Early Evidence for the Existence of Radicals

Before 1900 chemists considered carbon to be tetrasubstituted in virtually all organic compounds. Then, Gomberg s attempt to prepare hexaphenylethane from the reaction of triphenylmethyl chloride (5) with zinc dust (equation 5.1) gave colored solutions that reacted with reagents such as iodine (equation 5.2) to produce stable products. The results were interpreted in terms of an equilibrium between hexaphenylethane (6) and triphenylmethyl radicals.  

Hexaphenylethane was accepted as the structure for the dimer of the triphenylmethyl radical for over sixty years. In 1968, however, Lankamp and co-workers reported that the NMR spectrum of the supposed hexaphen- 

Forrester, A. R. Hay, J. M. Thomson, R. H. Organic Chemistry of Stable Free Radicals Academic Press New York, 1968 p. 1. 

For an overview of the chemistry of radicals, see (a) Kochi, J. K., Ed. Free Radicals, Vols. I and II John Wiley Sons New York, 1973 (b) Leffler, J. E. An Introduction to Free Radicals Wiley-Interscience New York, 1993 (c) Fossey, J. Lefort, D. Sorba, J. Free Radicals in Organic Chemistry John Wiley Sons Chichester, UK, 1995. 

The bond dissociation energy of hexaphenylethane has been calculated to be 16.6kcal/mol, suggesting that it might be possible to synthesize 6 Vreven, T. Morokuma, K. /. Phys. Chem. A 2002, 106, 6167. 

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