Hexaphenylethane, preparation

For a review, see Sholle, V.D. Rozantsev, E.G. Russ. Chem. Rev., 1973, 42, 1011. Gomberg, M. J. Am. Chem. Soc., 1900, 22, 757, Ber., 1900, 33, 3150. Hexaphenylethane has still not been prepared, but substituted compounds [hexakis(3,5-di-ferf-butyl-4-biphenylyl)ethane and hexakis(3,5-di-tert-butylphenyl)ethane] have been shown by X-ray crystallography to be nonbridged hexaarylethanes in the solid state Stein, M. Winter, W. Rieker, A. Angew. Chem. Int. Ed. Engl., 1978,17, 692 Yannoni, N. Kahr, B. Mislow, K. J. Am. Chem. Soc., 1988,110, 6670. In solution, both dissociate into free radicals. 

Hexaphenylethane has not, indeed, ever been prepared, and may well be not capable of existing under normal conditions due to the enormous steric crowding that would be present. The reasons for the relatively high stability of Ph3C- are discussed below (p. 311). 

The first organic free radical to be discovered was triphenylmethyl, the result of the effort of Gomberg to prepare hexaphenylethane.6 In geographical exploration, isolated white spaces on the map rarely contain anything strikingly different from the neighboring explored regions chemical exploration leads to all sorts of surprises. 

Gomberg was first to prepare tetraphenylmethane, a problem initiated during a leave of absence from the University of Michigan in 1896-1897 which was spent with A. von Baeyer, Munich and V. Meyer, Heidelberg, Germany. In order to further support its structure, he wanted to prepare hexaphenylethane and test its reactivity. After some initial problems, he realized that oxygen from the air somehow must interfere with the reaction... 

The existence of a carbon radical was first proposed by Moses Gomberg in 1900. Gomberg was trying to prepare hexaphenylethane from triphenylmethyl chloride according to the following equation ... 

By about 1925, then, there seems to have been general agreement that Gomberg had prepared hexaphenylethane, and that in solution this was in equilibrium with (essentially) the triphenylmethyl radical. In subsequent decades, mainly in the 1930 s, there appeared several publications on experiments with the alleged hexaphenylethane [13, 14, 15]. 

There are three aspects to what we can call the modem hexaphenylethane period the demonstration in 1968 that hexapheitylethane did not exist, the preparation in 1978 of the first genuine (unbridged) hexapheitylethane derivative, and the interplay at about this time of reliable computational and experimental methods of probing the stmcture of hexaphei rlethane and related compounds. 

The preparation in 1978 of the first genuine (unbridged) hexaphenylethane. With the revelation in 1968 that the dimer in equilibrium with tripheitylmethyl was the methylenecyclohexadiene 3, the hexaphenylethane problem, believed for decades to have been solved, arose a in in a sharply focused form synthesis. Around the time of the stmcture disproof of the traditional hexaphenylethane, derivatives of this compound were known, bnt in all of these steric ortho-type repulsions (Fig. 8.1) were redneed or eliminated by ortho bonding, as infor example 6 [22] and... 

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. ... 

Heating Me2P-PMe2 to 650°C and F2P-PF2 to 350°C will produce the phosphinyl radicals McjP and F2R respectively. Diphenylphosphinyl radicals, obtainable as a pink-coloured condensate by low-temperature ultra-violet irradiation of tetraphenyldiphosphine, are stable at 77 K. They may be compared with the diphenylamide and triphenylmethyl radicals, which are prepared from tetrap-henylhydrazine and hexaphenylethane, respectively. 

The issue of stability vs. persistence of free radicals is an important one that dates back to the birth of the field. In 1900 Gomberg prepared the triphenylmethyl or trityl radical according to Eq. 2.13. Under appropriate conditions, the free radical persists in solution indefinitely at room temperature. This initially controversial result was arguably the birth of reactive intermediate chemistry, and it spurred volumes of work. The trityl radical is in equilibrium with a dimer that, for decades, was assumed to be hexaphenylethane. However, nuclear magnetic resonance (NMR) and ultraviolet (UV) studies in 1968 revealed that the actual dimer was the unsymmetrical structure shown in Eq. 2.13, in which one trityl center added to the para position of a ring of another radical. 

Effect of Solvent Pretreatment on Conductivity at —8.9° of Hexaphenylethane Solutions Prepared in the Dark... 

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