[jnimagine]

The difference in energy between gauche and anti of n-butane is 3.6kJ/mol.  For the molecule iodobutane, I found the differnece in energy between the gauche and anti to be about 0.62KJ/mol...
What accounts for this difference???

and also, when you're asked to find the energy barriers to the rotation from anti to gauche and you have a graph of energy vs dihedral angle, you just find the difference in energy between anti and gauche right??

[FeLiXe]

I guess it depends on where the iodine is. you would either have sterical hindrance from the methyl group or from the iodine and no clear advantage of one conformation

the barriers are the difference between minimum and maximum on the potential energy curve

[jnimagine]

hmm... i'm confused..
I thought that iodobutane would have higher energy difference between gauche and anti... because of the interaction between CH3 and I.. which would be greater than the interaction of CH3 and just H in the case of n-butane...

[FeLiXe]

are you talking about 1-Iodo-n-butane or 2-Iodo-n-butane?

[jnimagine]

oops sorry my bad
i'm actually talking about 1-iodopropane... lol
i must have been crazy while i was typing the question...!

so to clarify, we're comparing difference in energy between gauche and anti conformations of n-butane(3.6kj/mol) and 1-idodopropane(0.62kj/mol)

sorry about the confusion!

[FeLiXe]

then i guess the idea is that 4 atoms are bigger than 1 atom even if that one atom is a very big atom
you can probably estimate it from van der waals radii

and maybe you have a little bit of a hydrogen bond like interaction that will stabilize it a little bit more

[nj_bartel]

then i guess the idea is that 4 atoms are bigger than 1 atom even if that one atom is a very big atom
you can probably estimate it from van der waals radii

and maybe you have a little bit of a hydrogen bond like interaction that will stabilize it a little bit more

It's actually more about bond length.

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

The extremely long bond length of C-I places the I beyond the range of most steric effects, even thought the atom is huge.

[FeLiXe]

i guess it's true that it's not just van-der-Waals radii but then it should be a combination of bond lengths, van der Waals radii, bond angles and maybe electrostatics

if bond lengths would be everything and you assume that steric hindrance decreases with increasing bond length then the CH3-I gauche conformation should be lower in energy than the anti conformation where CH3 and H are next to each other

[nj_bartel]

Well, first, to be clear, I never said bond lengths were the only factor - only that in this case, they were the reason iodine had so much less steric interaction, not that iodine is one atom while methyl is 4 atoms, as you said.

The I atomic radius is huge.  Two eclipsed I atoms will have a huge steric interference because they have the same bond length.  The steric interference is so small in the case in question because the I, though huge, is positioned more or less by itself above everything.

Same reason the 1,3-diaxial values for I are so small (when compared to H,H).

[FeLiXe]

ok, I think I see what you are meaning:iodine is so far away from the central bond that it does not interact much with the hydrogen which is closer

[jnimagine]

oh ok i think i get it guys
Thanks!
but one more thing...

What's an energy barrir to rotation?
I have data on the enrgy at anti and gauche conformations of 1-iodopropane.
I have also graphed the data (steric energy vs dihedral angle) 
from the graph, do I just subtract the trough at anti from the trough at gauche?  What about for gauche to gauche?  since the energies are the same at the two gauche conformations, would it be 0??

[FeLiXe]

an energy barrier is the difference between a minimum and a maximum on the potential curve

a difference between two minima is a reaction energy

[jnimagine]

ok so for example, for 1-iodopropane, the energy barrier from anti to gauche would be the energy at 120degrees which is the maximum - the energy at 180degrees which is the minimum?? so I don't even consider the energy at gauche conformation??
from gauche to anti, on the other hand,,, would be the maximum - energy at gauche?
If I do what I said above, I would get about 13kJ/mol... then this whole thread is wrong....
The question asked "what is the difference in energy between the gauche and anti conformations? and then we had to compare with the energy difference between gauche and anti for n-butane.. which was 3.6.."
then in fact, 1-iodopropane has a larger enrgy difference....

[FeLiXe]

yes that's how you get the energy barriers.
but why are you looking at barriers (i.e. min-max) for iodopropane and min-min energy differences for n-butane?