*AAMC 8 SPOILERS* Organic Chemistry MCAT question?

[TrillyBassily]

This question is from AAMC practice MCAT Exam #8.

The question asks about the most stable conformation of a cyclohexane ring with two substituents on the 1st and 4th carbons. I thought it was always the most stable to have substituents be equatorial on the ring, apparently however it is more stable to have one substituent be equatorial and the other be axial.

I'm not sure why this is the case, can someone please shed some light on the issue?

Here is a link to a picture of the question, along with the answer that the AAMC test provided:

http://postimg.org/image/k669p2x9v/

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[SnoopyMD]

Recall that the most stable cyclohexane formation is chair conformation, not a flat ring. If the ring were flat, both equatorial would be correct. In chair conformation however, that would actually place the 1,4 substituents closer than necessary. Draw it out to prove this to yourself.

 

[TrillyBassily]

Recall that the most stable cyclohexane formation is chair conformation, not a flat ring. If the ring were flat, both equatorial would be correct. In chair conformation however, that would actually place the 1,4 substituents closer than necessary. Draw it out to prove this to yourself.

Could it also have anything to do with the fact that in the drawing the two substituents are cis based on the wedges drawn and they could not be cis if both substituents were equatorial? Would I be correct in saying that?

 

[SnoopyMD]

Could it also have anything to do with the fact that in the drawing the two substituents are cis based on the wedges drawn and they could not be cis if both substituents were equatorial? Would I be correct in saying that?

That logic makes sense to me. That may be what they wanted you to pick up on from the structure given.

 

[gettheleadout]

There's a thread for this, read the sticky.