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Triphenylmethyl radical
Chemical compound From Wikipedia, the free encyclopedia
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The triphenylmethyl radical (often shortened to trityl radical after 1927 suggestion by Helferich et al.[1]) 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 trityl radical has been heavily exploited.[2]
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Preparation and properties
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The triphenylmethyl radical can be prepared by homolysis of triphenylmethyl chloride 1 by a metal like silver or zinc in benzene or diethyl ether. The radical 2 forms a chemical equilibrium with the quinoid-type dimer 3 (Gomberg's dimer). In benzene the concentration of the radical is 2%.[3]
Solutions containing the radical are yellow; 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.
The triphenylmethyl radical exhibits green photoluminescence. Further reaction of the quinoid dimer with another triphenylmethyl radical produces a quinoid radical that exhibits red photoluminescence.[4]
When exposed to air, the radical rapidly oxidizes to the peroxide, and the color of the solution changes from yellow to colorless. Likewise, the radical reacts with iodine to triphenylmethyl iodide.
While the triphenyl radical itself forms a quinoid dimer, alkyl substitution at the para ring positions inhibits dimer formation entirely.[5] Other derivatives of the triphenyl radical with certain substituted 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 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.[6][7] Other derivatives have been reported as the quinoid dimer [8]
The class of triaryl-methyl radicals have applications in the synthesis of organic magnets.[9]
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History
The radical was discovered by Moses Gomberg in 1900 at the University of Michigan.[10][11][12] He tried to prepare hexaphenylethane from triphenylmethyl chloride and zinc in benzene in a Wurtz reaction 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.[13][14][15] The triphenylmethyl radical, and the larger class of triarylmethyl radicals, are called Gomberg radicals.[4][9]
The correct quinoid 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).[16] It subsequently took until 1968 for its rediscovery when researchers at the Vrije Universiteit Amsterdam published proton NMR data.[17]
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