The triphenylmethyl radical (often shortened to trityl radical after 1927 suggestion by Helferich et al.) is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the formula (C₆H₅)₃C. It is a persistent radical. It was the first

radical 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 ...

ever to be described in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

. Because of its accessibility, the trityl radical has been heavily exploited.

Preparation and properties

The triphenylmethyl radical can be prepared by homolysis of

triphenylmethyl chloride Triphenylmethyl chloride or trityl chloride (TrCl) is a white solid with the chemical formula C19H15Cl. It is an alkyl halide, sometimes used to introduce the trityl protecting group. Preparation Triphenylmethyl chloride is commercially available. ...

1 by a metal like

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

diethyl ether Diethyl ether, or simply ether, is an organic compound with the chemical formula , sometimes abbreviated as . It is a colourless, highly Volatility (chemistry), volatile, sweet-smelling ("ethereal odour"), extremely flammable liquid. It belongs ...

. The radical 2 forms a

chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...

with the

quinoid In organic chemistry, quinoids are a class of chemical compounds that are derived from quinone. Unlike benzenoid structures, the quinoid part is not aromatic. See also * Benzenoid * Aromatic compound Aromatic compounds or arenes are organi ...

-type dimer 3 (

Gomberg's dimer Gomberg's dimer is the organic compound with the formula Ph2C=C6H5-CPh3, where Ph = phenyl, C6H5. It is a yellow solid that is air-stable for hours at room temperature and soluble in organic solvents. The compound achieved fame as the Dimer (chemi ...

). In benzene the concentration of the radical is 2%. Solutions containing the radical are

yellow Yellow is the color between green and orange on the spectrum of light. It is evoked by light with a dominant wavelength of roughly 575585 nm. It is a primary color in subtractive color systems, used in painting or color printing. In t ...

; when the temperature of the solution is raised, the yellow color becomes more intense as the equilibrium is shifted in favor of the radical rather than the colorless dimer, in accordance with

Le Chatelier's principle In chemistry, Le Chatelier's principle (pronounced or ) is a principle used to predict the effect of a change in conditions on chemical equilibrium. Other names include Chatelier's principle, Braun–Le Chatelier principle, Le Chatelier–Braun p ...

. The triphenylmethyl radical exhibits green

photoluminescence Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. phot ...

. Further reaction of the quinoid dimer with another triphenylmethyl radical produces a quinoid radical that exhibits red photoluminescence. When exposed to air, the radical rapidly oxidizes to the

peroxide In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...

, and the color of the solution changes from yellow to colorless. Likewise, the radical reacts with

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

to triphenylmethyl iodide. While the triphenyl radical itself forms a quinoid dimer, alkyl substitution at the ''para'' ring positions inhibits dimer formation entirely. Other derivatives of the triphenyl radical with certain

substituted Substitution may refer to: Arts and media *Substitution (poetry), a variation in poetic scansion *Substitution (theatre), an acting methodology Music *Chord substitution, swapping one chord for a related one within a chord progression *Tritone ...

phenyl groups do form dimers with a hexaphenylethane-like structure. For example, the tris(3,5-di-''tert''-butylphenyl) radical dimerizes to give hexakis(3,5-di-''t''-butylphenyl)ethane, with a

bond length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...

of 1.67 Å for the central carbon–carbon bond. Theoretical calculations on a very high level of theory indicate that van der Waals attraction between the ''tert''-butyl groups create a potential minimum that is absent in the unsubstituted molecule. Other derivatives have been reported as the quinoid dimer The class of tri

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 ...

-methyl radicals have applications in the synthesis of organic

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

History

The radical was discovered by Moses Gomberg in 1900 at the

University of Michigan The University of Michigan (U-M, U of M, or Michigan) is a public university, public research university in Ann Arbor, Michigan, United States. Founded in 1817, it is the oldest institution of higher education in the state. The University of Mi ...

. He tried to prepare hexaphenylethane from triphenylmethyl chloride and zinc in benzene in a

Wurtz reaction In organic chemistry, the Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction in which two alkyl halides are treated with sodium metal to form a higher alkane. : 2 R−X + 2 Na → R−R + 2 NaX The reaction is of little val ...

and found that the product, based on its behaviour towards iodine and oxygen, was far more reactive than anticipated. The discovered structure was used in the development of ESR spectroscopy and confirmed by it. The triphenylmethyl radical, and the larger class of triarylmethyl radicals, are called ''Gomberg radicals''. The correct

structure for the dimer was suggested as early as 1904 but this structure was soon after abandoned by the scientific community in favor of hexaphenylethane (4). It subsequently took until 1968 for its rediscovery when researchers at the

Vrije Universiteit Amsterdam The (abbreviated as ''VU Amsterdam'' or simply ''VU'' when in context) is a public university, public research university in Amsterdam, Netherlands, founded in 1880. The VU Amsterdam is one of two large, publicly funded research universities in ...

published

proton NMR Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or 1H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the stru ...

data.

References

{{reflist, 2

External links

Molecule of the Month
June 1997

Free radicals Phenyl compounds