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, J.; ...

, a radical-substitution reaction is a

substitution reaction A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions ar ...

involving

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 as a

reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these com ...

.March Jerry; (1985). Advanced organic chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, inc. The reaction always involves at least two steps, and possibly a third. : FreeRadicalSubstitutionGeneralDescr (1)

In the first step called initiation (2,3), a

is created by homolysis. Homolysis can be brought about by heat or ultraviolet light, but also by

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 ...

s such as

organic peroxide In organic chemistry, organic peroxides are organic compounds containing the peroxide functional group (). If the R′ is hydrogen, the compounds are called hydroperoxides, which are discussed in that article. The O−O bond of peroxides easily b ...

s or azo compounds. UV Light is used to create two free radicals from one diatomic species. The final step is called termination (6,7), in which the radical recombines with another radical species. If the reaction is not terminated, but instead the radical group(s) go on to react further, the steps where new radicals are formed and then react are collectively known as propagation (4,5). This is because a new radical is created, able to participate in secondary reactions.

Radical substitution reactions

free radical halogenation In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chlorofo ...

reactions, radical substitution takes place with

halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...

reagents and

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...

substrates. Another important class of radical substitutions involve aryl radicals. One example is the hydroxylation of

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

Fenton's reagent Fenton's reagent is a solution of hydrogen peroxide (H2O2) with ferrous iron (typically iron(II) sulfate, FeSO4) as a catalyst that is used to oxidize contaminants or waste waters as part of an advanced oxidation process. Fenton's reagent can be ...

. Many oxidation and reduction reactions in organic chemistry have free radical intermediates, for example the oxidation of

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...

s to

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

s with

chromic acid The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixtu ...

Coupling reaction A coupling reaction in organic chemistry is a general term for a variety of reactions where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R ...

s can also be considered radical substitutions. Certain aromatic substitutions takes place by

radical-nucleophilic aromatic substitution Radical-nucleophilic aromatic substitution or SRN1 in organic chemistry is a type of substitution reaction in which a certain substituent on an aromatic compound is replaced by a nucleophile through an intermediary free radical species: The subst ...

. Auto-oxidation is a process responsible for deterioration of paints and food, as well as production of certain lab hazards such as

diethyl ether peroxide Diethyl ether hydroperoxide is the organic compound with the formula C2H5OCH(OOH)CH3. It is a colorless, distillable liquid. Diethyl ether hydroperoxide and its condensation products are blamed for the explosive organic peroxides that slowly form ...

. More radical substitutions are listed below: * The Barton–McCombie deoxygenation involves substitution of a

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 hydroxy g ...

group for a proton. * The Wohl–Ziegler reaction involves allylic bromination of alkenes. * The

Hunsdiecker reaction The Hunsdiecker reaction (also called the Borodin reaction or the Hunsdiecker–Borodin reaction) is a name reaction in organic chemistry whereby silver salts of carboxylic acids react with a halogen to produce an organic halide. It is an ex ...

converts silver salts of carboxylic acids to alkyl halides. * The Dowd–Beckwith reaction involves ring expansion of cyclic β-keto esters. * The Barton reaction involves synthesis of nitrosoalcohols from nitrites. * The Minisci reaction involves generation of an alkyl radical from a carboxylic acid and a silver salt, and subsequent substitution at an aromatic compound

References

{{DEFAULTSORT:Radical Substitution Free radical reactions Reaction mechanisms