Ethane, ethene, and ethyne model considerations

The barrier to internal rotation in Si2He is 4.9 kJ mol i.e. less than half the barrier in ethane [4], The smaller barrier in disilane is, of course, consistent with a rationalization in terms of repulsion between C-H or Si-H bonds, but does not prove it. 

Ethene is a planar molecule. The coordination geometry of each carbon atom is approximately trigonal planar. The C-C bond distance is about 20 pm shorter than in ethane. The ethyne molecule is linear, the C-C bond distance is about 14 pm shorter than in ethene and about 33 pm shorter than in ethane. 

The CC bond strengths in the three compounds may be defined as the bond dissociation energies, i.e. as the standard energies of the reactions 

These energies are associated with uncertainties of about 5 kJ moP. Note that the bond dissociation energies vary as 1.0 2.0 2.6. 

Another measure of C-C bond strength is provided the thermochemical bond energies calculated from the energies of atomization in combination with the mean C-H bond energy in methane  

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