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18.2: Homolysis

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  • In chemistry , homolysis (from Greek ὅμοιος, homoios, “equal,” and λύσις, lusis, “loosening”) or homolytic fission is chemical bond dissociation of a molecule by a process where each of the fragments retains one of the originally bonded electrons. During homolytic fission of a neutral molecule with an even number of electrons, two free radicals will be generated. [1] That is, the two electrons involved in the original bond are distributed between the two fragment species. The energy involved in this process is called bond dissociation energy . Bond cleavage is also possible by a process called heterolysis .
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    Because the relatively high energy required to break bonds in this manner, homolysis only occurs under certain circumstances:

    • Ultraviolet radiation (e.g. sunlight causing sunburns)
    • Heat
      • Certain intramolecular bonds, such as the O–O bond of a peroxide, are weak enough to spontaneously homolytically dissociate with a small amount of heat.
      • High temperatures in the absence of oxygen (pyrolysis) can induce homolytic elimination of carbon compounds.[2]


    1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”) (1997). Online corrected version:  (2006–) “homolysis (homolytic)“.
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