Factors Affecting Stability of Ethane and Radicals

Aside from the interaction of the radical with the solvent there are several factors that can make the ethane unstable or stabilize the resulting free radical. They are the stabilization of the radical by resonance, steric strain in the ethane, and dipole-dipole repulsion in the ethane. Steric inhibition of both resonance and solvation in the radical favor association to the dimer. 

We can assume (for the moment) that a carbon free radical is planar with a threefold axis of symmetry as far as the remaining bonds to carbon are concerned. Quantum mechanics predicts that delocaliza- 

A structure that prevents coplanarity prevents resonance stabilization to a corresponding degree. Thus the triptycyl radical has none of the stability of triphenylmethyl and the benzene rings are tied back in a position with the very maximum deviation from coplanarity.16 

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. 

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