Hexaphenylethane hydrogenation

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. 

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). 

In the hexaphenylethane molecule the valence bonds of the two ethane carbon atoms are tetrahedral, but during the dissociation this configuration is replaced by the planar configuration which is present in the free radical. The planar structure may, however, be slightly distorted owing to the repulsion of the ortho hydrogen atoms of the benzene rings. 

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