Source of Barrier

From WikidChem

12/08/06 Slides 13-14

Slide 13: Experimental evidence has already established that the eclipsed conformation of ethane has 3 kcal/mole more energy than the staggered conformation of the molecule. So, the question now is -- what is the source of the energy barrier in the eclipsed ethane?

The first explanation is that the eclipsed form is destabilized. The aligned hydrogen atoms are as close as they can be and exhibit nuclear repulsion, the same for the electrons in the carbon-hydrogen bonds. However, when compared to the propane molecule (CH3CH2CH3), which should have even greater repulsion as predicted by this hypothesis, we see that the small size of the hydrogen atom makes this effect negligible. The energy barrier for propane is 3.4 kcal/mol, which is only slightly higher than the 3 kcal/mol for ethane. In addition to the proton-proton & electron-electron repulsions, there are also simultaneously occuring attractions between the hydrogen atom nuclei and electrons.

Finally, there is overlap between the sigma C-H bonding orbitals that also stabilizes the molecule. (Overlap between bonding MO should be destabilizing, if both are filled with electrons - JMM)

Slide 14: The second explanation is that the staggered form is stablized, so its energy is much lower. HOMO/LUMO mixing within the molecule causes stabilization through a process called hyperconjugation. As is evident in the molecular orbital picture, the sigma C-H and sigma C-H* orbitals of anti C-H groups involving the two carbon atoms overlap, and so can stabilize the electrons in the sigma bond. All of the other C-H bonding pairs can be similarly stabilized. However, this orbital mixing produces a resonance structure (carbon is double-bonded) that has charge separation on the hydrogen atoms, so this is not very likely.

"hyperconjugation" is the term used to described such HOMO/LUMO mixing involving a sigma bond.

This website explains hyperconjugation: external link: http://www.chem.ucalgary.ca/courses/351/Carey/Useful/hyperconjugation.html

As the website describes, staggered structures may also be stabilized by favorable overlap between a sigma C-H bonding orbital on one carbon and an unoccupied 2p orbital on an adjacent carbon. McBride did not mention this sort of hyperconjugation in class. -NZG