Radical ion
Encyclopedia
A radical ion is a free radical
species that carries a charge. Radical ion
s are encountered in organic chemistry
as reactive intermediates and in mass spectrometry
as gas phase ions. Positive radical ions are called radical cations whereas negative radical ions are called radical anions.
and 
. In mass spectrometry, the sign is written first, followed by the superscripted dot: 
and 
.
by alkali metal
s. For example the reaction of naphthalene
with sodium
in an aprotic solvent yields the naphthalene radical anion - sodium ion salt. In a ESR spectrum this compound shows up as a quintet
of quintets (25 lines). In the presence of a proton
source the radical anion is protonated and effectively hydrogenated like in the Birch reduction
.
The electron is transferred from the alkali metal ion to an unoccupied antibonding p-p п* orbital of the aromatic molecule. This transfer is usually only energetically favorable if the aprotic solvent efficiently solvates the alkali metal ion. Effectiveness for this is in the order diethyl ether
< THF < 1,2-dimethoxyethane
< HMPA. In principle any unsaturated molecule can form a radical anion, but the antibonding orbitals are only energetically accessible in more extensive conjugated systems. Ease of formation is in the order benzene
< naphthalene
< anthracene
< pyrene
, etc. On addition of a proton source, the structure of the resulting hydrogenated molecule is defined by the charge distribution of the radical anion. For instance, the anthracene radical anion forms mainly (but not exclusively) 9,10-dihydroanthracene.
An example of a non-carbon radical anion is the superoxide
anion, formed by transfer of one electron to an oxygen
molecule.
A very effective way to remove any traces of water from THF
is by reflux
with sodium
wire in the presence of a small amount of benzophenone
. Benzophenone is reduced to the ketyl
radical anion by sodium which gives the THF solution an intense blue color. However, any trace of water in THF will further reduce the ketyl to the colourless alcohol
. In this way, the color of the THF signals the dryness and the distilled THF contains less than 10 ppm of water. This treatment also effectively removes any peroxides in the THF. Radical anions of this type are also involved in the Acyloin condensation
.
Cyclooctatetraene
is reduced by elemental potassium
all the way to the dianion because the 10 electron system is aromatic. Quinone
is reduced to a semiquinone
radical anion. Semidiones are derived from the reduction of dicarbonyl compounds.
one electron is abstracted by an electron in the electron beam to create a radical cation M+.. This species represents the molecular ion or parent ion and will tell the precise molecular weight. On a typical mass spectrum
more signals show up because the molecular ion fragments into a complex mixture of ions and uncharged radical species. For example the methanol
radical cation fragments into a methyl cation CH3+ and a hydroxyl
radical. In naphthalene
the unfragmented radical cation is by far the most prominent peak in the mass spectrum. Secondary species are generated from proton
gain (M+1) and proton loss (M-1).
Polaron
s and bipolaron
s are radical cations encountered in doped conducting polymers.
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...
species that carries a charge. Radical ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s are encountered in organic chemistry
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...
as reactive intermediates and in mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
as gas phase ions. Positive radical ions are called radical cations whereas negative radical ions are called radical anions.
Notation
In organic chemistry, a radical ion is typically indicated by a superscript dot followed by the sign of the charge: and . In mass spectrometry, the sign is written first, followed by the superscripted dot: and .Radical anions
Many aromatic compounds can undergo one-electron reductionOne-electron reduction
A one-electron reduction in organic chemistry involves the transfer of an electron from a metal to an organic substrate. It serves to differentiate between true organic reductions and other reductions such as hydride transfer reactions that actually involve two-electron species.The first...
by alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
s. For example the reaction of naphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
with sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
in an aprotic solvent yields the naphthalene radical anion - sodium ion salt. In a ESR spectrum this compound shows up as a quintet
Quintet
A quintet is a group containing five members.It is commonly associated with musical groups, such as a string quintet, or a group of five singers, but can be applied to any situation where five similar or related objects are considered a single unit....
of quintets (25 lines). In the presence of a proton
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
source the radical anion is protonated and effectively hydrogenated like in the Birch reduction
Birch reduction
The Birch Reduction is an organic reaction which is particularly useful in synthetic organic chemistry. The reaction was reported in 1944 by the Australian chemist Arthur Birch working in the Dyson Perrins Laboratory in the University of Oxford, building on earlier work by Wooster and Godfrey in...
.
The electron is transferred from the alkali metal ion to an unoccupied antibonding p-p п* orbital of the aromatic molecule. This transfer is usually only energetically favorable if the aprotic solvent efficiently solvates the alkali metal ion. Effectiveness for this is in the order diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
< THF < 1,2-dimethoxyethane
Dimethoxyethane
Dimethoxyethane, also known as glyme, monoglyme, dimethyl glycol, ethylene glycol dimethyl ether, dimethyl cellosolve, and DME, is a clear, colorless, aprotic, and liquid ether that is used as a solvent. Dimethoxyethane is miscible with water.Dimethoxyethane is often used as a higher boiling...
< HMPA. In principle any unsaturated molecule can form a radical anion, but the antibonding orbitals are only energetically accessible in more extensive conjugated systems. Ease of formation is in the order 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....
< naphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
< anthracene
Anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon consisting of three fused benzene rings. It is a component of coal-tar. Anthracene is used in the production of the red dye alizarin and other dyes...
< pyrene
Pyrene
Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This colourless solid is the smallest peri-fused PAH...
, etc. On addition of a proton source, the structure of the resulting hydrogenated molecule is defined by the charge distribution of the radical anion. For instance, the anthracene radical anion forms mainly (but not exclusively) 9,10-dihydroanthracene.
An example of a non-carbon radical anion is the superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
anion, formed by transfer of one electron to an oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
molecule.
A very effective way to remove any traces of water from THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
is by reflux
Reflux
Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations...
with sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
wire in the presence of a small amount of benzophenone
Benzophenone
Benzophenone is the organic compound with the formula 2CO, generally abbreviated Ph2CO. Benzophenone is a widely used building block in organic chemistry, being the parent diarylketone.-Uses:...
. Benzophenone is reduced to the ketyl
Ketyl
A ketyl group in organic chemistry is an anion radical with the general structure C-O. in which an oxygen radical is bonded directly to carbon. This radical is very unstable and appears in chemical reactions as a reactive intermediate...
radical anion by sodium which gives the THF solution an intense blue color. However, any trace of water in THF will further reduce the ketyl to the colourless alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
. In this way, the color of the THF signals the dryness and the distilled THF contains less than 10 ppm of water. This treatment also effectively removes any peroxides in the THF. Radical anions of this type are also involved in the Acyloin condensation
Acyloin condensation
Acyloin condensation is a reductive coupling of two carboxylic esters using metallic sodium to yield an α-hydroxyketone, also known as an acyloin....
.
Cyclooctatetraene
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
is reduced by elemental potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
all the way to the dianion because the 10 electron system is aromatic. Quinone
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...
is reduced to a semiquinone
Semiquinone
Semiquinone is a free radical resulting from the removal of one hydrogen atom with its electron during the process of dehydrogenation of a hydroquinone to quinone or alternatively the addition of a single H atom to a quinone....
radical anion. Semidiones are derived from the reduction of dicarbonyl compounds.
Radical cations
Cationic radical species do also exist but are much less stable. They appear prominently in mass spectrometry. When a gas-phase molecule is subjected to electron ionizationElectron ionization
Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions...
one electron is abstracted by an electron in the electron beam to create a radical cation M+.. This species represents the molecular ion or parent ion and will tell the precise molecular weight. On a typical mass spectrum
Mass spectrum
A mass spectrum is an intensity vs. m/z plot representing a chemical analysis. Hence, the mass spectrum of a sample is a pattern representing the distribution of ions by mass in a sample. It is a histogram usually acquired using an instrument called a mass spectrometer...
more signals show up because the molecular ion fragments into a complex mixture of ions and uncharged radical species. For example the methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
radical cation fragments into a methyl cation CH3+ and a hydroxyl
Hydroxyl
A 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...
radical. In naphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
the unfragmented radical cation is by far the most prominent peak in the mass spectrum. Secondary species are generated from proton
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
gain (M+1) and proton loss (M-1).
Polaron
Polaron
A polaron is a quasiparticle composed of a charge and its accompanying polarization field. A slow moving electron in a dielectric crystal, interacting with lattice ions through long-range forces will permanently be surrounded by a region of lattice polarization and deformation caused by the moving...
s and bipolaron
Bipolaron
- Bipolarons in physics :In physics, a bipolaron is a bound pair of two polarons. An electron in a material may cause a distortion in the underlying lattice. The combination of electron and distortion is known as a polaron...
s are radical cations encountered in doped conducting polymers.