The most important reaction types involving free radicals are:
* Free-radical substitution, for instance free-radical halogenation and autoxidation.
* Free-radical addition reactions
* Intramolecular free radical reactions (substitution or addition) such as the Hofmann–Löffler reaction or the Barton reaction
* Free radical rearrangement reactions are rare compared to rearrangements involving carbocations and restricted to aryl migrations.
* Fragmentation reactions or homolysis, for instance the Norrish reaction, the Hunsdiecker reaction and certain decarboxylations. For fragmentations taking place in mass spectrometry see mass spectrum analysis.
* Electron transfer. An example is the decomposition of certain peresters by Cu(I) which is a one-electron reduction reaction forming Cu(II), an alkoxy oxygen radical and a carboxylate. Another example is Kolbe electrolysis.
* Radical-nucleophilic aromatic substitution is a special case of nucleophilic aromatic substitution.
* Carbon–carbon coupling reactions, for example manganese-mediated coupling reactions.
* Elimination reactions
Free radicals can be formed by photochemical reaction and thermal fission reaction or by oxidation reduction reaction. Specific reactions involving free radicals are combustion, pyrolysis and cracking. Free radical reactions also occur within and outside of cells, are injurious, and have been implicated in a wide range of human diseases (see 13-Hydroxyoctadecadienoic acid, 9-hydroxyoctadecadienoic acid, reactive oxygen species, and Oxidative stress) as well as many of the maladies associated with ageing (see ageing).