[jmg12]

Okay I understand the concept that the bulkier group is better off in an equatorial position. However, when doing this lab, I received some odd numbers using Spartan 3D. If you take a look at the picture and figures D E F. The energies are as follows:
D = 287.2505 kJ/mol
E = 258.6099 kJ/mol
F = 241.0435 kJ/ mol

Now why in the world is F more stable than E, especially when the R group (CCl3) is in an axial position. Someone please explain.

http://i729.photobucket.com/albums/ww295/jgsupertramp/2011-10-12_11-55-41_167.jpg
Thank you!

[Jerry]

Okay I understand the concept that the bulkier group is better off in an equatorial position. However, when doing this lab, I received some odd numbers using Spartan 3D. If you take a look at the picture and figures D E F. The energies are as follows:
D = 287.2505 kJ/mol
E = 258.6099 kJ/mol
F = 241.0435 kJ/ mol

Now why in the world is F more stable than E, especially when the R group (CCl3) is in an axial position. Someone please explain.

http://i729.photobucket.com/albums/ww295/jgsupertramp/2011-10-12_11-55-41_167.jpg
Thank you!

Think about R's position relative to the other substituents.  Sometimes, there are situations where axial is preferred because of other strain that is involved in the molecule.  So what do you see in relation to where the R group is when axial vs. equatorial? 

[jmg12]

That it's on the same side (cis) in structure F? I just don't get why the R group in an axial would be preferred. Wouldn't it be interacting with the hydrogens in the 1,3 position. And if it was in equatorial it wouldn't be interacting with much of anything because it has so much room. I don't see any strain throughout the molecule except when the R group goes into the axial. I don't get it lol.

[Jerry]

That it's on the same side (cis) in structure F? I just don't get why the R group in an axial would be preferred. Wouldn't it be interacting with the hydrogens in the 1,3 position. And if it was in equatorial it wouldn't be interacting with much of anything because it has so much room. I don't see any strain throughout the molecule except when the R group goes into the axial. I don't get it lol.

All right, I'm not sure how to exactly phrase it so that I don't just fully give it away, but think about a cyclohexane that has two substituents.  Both of the substituents are equatorial.  So you can have 1,2 or 1,3 or 1,4 diequatorial cyclohexane molecules.  Do you think there are any differences in the stability of those molecules?

[jmg12]

Is the 1,2 least stable? So because, the CCl3 group has two methyl groups in equatorial positions on its left and right...it will pop up to the axial position for less steric interactions???Please tell me that's right lol I've been stuck on this for HOURS.

[Jerry]

Is the 1,2 least stable? So because, the CCl3 group has two methyl groups in equatorial positions on its left and right...it will pop up to the axial position for less steric interactions???Please tell me that's right lol I've been stuck on this for HOURS.

I wouldn't say that it pops up to the axial position just randomly or anything, but it might be the case that having 1,2,3 tri-equatorial substituents has more strain than 1,3 di-equatorial with a 2 "mono" axial.  Although having axial substituents is usually less preferred, there are some molecules that show less strain when a substituent is axial instead of equatorial. 

In order of decreasing stability with the first listed being the most stable:
1,4 > 1,3 > 1,2 for equatorial substituents.

[jmg12]

Okay gotcha, thanks a lot.