Hexaphenyl ethane

Note on terminology Many authors write stable when they mean unreactive or inert but stable is the opposite of unstable , and the opposite of reactive is unreactive or inert . Methyl radicals are very stable, but very reactive hexaphenyl ethane is unstable, but very inert. 

The hexaphenyl ethane obtained was far more reactive than was to be expected, and on account of its properties Gomberg named it triplienyl methyl. Since then much work and discussion have centred around this substance, which is now generally regarded as being hexaphenyl-ethane. 

This assumption has its parallel in the case of triphenylmethyl, C(C6H5)3, which in solution has the simple formula, but is regarded as associated in the solid condition to hexaphenyl ethane, (C6H5)8C — C(C6H5)3. ... 

Studies of the dissociation energy of compounds of the hexa-aryl ethane type to triaryl methane and similar radicals have been made in solution. Ziegler and Ewald, for example, measured the heat of dissociation of hexaphenyl ethane from the temperature dependence of the equilibrium constant. I he equilibrium constant was measured using the extinction coefficient. They obtained a value of 10-12 kcal, depending slightly on the solvent used. Other results for this compound are quoted by Szwarc ] ut they lie within these limits. The values for the dissociation in the vapour phase cannot ]>e obtained, and the corrections required may... 

There are instances in which the mode of fission and therefore reaction mechanism changes with the environment or experimental conditions. This can be illustrated with the dissociation of hexaphenyl-ethane, the bromination of bromobenzene, and the reactions of diazonium ions ... 

In other instances, free radicals appear to be involved. When one mole of magnesium is added to two moles of triphenylmethyl bromide, the solution contains triphenylmethyl free radicals and hexaphenyl-ethane but no Grignard reagentAddition of a second mole of magnesium produces the Grignard reagent in almost quantitative yield ... 

Fm. 19. Spectrum of triphenylmethyl radicals, obtained by grinding hexaphenyl-ethane with AbO,. Curve 1 at 25 C curve 2 at 140°C. Diffuse reflexion spectra, from KortUm et al. (62). 

This radical is relatively stable (we shall see why shortly), but reacts with itself reversibly in solution. The product of the dimerization of triphenylmethyl was for 70 years believed to be hexaphenyl ethane but, in 1970, NMR showed that it was, in fact, an unsymmetrical dimer. 

However, the situation is not that simple. Paradoxically, hexaarylethanes, the apparently overcrowded molecules, can be stabilizedby increasing steric bulk. Introduction of 12( ) bulky tert-Bu substituents shifted the equilibrium towards the non-dissociated state (ethane) whereas equilibrium in the less sterically crowded parent hexaphenyl ethane favors dissociation (radicals) (Figure 2.32). Dispersion interactions were shown to be the source of these counterintuitive equilibria. ... 

Conant and Chow, on refluxing a benzene solution of hexaphenyl-ethane and maleic anhydride, under N2, obtained a 25% yield of the adduct 57 (isolated as acid). 

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