Radical substitution
Encyclopedia
In organic chemistry
, a radical substitution reaction is a substitution reaction
involving free radical
s as a reactive intermediate
.
The reaction always involves at least two steps, and possibly a third.
In the first step called initiation (2,3) a free radical is created by homolysis
. Homolysis can be brought about by heat or light but also by radical initiator
s such as organic peroxide
s or azo compound
s. 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 is collectively known as propagation (4,5) because a new radical is created available for secondary reactions.
reactions radical substitution takes place with halogen
reagents and alkane
substrates. Another important class of radical substitutions involve aryl radical
s. One example is the hydroxylation of benzene
by Fenton's reagent
. Many oxidation and reduction reactions in organic chemistry have free radical intermediate
s, for example the oxidation of aldehyde
s to carboxylic acid
s with chromic acid
. Coupling reaction
s can also be considered radical substitutions. Certain aromatic substitutions takes place by radical-nucleophilic aromatic substitution
. Auto-oxidation is a process responsible for deterioration of paints and food and lab hazards such as diethyl ether peroxide
.
More radical substitutions are listed below:
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, a radical substitution reaction is a substitution reaction
Substitution reaction
In a substitution reaction, a functional group in a particular chemical compound is replaced by another group. In organic chemistry, the electrophilic and nucleophilic substitution reactions are of prime importance...
involving free radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
s as a reactive intermediate
Reactive intermediate
In chemistry a reactive 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 compounds be isolated and stored, e.g. low temperatures, matrix isolation...
.
The reaction always involves at least two steps, and possibly a third.
In the first step called initiation (2,3) a free radical is created by homolysis
Homolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...
. Homolysis can be brought about by heat or light but also by radical initiator
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 initiators are utilized in industrial processes such...
s such as organic peroxide
Organic peroxide
Organic peroxides are organic compounds containing the peroxide functional group . If the R' is hydrogen, the compound is called an organic hydroperoxide. Peresters have general structure RCOOR. The O-O bond easily breaks and forms free radicals of the form RO·...
s or azo compound
Azo compound
Azo compounds are compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene , HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene." The more...
s. 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 is collectively known as propagation (4,5) because a new radical is created available for secondary reactions.
Radical substitution reactions
In free radical halogenationFree 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 heat or UV light. The reaction is used for the industrial synthesis of chloroform , dichloromethane , and hexachlorobutadiene...
reactions radical substitution takes place with halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
reagents and alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
substrates. Another important class of radical substitutions involve aryl radical
Aryl radical
An Aryl radical in organic chemistry is an reactive intermediate and an arene compound incorporating one free radical carbon atom as part of the ring structure. As such it is the radical counterpart of the Arenium ion. The parent compound is the phenyl radical C6H5....
s. One example is the hydroxylation of benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
by Fenton's reagent
Fenton's reagent
Fenton's reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants or waste waters. Fenton's reagent can be used to destroy organic compounds such as trichloroethylene and tetrachloroethylene ....
. Many oxidation and reduction reactions in organic chemistry have free radical intermediate
Reactive intermediate
In chemistry a reactive 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 compounds be isolated and stored, e.g. low temperatures, matrix isolation...
s, for example the oxidation of aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s to carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s with chromic acid
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 mixture for glass. Chromic acid may also refer to the...
. Coupling reaction
Coupling reaction
A coupling reaction in organic chemistry is a catch-all term for a variety of reactions where two hydrocarbon fragments are coupled with the aid of a metal catalyst...
s can also be considered radical substitutions. Certain aromatic substitutions takes place by radical-nucleophilic aromatic substitution
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:...
. Auto-oxidation is a process responsible for deterioration of paints and food and lab hazards such as diethyl ether peroxide
Diethyl ether peroxide
Diethyl ether peroxides are a class of organic peroxides that slowly form in diethyl ether upon storage under air, light, or in the presence of metal by autoxidation.- Diethyl ether hydroperoxide :...
.
More radical substitutions are listed below:
- The Barton-McCombie deoxygenationBarton-McCombie deoxygenationThe Barton–McCombie deoxygenation is an organic reaction in which an hydroxy functional group in an organic compound is replaced by a hydride to give an alkyl group . It is named for the British chemists Sir Derek Harold Richard Barton and Stuart W. McCombie.This deoxygenation reaction is a...
is a way to substitute a hydroxylHydroxylA hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
group for a proton. - The Wohl-Ziegler reactionWohl-Ziegler reactionThe Wohl-Ziegler reactionis a chemical reaction that involves the allylic or benzylic bromination of hydrocarbons using an N-bromoimide and a radical initiator.Best yields are achieved with N-bromosuccinimide in carbon tetrachloride solvent...
involves the allylic bromination of alkenes. - The Hunsdiecker reactionHunsdiecker reactionThe Hunsdiecker reaction is the organic reaction of silver salts of carboxylic acids with halogens to give organic halides. It is an example of a halogenation reaction...
converts silver salts of carboxylic acids to alkyl halides. - The Dowd-Beckwith reactionDowd-Beckwith ring expansion reactionThe Dowd–Beckwith ring expansion reaction is an organic reaction in which a cyclic β-keto ester is expanded by up to 4 carbons in a free radical ring expansion reaction through an α-alkylhalo substituent . The radical initiator system is based on AIBN and tributyltin hydride. The cyclic β-keto...
involves ring expansion of cyclic β-keto esters. - The Barton reactionBarton reactionThe Barton Reaction involves the photolysis of a nitrite to form a δ-nitroso alcohol. It is named for the British chemist Sir Derek Harold Richard Barton...
involves synthesis of nitrosoalcohols from nitrites. - The Minisci reactionMinisci reactionThe Minisci-Reaction is a named reaction in organic chemistry. It is a radical substitution to an aromatic compound, in particular to a heteroaromatic base, that introduces an alkyl group. The reaction was published about in 1971 by F. Minisci.. The aromatic compound is generally electron-deficient...
involves generation of an alkyl radical from a carboxylic acid and a silver salt and subsequent substitution at an aromatic compound