Free-radical Reaction
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A free-radical reaction is any
chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
involving
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...
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 shortened to trityl radical after 1927 suggestion by Burckhardt Helferich, Helferich et al.) is an organic compound with the formula (C6H5)3C. It is a persistent radical. It was the first radical (chemistry), rad ...
by Moses Gomberg (1900) and the lead-mirror experiment described by Friedrich Paneth 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. It is a methyl radical synthon. Its use in gasoline is being phased out for environmental considerations. Th ...
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 Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
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 branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
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 in ...
s such as peroxides or azobis compounds. Many radical reactions are chain reactions with a
chain initiation Chain-growth polymerization (American English, AE) or chain-growth polymerisation (British English, BE) is a polymerization technique where monomer molecules add onto the active site on a growing polymer chain one at a time. There are a limited n ...
step, a
chain propagation In chemistry, chain propagation (sometimes just referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a Chain reaction#Chemical chain reactions, chemical chain reaction. For exa ...
step and a
chain termination In polymer chemistry, 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 ...
step.
Reaction inhibitor A reaction inhibitor is a substance that decreases the rate of, or prevents, a chemical reaction. A catalyst or an Enzyme activator, in contrast, is a substance that increases the rate of a chemical reaction. Examples * Added acetanilide slows the ...
s slow down a radical reaction and
radical disproportionation Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...
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 different from chemical thermodynamics, which deals with the direction in which a ...
of a radical reaction depend on all these individual reactions. In
steady state In systems theory, a system or a 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 properties ''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 u ...
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 bond, covalent, single bond, meaning that carbon shares its outer valence el ...
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 Work ...
< secondary <
tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
<
benzyl In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure . Benzyl features a benzene ring () attached to a methylene group (). Nomenclature In IUPAC nomenclature, the prefix benzyl refers to a substituent ...
<
allyl In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . In 1844, Theodor Wertheim isolated a ...
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 ...
Many stabilizing effects can be explained as
resonance effect In chemistry, resonance, also called mesomerism, is a way of describing Chemical bond, bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance struc ...
s, an effect specific to radicals is the captodative effect.


Reactions

The most important reaction types involving free radicals are: * Free-radical substitution, 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. These reactions can happen due to the free radicals having an unpaired electron in their valence shell, making them highly reactive. Radical addit ...
reactions * Intramolecular free radical reactions (substitution or addition) such as the Hofmann–Löffler reaction or the Barton reaction * 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 at ...
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 ...
migrations. * Fragmentation reactions or homolysis, for instance the Norrish reaction, 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 ...
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 ...
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, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions. Electrochemical processes are ET reactio ...
. An example is the decomposition of certain peresters by Cu(I) which is a
one-electron reduction Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
reaction forming Cu(II), an
alkoxy In chemistry, the alkoxy group is an alkyl group which is Single bond, singularly bonded to oxygen; thus . Denoted usually with apostrophe('). The range of alkoxy groups is vast, the simplest being methoxy (). An ethoxy group () is found in the ...
oxygen radical and a
carboxylate In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an anion, an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,... ...
. 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 is a special case of
nucleophilic aromatic substitution A nucleophilic aromatic substitution (SNAr) 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 c ...
. * Carbon–carbon coupling reactions, for example manganese-mediated coupling reactions. *
Elimination reactions An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one- or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 r ...
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. Combustion ...
,
pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...
and
cracking Cracking may refer to: * Cracking, the formation of a fracture or partial fracture in a solid material studied as fracture mechanics ** Performing a sternotomy * Fluid catalytic cracking, a catalytic process widely used in oil refineries for crac ...
. 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 13-Hydroxyoctadecadienoic acid (13-HODE) is the commonly used term for 13(''S'')-hydroxy-9''Z'',11''E''-octadecadienoic acid (13(''S'')-HODE). The production of 13(''S'')-HODE is often accompanied by the production of its stereoisomer, 13(''R'') ...
,
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 and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
, and
Oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...
) as well as many of the maladies associated with ageing (see
ageing Ageing (or aging in American English) is the process of becoming older until death. The term refers mainly to humans, many other animals, and fungi; whereas for example, bacteria, perennial plants and some simple animals are potentially biol ...
).Biochim Biophys Acta. 2014 Feb;1840(2):809–17. doi: 10.1016/j.bbagen.2013.03.020


See also

*
Radical clock In chemistry, a radical clock is a chemical compound that assists in the indirect methodology to determine the kinetics (chemistry), kinetics of a free-radical reaction. The radical-clock compound itself reacts at a known rate, which provides a cali ...


References

{{Reflist Organic reactions