In a sample of cis-1,2-dichlorocyclohexane at room temperature, the chlorines will:
A. both be equatorial whenever the molecule is in the chair conformation.
B. both be axial whenever the molecule is in the chair conformation.
C. alternate between both equatorial and both axial whenever the molecule is in the chair conformation.
D. both alternate between equatorial and axial but will never exist both axial or both equatorial at the same time.
I thought the answer would be A because on pg. 28 says that "most substituent groups are favored in the equatorial position." In addition, the molecule is cis which means the substituents are on the same side. Trans means the substituents are on opposite sides.
Thus, I was confused when examkrackers said the answer was D. Stating that "Cis groups on cyclohexane will never be both equatorial or both axial while in the chair configuration."
Is their answer correct. Can someone explain this further to me or show me where I can get more information? Does this mean that trans groups on cyclohexane will be either both equatorial or axial (with the equatorial position being favored)?
Thank you for all your help.
Sincerely,
Verónica
Their answer is correct.
Here's a little trick I picked up that should help you do these types of problems.
1) Draw a hexagon (your cyclohexane)
2) Place the substituents in their spots, draw in the appropriate shading for whether the substituents are cis/trans. For your problem, the chlorines are cis meaning they are on the same side (so either both sticking into the page or both sticking out of the page, doesn't matter, just keep it consistent with other groups). If they were trans, they would be pointing opposite ways, again it doesn't matter just keep it consistent.
3) When looking at your hexagon, the axial and equatorial positions alternate (E-A-E-A, etc.) as you go around the ring. If the group pointing INTO the page at one spot is axial, then the groups pointing INTO the page at the surrounding spots will both be equatorial. Likewise, if the group pointing OUT of the page at one spot is axial, then the groups pointing OUT of the page at the surrounding spots will both be equatorial.
4) Knowing this, we can do things like answer your question or draw the correct chairs
So in your problem, you have two groups pointing the SAME way next to each other so one will be axial and the other will be equatorial. This alone eliminates A, B and C since they can never BOTH be at the same position.
Since they are the SAME group, neither is favored to be in the equatorial position. Therefore, when in the chair conformation they sort of go into an "in-between" state that is a mix of both axial and equatorial (i'd visualize it as both groups constantly switching back and forth between axial and equatorial so at one point in time, it's never really either.)
Hopes this helps. I find drawing the 2-D hexagon helps me set up the correct chair conformations and answer questions like these.
EDIT: for your follow-up post, if it was cis-1,3 dichlorocyclohexane:
If we follow the trick above, we have a hexagon with chlorine pointing the same way with an empty spot in between. Therefore, they will both have the same orientation, either both axial or both equatorial. Since the chlorine atom is bulkier than the hydrogen atom (as is pretty much everything you'll encounter), it is favored to have the chlorines in the equatorial position. In this case, it is possible to have both in the equatorial position which would most definitely be favored. The answer would be A.
I can't think of any situation where C would be the answer when there are only two substituent groups. Would only make sense to me if they were asking about two specific substituents in a cyclohexane with multiple substituent groups.
EDIT2:
Thus, I was confused when examkrackers said the answer was D. Stating that "Cis groups on cyclohexane will never be both equatorial or both axial while in the chair configuration."
Is their answer correct. Can someone explain this further to me or show me where I can get more information? Does this mean that trans groups on cyclohexane will be either both equatorial or axial (with the equatorial position being favored)?
Hopefully now you understand the first part there. Cis groups will never be both equatorial or both axial because they are pointing the same direction and the trick above thus indicates one would be E and the other A (which one depends on the groups themselves).
Trans groups will be both equatorial or both axial. If they are the only substituents, then yes the equatorial positions will ALWAYS be favored. If there are other substituents, it depends on how bulky those groups are. There are certain groups which are considered "ring-locking", like tert-butyl, which will (with rare exceptions) always be in the equatorial position. Using the trick above, we can figure out the orientation of the other groups.
It's a bit hard to explain well using text only so please do let me know if you understand and if not, I will do my best to put my scanner to use and draw it out a bit more.
