Free-radical reaction

The chemical kinetics of a radical reaction depend on all these individual reactions. In steady state the concentrations of initiating (I^.) and terminating species T^. are negligent and rate of initiation and rate of termination are equal. The overall reaction rate can be written as:[4]

${\displaystyle \ {\text{rate}}=k_{\text{obs}}[I]^{3/2}\,}$

with a broken-order dependence of 1.5 with respect to the initiating species.

The reactivity of different compounds toward a certain radical is measured in so-called competition experiments. Compounds bearing carbon–hydrogen bonds react with radicals in the order primary < secondary < tertiary < benzyl < allyl reflecting the order in C–H bond dissociation energy[4]

Many stabilizing effects can be explained as resonance effects, an effect specific to radicals is the captodative effect.

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.[5] 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).[6][7][8]

• Radical clock