A free-radical reaction is any

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

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. This reaction type is abundant in

organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, Mechanistic Organ ...

s. Two pioneering studies into free radical reactions have been the discovery of the

triphenylmethyl radical The triphenylmethyl radical (often shorted to trityl radical) is an organic compound with the formula (C6H5)3C. It is a persistent radical. It was the first radical ever to be described in organic chemistry. Because of its accessibility, the tri ...

Moses Gomberg Moses Gomberg (February 8, 1866 – February 12, 1947) was a chemistry professor at the University of Michigan. He was elected to the National Academy of Sciences and served as president of the American Chemical Society. Early life and education ...

(1900) and the lead-mirror experiment described by

Friedrich Paneth Friedrich Adolf Paneth (31 August 1887 – 17 September 1958) was an Austrian-born British chemist. Fleeing the Nazis, he escaped to Britain. He became a naturalized British citizen in 1939. After the war, Paneth returned to Germany to bec ...

in 1927. In this last experiment

tetramethyllead Tetramethyllead, also called tetra methyllead and lead tetramethyl, is a chemical compound used as an antiknock additive for gasoline. Its use is being phased out for environmental considerations. The National Institute for Occupational Safety a ...

is decomposed at elevated temperatures to methyl radicals and elemental lead in a

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

tube. The gaseous methyl radicals are moved to another part of the chamber in a carrier gas where they react with lead in a mirror film which slowly disappears. When radical reactions are part of

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

the radicals are often generated from

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 ini ...

s such as peroxides or azobis compounds. Many radical reactions are chain reactions with a

chain initiation Chain-growth polymerization ( AE) or chain-growth polymerisation ( BE) is a polymerization technique where unsaturated monomer molecules add onto the active site on a growing polymer chain one at a time. There are a limited number of these active ...

step, a

chain propagation Chain propagation (sometimes referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a chemical chain reaction. For example, in the chlorination of methane, there is a two-step p ...

step and a

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, ...

step.

Reaction inhibitor A reaction inhibitor is a substance that decreases the rate of, or prevents, a chemical reaction. A catalyst, in contrast, is a substance that increases the rate of a chemical reaction. Examples * Added acetanilide slows the decomposition of drug ...

s slow down a radical reaction and

radical disproportionation Radical disproportionation encompasses a group of reactions in organic chemistry in which two radicals react to form two different non-radical products. Radicals in chemistry are defined as reactive atoms or molecules that contain an unpaired elect ...

is a competing reaction. Radical reactions occur frequently in the gas phase, are often initiated by light, are rarely acid or base catalyzed and are not dependent on polarity of the reaction medium. Reactions are also similar whether in the gas phase or solution phase.

Kinetics

The

chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in wh ...

of a radical reaction depend on all these individual reactions. In

steady state In systems theory, a system or a Process theory, process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those p ...

the concentrations of initiating (I^.) and terminating species T^. are negligent and rate of initiation and rate of termination are equal. The overall

reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...

can be written as:''Advanced Organic Chemistry'' F.A. Carey R.J. Sundberg :

\ \text = k_\,

with a broken-order dependence of 1.5 with respect to the initiating species. The reactivity of different compounds toward a certain radical is measured in so-called competition experiments. Compounds bearing

carbon–hydrogen bond In chemistry, the carbon-hydrogen bond ( bond) is a chemical bond between carbon and hydrogen atoms that can be found in many organic compounds. This bond is a covalent, single bond, meaning that carbon shares its outer valence electrons with up ...

s react with radicals in the order

primary Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Works * ...

secondary Secondary may refer to: Science and nature * Secondary emission, of particles ** Secondary electrons, electrons generated as ionization products * The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...

tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

benzyl In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure . Benzyl features a benzene ring () attached to a methylene group () group. Nomenclature In IUPAC nomenclature, the prefix benzyl refers to a substi ...

allyl In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, ...

reflecting the order in C–H

bond dissociation energy The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical s ...

Many stabilizing effects can be explained as

resonance effect In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

s, an effect specific to radicals is the captodative effect.

Reactions

The most important reaction types involving free radicals are: *

Free-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 Wiley ...

, for instance

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 chloroform ( ...

and

autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...

. *

Free-radical addition In organic chemistry, free-radical addition is an addition reaction which involves free radicals. 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 (al ...

reactions * Intramolecular free radical reactions (substitution or addition) such as the

Hofmann–Löffler reaction The Hofmann–Löffler reaction (also referred to as Hofmann–Löffler–Freytag reaction, Löffler–Freytag reaction, Löffler–Hofmann reaction, as well as Löffler's method) is an organic reaction in which a cyclic amine 2 (pyrrolidine or, i ...

or the

Barton reaction The Barton reaction, also known as the Barton nitrite ester reaction, is a photochemical reaction that involves the photolysis of an alkyl nitrite to form a δ-nitroso alcohol. Discovered in 1960, the reaction is named for its discoverer, Nobel Lau ...

* Free radical

rearrangement reaction In organic chemistry, a rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another ...

s are rare compared to rearrangements involving carbocations and restricted to

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...

migrations. * Fragmentation reactions or homolysis, for instance the

Norrish reaction A Norrish reaction in organic chemistry is a photochemical reaction taking place with ketones and aldehydes. Such reactions are subdivided into Norrish type I reactions and Norrish type II reactions. The reaction is named after Ronald George Wreyfo ...

, 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 exam ...

and certain

decarboxylation Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is t ...

s. For fragmentations taking place in mass spectrometry see

mass spectrum analysis Mass spectral interpretation is the method employed to identify the chemical formula, characteristic fragment patterns and possible fragment ions from the mass spectra. Mass spectra is a plot of relative abundance against mass-to-charge ratio. It i ...

. *

Electron transfer Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons. Electrochemical processes ar ...

. An example is the decomposition of certain peresters by Cu(I) which is a

one-electron reduction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

reaction forming Cu(II), an

alkoxy In chemistry, the alkoxy group is an alkyl group which is singularly bonded to oxygen; thus . The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the organic compound ethyl phenyl ether (, also kn ...

oxygen radical and a

carboxylate In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...

. Another example is

Kolbe electrolysis __NOTOC__ The Kolbe electrolysis or Kolbe reaction is an organic reaction named after Hermann Kolbe. The Kolbe reaction is formally a decarboxylative dimerisation of two carboxylic acids (or carboxylate ions). The overall reaction is: : If a ...

. *

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 ...

is a special case of

nucleophilic aromatic substitution A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. Aromatic rings are usually nucleophilic, but some aromatic compou ...

. * Carbon–carbon coupling reactions, for example

manganese-mediated coupling reactions Manganese-mediated coupling reactions are radical coupling reactions between enolizable carbonyl compounds and unsaturated compounds initiated by a manganese(III) salt, typically manganese(III) acetate. Copper(II) acetate is sometimes used as a co-o ...

. *

Elimination reactions Elimination may refer to: Science and medicine *Elimination reaction, an organic reaction in which two functional groups split to form an organic product *Bodily waste elimination, discharging feces, urine, or foreign substances from the body ...

Free radicals can be formed by photochemical reaction and thermal fission reaction or by oxidation reduction reaction. Specific reactions involving free radicals are

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...

pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

and cracking. Free radical reactions also occur within and outside of cells, are injurious, and have been implicated in a wide range of human diseases (see 13-Hydroxyoctadecadienoic acid,

9-hydroxyoctadecadienoic acid 9-Hydroxyoctadecadienoic acid (or 9-HODE) has been used in the literature to designate either or both of two stereoisomer metabolites of the essential fatty acid, linoleic acid: 9(''S'')-hydroxy-10(''E''),12(''Z'')-octadecadienoic acid (9(''S'')- ...

reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

, and

Oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily Detoxification, detoxify the reactive intermediates or to repair the resulting damage. Disturbances ...

) as well as many of the maladies associated with ageing (see

ageing Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In ...

).Biochim Biophys Acta. 2014 Feb;1840(2):809–17. doi: 10.1016/j.bbagen.2013.03.020

References

{{Reflist Organic chemistry

Kinetics

Reactions

See also

References