Hexaphenylethane

Hexaphenylethane, 1, is interesting beeause all attempts to make it have failed, and its synthesis would represent the denouement to a historical saga and shed light on certain questions of theoretical interest. 

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From such crude data as are to be found in the literature we can calculate approximate values of the equilibrium constants, and hence of the free energies of dissociation for the various hexaarylethanes. From our quantum-mechanical treatment, on the other hand, we obtain only the heats of dissociation, for which, except in the single case of hexaphenylethane, we have no experimental data. Thus, in order that we may compare our results with those of experiment, we must make the plausible assumption that the entropies of dissociation vary only slightly from ethane to ethane. Then at a given temperature the heats of dissociation run parallel to the free energies and can be used instead of the latter in predicting the relative degrees of dissociation of the different molecules. 

From Table III we see that the difference between the free radical resonance energies of tribiphenylmethyl and triphenylmethyl is 0.07a. Hence X]/X2 = 37 = 2.2 X103. Ziegler and Ewald8 found that at 20°C the value of the dissociation constant for hexaphenylethane in benzene solution is 4.1 X10-4 and consequently we calculate for hexabiphenylethane a value of X = 2.2X103 X4.1 X 10 4 = 0.90. This value is probably too low as the compound is reported to be completely dissociated the error may not be large, however, since a dissociation constant of 0.90 would lead to 91 percent dissociation in 0.05M solution. 

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. 

Steric effects have been discussed in free radical chemistry ever since the discovery of the first free radical, triphenylmethyl 1 by M. Gomberg in 19001. To what extent is the dissociation of its dimer, which was believed to be hexaphenylethane 23 till 19682 determined by electronic stabilization of triphenylmethyl 1 or by steric strain in its dimer ... 

The importance of establishing the correct structure of the reaction product is best illustrated by the confusion that can result when this has been assumed, wrongly, as self-evident, or established erroneously. Thus the yellow triphenylmethyl radical (3, cf. p. 300), obtained from the action of silver on triphenylmethyl chloride in 1900, readily forms a colourless dimer (m.w. = 486) which was—reasonably enough—assumed to be hexaphenylethane (4) with thirty aromatic ... 

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 decomposition of hexaphenylethane to triphenylmethyl radicals in liquid chloroform has been studied at 0 °C. 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

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. 

Treatment of triphenylmethyl chloride with silver gave not the expected hydrocarbon but an oxygen-containing compound later found to be the peroxide. The reaction run in an inert atmosphere did give a hydrocarbon, but one with unusual properties. It reacted rapidly with oxygen, chlorine, and bromine, and was quite different from tetra-phenylmethane or what was expected of hexaphenylethane. Gomberg... 

The equilibrium constant of hexaphenylethane dissociation, in striking contrast to the rate constant for dissociation, varies considerably with solvent. The radical with its unpaired electron and nearly planar structure probably complexes with solvents to a considerable extent while the ethane does not. Since the transition state is like the ethane and its solvation is hindered, the dissociation rate constants change very little with solvent.12 13 From an empirical relationship that happens to exist in this case between the rate and equilibrium constants in a series of solvents, it has been calculated that the transition state resembles the ethane at least four times as much as it resembles the radical. These are the proportions that must be used if the free energy of the transition state in a given solvent is to be expressed as a linear combination of the free energies of the ethane and radical states.14... 

The question then arises as to which is the more responsible for the dissociation of hexaphenylethane, steric strain in the dimer or resonance stabilization in the radical Because bulky groups in the... 

An argument from heats of hydrogenation concludes that resonance is responsible for about two-thirds of the difference in stability between the central bond of hexaphenylethane and normal carbon-carbon bonds. It can be calculated from other thermochemical data that the heat of hydrogenation of ethane to two moles of methane is —13 kcal. In contrast the heat of hydrogenation of hexaphenylethane has been shown to be —35 kcal. per mole. 

Since triphenylmethane is not stabilized by any resonance not already present in hexaphenylethane, the difference between the two heats of hydrogenation, or 22 kcal., might be a measure of the steric effect alone. The difference in the heats of dissociation into radicals when ethane and hexaphenylethane are compared is 62 kcal. This leaves about 40 kcal. to be accounted for as resonance stabilization of the radical.16 This degree of resonance stabilization for the triphenylmethyl radical does not violate quantum mechanical expectations. 

Solutions of hexaphenylethane in liquid sulfur dioxide conduct electricity, suggesting an ionization into triphenylmethyl positive and negative ions. Since the spectrum of triphenylmethide ion was missing from the spectrum of the solution the following equilibrium was postulated ... 

But the hexaphenylethane analog with lead atoms in the place of the central carbon atoms appears to be about 50% dissociated.36... 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

Many radical reactions do show the expected small and non-specific response to substituents. Reaction 14 of Table XIV is an example it has a value of p not significantly different from zero and shows almost a random response to the polar nature of the substituent.438 The dissociation of hexaphenylethanes is obscured by experimental uncertainties but seems to be increased by both electron-releasing and electron-withdrawing substituents. 

Admittedly, PI13C—CPI13 (literally, hexaphenylethane as drawn) has largely uncharacterized features as well. For example, the considerable weakness of the central C—C bond is not paralleled by the central C—C bond in tetraphenylmethane and 1,1,1,2-tetraphenylethane, the sole thermo-chemically characterized species in which there is a C—(Cb)3 (C ) structural group. [The enthalpy of formation of the latter species is from H.-D. Beckhaus, B. Dogan, J. Schaetzer, S. Hellmann and C. Riichardt, Chem. Ber., 133, 137 (1990).]... 

Schmidlin s experiment here described shows very clearly the equilibrium between hexaphenylethane and triphenylmethyl. The disappearance of the colour on shaking the substance with air indicates that the yellow radicle, present in equilibrium, is removed as (colourless) peroxide. The re-establishment of the equilibrium by renewed dissociation of (colourless) hexaphenylethane proceeds so slowly that the formation of the yellow radicle in the decolorised solution can be observed without difficulty. 

As the odd number of hydrogen atoms already shows, triphenylmethyl, C19H16, known in solution only, contains a tervalent carbon atom. In contrast to the colourless hexaphenylethane, which can be isolated in the crystalline state, triphenylmethyl is intensely yellow. Its absorption spectrum exhibits characteristic bands (examine the spectrum in a spectroscope). 

When the molecular weight of hexaphenylethane is determined cryoscopically in benzene solution, the value corresponding to this hydrocarbon is obtained with close approximation. Indeed, only 2-3 per cent of the dissolved molecules are split into the triphenylmethyl halves. 

The dissociation of hexaphenylethane can also be demonstrated colorimetrically. Whereas, in general, coloured solutions undergo no change in intensity of colour on dilution, since the number of coloured molecules observed in the colorimeter remains the same (Beer s law), the intensity must increase if the coloured molecules become more numerous as a result of progressive dissociation following dilution (Piccard). 

The decomposition of hexaphenylethane is to be attributed to the inadequate binding force between the two ethane carbon atoms which are each over-much occupied by three phenyl groups. If these are progressively replaced by diphenyl groups the combining power of the fourth valency becomes smaller and smaller, and is finally reduced to zero in p-tridiphenyl-methyl (Schlenk). 

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