Kneecoal

10+ Year Member
Mar 2, 2009
870
0
Long Island, NY
Status
Pre-Dental
It asks which of the isomers is most stable. Cyclohexane with 3 methyl groups - 2 are right next to each other, and the last one is 2 over from that. The answer is the one with all 3 equatorial because large groups all equatorial to each other presents a molecule with the lowest energy.

But I thought I remembered reading somewhere - is there a case where a cyclohexane molecule is at lowest energy when its extra groups are all axial but facing opposite each other? Or am I making this up?
 
Jun 14, 2009
800
3
Status
Pre-Dental
It asks which of the isomers is most stable. Cyclohexane with 3 methyl groups - 2 are right next to each other, and the last one is 2 over from that. The answer is the one with all 3 equatorial because large groups all equatorial to each other presents a molecule with the lowest energy.

But I thought I remembered reading somewhere - is there a case where a cyclohexane molecule is at lowest energy when its extra groups are all axial but facing opposite each other? Or am I making this up?
My understanding is that equatorial is always more stable than axial, because the methyl groups will be sterically bothered by other axial H's. Remember that two equatorial groups next to each other are considered "trans" to each other as well.

Since there's only two ways for axial groups to stick out of the ring (up and down), a theoretically most-stable multisubstituted axial conformation would have to be trans 1,2 or 1,4 disubstituted, but even then the equatorial conformation is more stable.

Perhaps you might be thinking about a case in which the substituents are not methyl groups? For example, a t-butyl group will prefer equatorial and will force a methyl group into the axial position to accommodate it.
 
May 15, 2009
1,922
3
CA
Status
Dental Student
It asks which of the isomers is most stable. Cyclohexane with 3 methyl groups - 2 are right next to each other, and the last one is 2 over from that. The answer is the one with all 3 equatorial because large groups all equatorial to each other presents a molecule with the lowest energy.

But I thought I remembered reading somewhere - is there a case where a cyclohexane molecule is at lowest energy when its extra groups are all axial but facing opposite each other? Or am I making this up?
Obviously, when axial groups are on the opposite sides of the ring, the molecule will be more stable, because there's less steric hindrance between those groups. However, this doesn't change the fact that molecules with BULKY GROUPS in the equatorial position are the most stable!! It's really the same principle, if you look at a model of cyclohexane you see that groups located in the equatorial position are located far from one another (as oppose to those located in axial positions), and therefore, experience the least amount of steric hindrance, which in turn stabilizes the molecule.