Hexaphenylethane, dissociation

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... 

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 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... 

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. 

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

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. 

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. 

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). 

Ever since the discovery by Gomberg in 1900 of the dissociation of hexaphenylethane into triphenylmethyl radicals the search for a theoretical explanation of the phenomenon has been carried on. The modern theory of the stability of the aromatic free radicals attributes it in the main to the resonance of the free valence among many atoms.28... 

It has been found that hexa-p-alkylphenylethanes in solution dissociate to a somewhat greater extent than hexaphenylethane itself, the magnitude of the enhancement of the degree of dissociation by the... 

We have an interesting problem in the cause of the ready dissociation of, for example, hexaphenylethane into triphenyl-methyl radicals, so that a solution of this substance is dissociated to a considerable extent (Dissociation energy is 11 kcal, Table 18 B, p. 191). 

Previously the dissociation of hexaphenylethane had been attributed to a Valenzbeanspruchung but tri- and tetra-phenylmethane are, however, very stable substances. 

It is just as impossible to explain the low heats of dissociation exclusively by steric effects. In hexaphenylethane this does, however, play a certain part in producing the so extraordinarily great lowering of the heat of dissociation. It has been shown by Coops et al.25 by means of the heats of combustion... 

The molecular compounds of hexaphenylethane with aromatic and aliphatic hydrocarbons are remarkable. The nature of the bond in this case is not yet clear is there a connection with the tendency to dissociation of the C—C bond ... 

Short-lived Ge-centered free radicals of R3Ge belong to the hypovalent (trivalent) germanium derivatives. In 1953-1957, Gihnan and coworkers , based on the dissociation of hexaphenylethane to free Ph3C radicals, tried to obtain the Ph3Ge radical by dissociation of Ph3GeMPh3 (M = C, Si, Ge, Sn). 

Fig. 5-9. Correlation oi g kjkf) [167] and the cohesive pressure 5 [238] in the dissociation of 1-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene ( hexaphenylethane ) at 0 °C cf. Eq. (5-56) in Section 5.3.4 (rate constants relative to acetonitrile as slowest standard solvent) ...

Hexaphenylethane 23 is unstable and dissociates into two trityl radicals. Similarly, the para-t-hutyl derivative 24 is also unstable. The standard explanation for the instabilities of these two crowded compounds is steric congestion. That argument, however, fails to account for the stability (a melting point of 214 C ) of the seemingly even more crowded all-mefa-f-butyl derivative 25. The C-C distance in the ethyl fragment is very long (1.67 A), which is perhaps an indication of its steric congestion. 

In certain cases, the number of carbon atoms which may contain the unpaired electron may, through the resonance with ionic structures, be increased. If this is so the free radical will be stabilized and its formation facilitated. Thus hexa-/ nitrophenylethane, (N02GgH4)3G—C(CgH4N02)3 dissociates completely into the radical (N02G6H4)3C under conditions in which hexaphenylethane only partially dissociates. In the first case the odd electron is not only shared between the ortho and para carbon atoms of the ring systems but also with the other carbon atoms, as for example, in the structure... 

Early investigators believed that solutions of hexaphenylethane in liquid sulfur dioxide contained both triphenylmethyl cations and triphenylmethyl anions [CcHb C ]. That equation 5 represents the correct nature of the dissociation has been demonstrated by the work of Anderson (J. Am. Chem. Soc.f 67, 1674 [1935]). He has shown that the absorption curves of hexaphenylethane and triphenylmethyl bromide in liquid sulfur dioxide are almost identical and that in dilute solution the same number of triphenylmethyl cations is obtained from one mole of hexaphenylethane as from two moles of triphenylmethyl bromide. 

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