triphenylmethyl radical

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%.{{March6th}}

Image:TriphenylmethylRadical.png

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.{{cite journal |doi=10.1070/MC2000v010n01ABEH001115 |title=Red photoluminescence in the synthesis of triphenylmethyl radicals by the Gomberg method |date=2000 |last1=Bulgakov |first1=Ramil G. |last2=Kuleshov |first2=Sergei P. |last3=Valiullina |first3=Zemfira S. |journal=Mendeleev Communications |volume=10 |pages=22–23 }}

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.

Image:Triphenylmethyl radical oxidation.png

While the triphenyl radical itself forms a quinoid dimer, alkyl substitution at the para ring positions inhibits dimer formation entirely.{{cite journal|doi=10.1070/RC1973v042n12ABEH002781|title=Advances in the chemistry of stable hydrocarbon radicals|year=1973|journal=Russian Chemical Reviews|volume=42|p=1015|first1=V. D.|last1=Sholle|first2=E. G.|last2=Rozantsev}} 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.{{Citation|last=Lewars|first=Errol|title=Modeling Marvels|year=2008|chapter=8. Hexaphenylethane|publisher=Springer|bibcode=2008moma.book.....L}}{{cite journal|last1=Grimme|first1=Stefan|last2=Schreiner|first2=Peter R.|year=2011|title=Steric crowding can stabilize a labile molecule: Solving the hexaphenylethane riddle|journal=Angewandte Chemie International Edition|volume=50|issue=52|pages=12639–12642|doi=10.1002/anie.201103615|pmid=22025456}} Other derivatives have been reported as the quinoid dimer {{cite journal|last1=Uchimura|first1=Y.|last2=Takeda|first2=T.|last3=Katoono|first3=R.|last4=Fujiwara|first4=K.|last5=Suzuki|first5=T.|date=2015|title=New Insights into the Hexaphenylethane Riddle: Formation of an α,o-Dimer.|journal=Angewandte Chemie International Edition|volume=54|issue=13|pages=4010–4013|doi=10.1002/anie.201500122|pmid=25704856}}

The class of triaryl-methyl radicals have applications in the synthesis of organic magnets.{{cite journal |doi=10.1070/RC2006v075n10ABEH003621 |title=From the Gomberg radical to organic magnets |date=2006 |last1=Shishlov |first1=Nikolay M. |journal=Russian Chemical Reviews |volume=75 |issue=10 |pages=863–884 }}

The radical was discovered by Moses Gomberg in 1900 at the University of Michigan.{{cite journal | title = An instance of trivalent carbon: triphenylmethyl | authorlink = Moses Gomberg|first=M. |last=Gomberg | journal = Journal of the American Chemical Society | year = 1900 | volume = 22 | issue = 11 | pages = 757–771 | doi = 10.1021/ja02049a006| url = https://zenodo.org/record/1428920}}{{cite journal | title = On trivalent carbon|first=M. |last=Gomberg | journal = Journal of the American Chemical Society | 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 |first=M. |last=Gomberg | journal = Journal of the American Chemical Society | year = 1902 | volume = 24 | issue = 7 | pages = 597–628 | doi = 10.1021/ja02021a001| url = https://zenodo.org/record/1428904}} 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.{{cite journal | title = Electron distribution in triphenylmethyl: Hyperfine structure of the paramagnetic resonance absorption of (C₆H₅)₃C^13*| first1= S. I. |last1=Weissman |first2=John C. |last2=Sowden | journal = Journal of the American Chemical Society| volume = 75 | issue = 2|year = 1953|pages = 503| doi = 10.1021/ja01098a522}}{{cite journal | title = Electron spin resonance studies of substituted triphenylmethyl radicals | first1= J. |last1= Sinclair |first2= D.|last2= Kivelson | journal = Journal of the American Chemical Society| volume = 90 | issue = 19 |year = 1968|pages = 5074–5080 | doi = 10.1021/ja01021a004}}{{cite web |url= http://www.chm.bris.ac.uk/motm/triphenylmethyl/tripesr1.html|title= ESR spectrum of the triphenylmethyl radical|last= |first= |date= |website= |publisher= School of Chemistry, University of Bristol|access-date= August 5, 2018}} The triphenylmethyl radical, and the larger class of triarylmethyl radicals, are called Gomberg radicals.

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).{{cite journal | title = The hexaphenylethane riddle | first= J. M. |last=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 published proton NMR data.{{cite journal | title = A new interpretation of the monomer–dimer equilibrium of triphenylmethyl- and alkyl-substituted-diphenyl methyl-radicals in solution | first1= H. |last1=Lankamp |first2=W. Th. |last2=Nauta |first3= C.|last3= MacLean | journal = Tetrahedron Letters | volume = 9 | issue = 2 |year = 1968 | pages = 249–254 | doi = 10.1016/S0040-4039(00)75598-5}}

• Triphenylmethyl hexafluorophosphate
• Triphenylmethane
• Triarylmethane dye
• Trivalent group 14 radicals

{{reflist|2}}

• [http://www.chm.bris.ac.uk/motm/triphenylmethyl/main1.html Molecule of the Month], June 1997
• [http://www.faidherbe.org/site/cours/dupuis/triphen2.htm Triphenylmethyl radical : properties and synthesis]

Category:Free radicals

Category:Phenyl compounds