- Triphenylmethyl radical
The triphenylmethyl radical is a
persistent radical and the first ever radical described inorganic chemistry . It can be prepared byhomolysis of triphenylmethylchloride 1 ("scheme 1") by a metal likesilver orzinc inbenzene ordiethyl ether . The radical 2 forms achemical equilibrium with thequinoid typedimer 3. In benzene the concentration of the radical is 2% ["Advanced Organic Chemistry" J. March, John Wiley & Sons ISBN 0-471-88841-9] .Solutions containing the radical are
yellow and when the temperature of the solution is increased the yellow color becomes more intense as the equilibrium is shifted in favor of the radical followingLe Chatelier's principle . Conversely when the solution is cooled it becomes less yellow.When exposed to air the radical rapidly oxidizes to the
peroxide ("Scheme 2") and the color of the solution changes from yellow to colorless. Likewise, the radical reacts withiodine to triphenylmethyliodide.The radical was discovered by
Moses Gomberg in 1900. [cite journal | title = An instance of trivalent carbon: triphenylmethyl | author = M. Gomberg | journal =J. Am. Chem. Soc. | year = 1900 | volume = 22 | issue = 11 | pages = 757–771 | doi = 10.1021/ja02049a006] [cite journal | title = On trivalent carbon | author = M. Gomberg | journal =J. Am. Chem. Soc. | year = 1901 | volume = 23 | issue = 7 | pages = 496–502 | doi = 10.1021/ja02033a015 (Note: radical is also called a "cadicle")] [cite journal | title = On trivalent carbon | author = M. Gomberg | journal =J. Am. Chem. Soc. | year = 1902 | volume = 24 | issue = 7 | pages = 597–628 | doi = 10.1021/ja02021a001] He tried to prepare hexaphenylethane from triphenylmethylchloride andzinc inbenzene in aWurtz reaction and found that the product, based on its behaviour towards iodine and oxygen, was far more reactive than anticipated.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 which is structure 4 in scheme 1 [cite journal | title = The hexaphenylethane riddle | author = J. M. McBride | journal = Tetrahedron | volume = 30 | issue = 14 | year = 1974 | pages = 2009–2022 | doi = 10.1016/S0040-4020(01)97332-6] . It subsequently took until 1968 for its rediscovery when researchers at the
Vrije Universiteit Amsterdam publishedproton NMR data [cite journal | title = A new interpretation of the monomer-dimer equilibrium of triphenylmethyl- and alkylsubstituted-diphenyl methyl-radicals in solution | author = H. Lankamp, W. Th. Nauta and C. MacLean | journal =Tetrahedron Letters | volume = 9 | issue = 2 |year = 1968 | pages = 249–254 | doi = 10.1016/S0040-4039(00)75598-5] . In hindsight the substituted ethane molecule does not make sense at all because it is simply too sterically overcrowded.Miscellany
* Gomberg concluded his 1900 article with the sentence "This work will be continued and I wish to reserve the field for myself." He ended his 1901 article by writing, "It is my intention to extend this study to other oxygen compounds, as well as to nitrogen derivatives, and I beg to reserve this field for further work." It is true that nineteenth-century chemists did not intrude on each other's research; to his dismay, Gomberg found out that this was not the case in the twentieth century.
External links
*
Molecule of the Month June 1997 [http://www.chm.bris.ac.uk/motm/triphenylmethyl/main1.html Link]
* Experimental procedures [http://www.faidherbe.org/site/cours/dupuis/triphen2.htm Link]References
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