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Free-radical addition

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Title: Free-radical addition  
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Subject: Thiol-ene reaction, Free radical reactions, Magnussen model, Addition reactions, Free-radical reaction
Collection: Addition Reactions, Free Radical Reactions, Reaction Mechanisms
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Free-radical addition

Free-radical addition is an 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 1
  • Self-terminating oxidative radical cyclizations 2
  • See also 3
  • References 4

Addition of mineral acid to an alkene

To illustrate, consider the alkoxy radical-catalyzed, anti-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, stoichiometric amounts.[2]

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

Self-terminating radical cyclization

The radical species in effect is a single oxygen atom sulfate radical ions (from ammonium persulfate) and hydroxyl radicals.

See also


  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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