Posted: Sun Mar 19, 2017 2:58 am
For two chair conformations of Trans-1,4-dimethylcyclohexane (pg 112), why's one chair conformation more stable than the other one? Aren't they same?
Re: Chair conformations
Posted: Sun Mar 19, 2017 3:26 am
This is simply due to the orientation of the CH3 substituents as Dr. Lavelle had previously stated that the equitorial position of the methyl makes the structure more stable than the axial position.
Re: Chair conformations [ENDORSED]
Posted: Mon Mar 20, 2017 4:22 pm
Looking at the top of Pg. 112, the one on the left is trans-1,4-dimethylcyclohexane. "Trans" means "opposite"; we can see that the two methyl substituents are on opposite sides of the cyclohexane ring. The one on the right is cis-1,4-dimethylcyclohexane. "Cis" means "same"; we can see, accordingly, that the two methyl substituents are on the same side of the cyclohexane ring.
Trans-1,4-dimethylcyclohexane is more stable (meaning that it has lower energy) than cis-1,4-dimethylcyclohexane. This is due to the steric and torsional strain that the substituents can encounter if they are placed close to each other. Because trans-1,4-dimethylcyclohexane has the methyl groups farther away from each other than cis-1,4-dimethylcyclohexane, it has lower energy and is therefore more stable.
Now, we can see that there are two different chair conformations for trans-1,4-dimethylcyclohexane. Substituents have higher preference for equatorial positions (as stated on Pg. 111), so we always want substituents in equatorial positions. This way, as we can see in the drawn-out chair conformations, the two methyl groups in trans-1,4-dimethylcyclohexane are farthest away from each other as possible, leading to lowest possible energy and the most stable conformation for this molecule.