[nj_bartel]

What are the interactions going on here?

[James Newby]

Have you drawn the Newman projection for eclipsed ethane?

wiki of newman projection:

http://en.wikipedia.org/wiki/Newman_projection

[nj_bartel]

I haven't drawn it recently, but I know what it looks like    The hydrogens are, I believe, too far from one another for the interaction to be steric.

[James Newby]

Well its the only interaction that can be occuring.  If you compare the eclipsed form to the staggered form, which one will be higher in energy?

[nj_bartel]

Eclipsed.  When I was taught this in organic I, my professor said the reason for the strain in eclipsed ethane is above the level of the course, but would be willing to explain it after class if anyone was curious.  I was a dummy and didn't want to confuse myself on the material, so I didn't bother asking.  I think it had something to do with orbital interactions.

Edit: Hm, maybe he was just referencing electrostatic repulsion.  Just going to send him an email to ask.

[James Newby]

Its conformational strain.  Its only a small amount, and almost any other effect will dominate over it.

http://research.cm.utexas.edu/nbauld/teach/ethane.html just the top section

This is just to make sure you are aware of bonds rotating.  Have you done any studies on 6 membered carbon rings yet?

[nj_bartel]

From my professor -

The BMO electronic repulsion you mention is only part of the story.  Keep in mind that conformational strain may be defined as the energy required to promote a bond rotation away from the lowest energy conformation.  In a staggered, antiperiplanar conformation there is effectively no electrostatic repulsion between the bonding MOs of the atoms or functional groups sharing the 180 degree dihedral angle, AND there is a favorable s-s* delocalization (i.e., hyperconjugation) between the groups under consideration.  In a synperiplanar conformation where the dihedral angle approaches 0 degrees, there may be some steric repulsion (not w/ hydrogens) as well as a modest electrostatic repulsion between the relevant BMOs.  However, the lack of a benefical hyperconjugative stabilization (no bonding s-s* interaction is possible) creates the greatest energy penalty relative to the antiperiplanar orientation where that stabilizing interaction is maximized (see attached diagram). 'Synperiplanar' and 'antiperiplanar' are not used to described conformers in ethane (only 'eclipsed' and 'staggered' are generally used), but I've employed the periplanar terminology so that you can extend the explanation to alkane homologs, such as n-butane, that have multiple eclipsed and staggered forms possessing different strain energies.

[Squirmy]

That's interesting about the s-s* interaction...I'd never considered that for ethane. I always thought it was just the the electrostatic repulsion of the eclipsed bonds. I think I might stick with that for students, though