Everything about Free Radical Addition totally explained
Free radical addition is an
addition reaction in
organic chemistry involving
free radicals .
The basic steps with examples of the free radical addition (also known as radical chain mechanism) are:
Free radical addition depends 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 doesn't 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 with
intramolecular ring closure and the radical species are inorganic rather than carbon based. This type of reaction is self-terminating because propagating isn't possible and the initiator is used in
stoichiometric amounts. .
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 isn't reactive enough for propagation and the
ketone is formed.
»
The radical species in effect is a single oxygen atom
synthon. Other inorganic radical that show this type of reactivity are
sulfate radical ions (from
ammonium persulfate) and
hydroxyl radicals.
Further Information
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