Free-radical addition

In organic chemistry, free-radical addition is an addition reaction which involves free radicals.[1] The addition may occur between a radical and a non-radical, or between two radicals.

The basic steps with examples of the free-radical addition (also known as radical chain mechanism) are:

Free-radical reactions depend on a reagent having a (relatively) weak bond, allowing it to homolyse to form radicals (often with heat or light). Reagents without such a weak bond would likely proceed via a different mechanism. An example of an addition reaction involving aryl radicals is the Meerwein arylation.

Addition of mineral acid to an alkene

To illustrate, consider the alkoxy radical-catalyzed, anti-Markovnikov reaction of hydrogen bromide to an alkene. In this reaction, a catalytic amount of organic peroxide is needed to abstract the acidic proton from HBr and generate the bromine radical, however a full molar equivalent of alkene and acid is required for completion.

Note that the radical will be on the more substituted carbon. Free-radical addition does not occur with the molecules HCl or HI. Both reactions are extremely endothermic and are not chemically favored.

Self-terminating oxidative radical cyclizations

In one specific type of radical addition called self-terminating oxidative radical cyclization, alkynes are oxidized to ketones through intramolecular radical cyclization where the radical species are inorganic rather than carbon based. This type of reaction is self-terminating because propagating is not possible and the initiator is used in stoichiometric amounts.[2]

As an example, a nitrate radical is generated by photolysis of ammonium cerium(IV) nitrate (CAN) which reacts with an alkyne to generate; first, a very reactive vinyl radical; then, via 1,5-hydrogen atom transfer (HAT) and 5-exo-trig ring-closure, a ketyl radical. The produced ketyl dislodges a nitrite radical which is not reactive enough for propagation and thus, the ketone is formed.

A reaction scheme showing a self terminating radical cyclization

The radical species in effect is a single oxygen atom synthon. Other inorganic radicals that show this type of reactivity are sulfate radical ions (from ammonium persulfate) and hydroxyl radicals.

See also

References
  1. L.G. Wade's Organic Chemistry 5th Ed. (p 319) – Mechanism supplements original.
  2. Self-Terminating, Oxidative Radical Cyclizations Tim Dreessen, Christian Jargstorff, Lars Lietzau, Christian Plath, Arne Stademann and Uta Wille Molecules 2004, 9, 480–497 Online article
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