[Kalibasa]

Hi! I've been studying bond rotations and I think I get it pretty well, but I have a couple questions.

1) While I understand eclipsed versus staggered formations, I don't understand why a molecule would stay in eclipsed formation if it's higher energy. Wouldn't the molecule automatically want to be in staggered formation? I don't see *why* a bond would rotate if the energy is different- how is this ideal for the molecule?

2) What is an energy barrier? My book only makes a brief mention of this. My guess is that molecules are only briefly in eclipsed formation (this is while they're on their way to another staggered formation), and that the energy barrier is just the energy it takes to overcome this temporary hurdle of an eclipsed formation. Is this right?

3) If that's the case, there would be a huge number of staggered molecules compared to eclipsed ones. So if you go to molecules like butane, how would gauche conformations fit into this ratio; how many molecules would be gauche compared to anti and eclipsed?

Thank you!!!

[sjb]

Hi! I've been studying bond rotations and I think I get it pretty well, but I have a couple questions.

1) While I understand eclipsed versus staggered formations, I don't understand why a molecule would stay in eclipsed formation if it's higher energy. Wouldn't the molecule automatically want to be in staggered formation? I don't see *why* a bond would rotate if the energy is different- how is this ideal for the molecule?

Put simply, the molecule doesn't *want* to do anything, but as a result of its internal energy, has to make use of this somehow. It may be just flying around faster, or it may be in internal changes, such as bond rotation

2) What is an energy barrier? My book only makes a brief mention of this. My guess is that molecules are only briefly in eclipsed formation (this is while they're on their way to another staggered formation), and that the energy barrier is just the energy it takes to overcome this temporary hurdle of an eclipsed formation. Is this right?

At a simple level, yes, this is right.

3) If that's the case, there would be a huge number of staggered molecules compared to eclipsed ones. So if you go to molecules like butane, how would gauche conformations fit into this ratio; how many molecules would be gauche compared to anti and eclipsed?

Thank you!!!

Butane is perhaps a more complex example, as there are two kinds of eclipsed conformations here, one where the methyls are eclipsed fully, and one where the methyls only eclipse a hydrogen. Moving back to a simpler situation, like ethane for instance, there is only one form of eclipsed conformation. In any event, the Boltzmann distribution (http://en.wikipedia.org/w/index.php?title=Boltzmann_distribution&oldid=309975779) will let you know the distribution of molecules amongst these states, assuming you can work out things like the energy gap.

[Kalibasa]

Great, thanks!