, which keeps the reaction going, are formed.

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To summarize, thisfree‐radical chain reactioninitially contains few free radicals and many molecules of reactants. As the reaction proceeds, the number of free radicals increases, while the number of reactant molecules decreases. Near the end of the reaction, many more free radicals exist than reactant molecules. At this stage of the overall reaction, termination steps become the predominant reactions. All of the halogenation mechanism reactions occur very rapidly, and the formation of the products takes only microseconds.