In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
, free-radical addition is an
addition reaction
In organic chemistry, an addition reaction is, in simplest terms, an organic reaction where two or more molecules combine to form a larger one (the adduct)..
Addition reactions are limited to chemical compounds that have multiple bonds, such as ...
which involves
free radical
A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing.
Ageing
Ailments of unknown cause
Biogerontology
Biological processes
Causes of death
Cellular processes
Gerontology
Life extension
Metabo ...
s. 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
Mechanism may refer to:
* Mechanism (engineering), rigid bodies connected by joints in order to accomplish a desired force and/or motion transmission
*Mechanism (biology), explaining how a feature is created
*Mechanism (philosophy), a theory that ...
) are:
*
Initiation by a
radical initiator
In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions. These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical i ...
: A radical is created from a non-radical precursor.
*
Chain propagation: A radical reacts with a non-radical to produce a new radical species
*
Chain termination
Chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.
Mechanisms of termination
In polymer chemistry, ...
: Two radicals react with each other to create a non-radical species
Free-radical reactions depend on a reagent having a (relatively) weak
bond
Bond or bonds may refer to:
Common meanings
* Bond (finance), a type of debt security
* Bail bond, a commercial third-party guarantor of surety bonds in the United States
* Chemical bond, the attraction of atoms, ions or molecules to form chemica ...
, 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 Markovnikov (russian: Марковников) is a Russian masculine surname, which originates from ''морковь'' (''carrot''); its feminine counterpart is Markovnikova. It may refer to
* Vladimir Markovnikov (1837–1904), Russian chemist
* Ni ...
reaction of
hydrogen bromide
Hydrogen bromide is the inorganic compound with the formula . It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room tempe ...
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
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
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
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...
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
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
amounts.
[''Self-Terminating, Oxidative Radical Cyclizations'' Tim Dreessen, Christian Jargstorff, Lars Lietzau, Christian Plath, Arne Stademann and Uta Wille Molecules 2004, 9, 480–49]
Online article
/ref>
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
Vinyl may refer to:
Chemistry
* Polyvinyl chloride (PVC), a particular vinyl polymer
* Vinyl cation, a type of carbocation
* Vinyl group, a broad class of organic molecules in chemistry
* Vinyl polymer, a group of polymers derived from vinyl ...
radical; then, via 1,5- hydrogen atom transfer (HAT) and 5-exo-trig ring-closure, a ketyl
A ketyl group in organic chemistry is an anion radical that contains a group R2C−O•. It is the product of the 1-electron reduction of a ketone.
Another mesomeric structure has the radical position on carbon and the negative charge on oxyge ...
radical. The produced ketyl dislodges a nitrite radical which is not reactive enough for propagation and thus, the ketone is formed.
:
The radical species in effect is a single oxygen atom synthon
In retrosynthetic analysis, a synthon is a hypothetical unit within a target molecule that represents a potential starting reagent in the retroactive synthesis of that target molecule. The term was coined in 1967 by E. J. Corey. He noted in 1 ...
. Other inorganic radicals that show this type of reactivity are sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
radical ions (from ammonium persulfate
Ammonium persulfate (APS) is the inorganic compound with the formula (NH4)2S2O8. It is a colourless (white) salt that is highly soluble in water, much more so than the related potassium salt. It is a strong oxidizing agent that is used as a catalys ...
) and hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
radicals.
See also
* The other radical reactions: radical substitution
In organic chemistry, a radical-substitution reaction is a substitution reaction involving free radicals as a reactive intermediate.March Jerry; (1985). Advanced organic chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wile ...
and radical polymerization
In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks ( repeat units). Free radicals can be formed by a number of different mechani ...
.
References
{{DEFAULTSORT:Free Radical Addition
Addition reactions
Free radical reactions
Reaction mechanisms