Okay, I think I'm getting confused...I feel like what I see on full lengths contradict this, but if two substituants are cis on the cyclohexane then when they are on the chair they are at different positions (like, one on equitorial, one on axial). and if they are trans they are the same configuration (both axial, both equitorial). is this correct?? Thanks a lot!!
Isomer question
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mycurlyearly said:
You can't interchange cis/trans and axial/equitorial. They are two different conventions. Cis/trans generally refers to substituents on a double bond, axial/equitorial refer to substituents on an alkane ring.
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mycurlyearly said:
Are you talking about cis/trans in the "flat" conformation?? And how to determine if they become axial or equatorial if the molecule relaxes to the chair conformation?
I recommend you play around with some models - you are better able to see the relationships that way. Unfortunately, you can't generalize like you did here - it depends on whether you are talking about substituents in the 1,2 position, 1,3 position, etc.
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Here's my 2% of $1:
I came across the same quesion in one of the full length MCATS (AAMC 7?) and solved it by drawing out all the substituents on each carbon in chair conformation. Then I imagined what cis or trans would look like in that configuration. Cis has both substituents above or below the plane of the ring and trans has one above and one below (even in chair conformation, you can imagine what it'd look like if you flattened the ring out) and then I just matched what I expected to see to what I actually see in the question.
I hope that was clear enough to be helpful. Good luck!
I came across the same quesion in one of the full length MCATS (AAMC 7?) and solved it by drawing out all the substituents on each carbon in chair conformation. Then I imagined what cis or trans would look like in that configuration. Cis has both substituents above or below the plane of the ring and trans has one above and one below (even in chair conformation, you can imagine what it'd look like if you flattened the ring out) and then I just matched what I expected to see to what I actually see in the question.
I hope that was clear enough to be helpful. Good luck!
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mycurlyearly said:
I belive you are right.
According to Wikipedia:
In stepping round the ring, it can be seen that the up-axial positions alternate with up-equatorial positions, so that for trans-1,2-cyclohexane, the substituents must either both be axial or both be equatorial to remain on opposite sides of the ring. Similarly, for cis-1,2-cyclohexane, the substituent at 1 must be equatorial and the 2 substituent axial, or vice versa. Each conformation will differ in its stability depending upon the identity of the functional groups.
http://en.wikipedia.org/wiki/Cyclohexane_conformation
It is also important to note that this does not hold unless they are in the 1, 2 position (same as 1, 6). Take a look at the following website for 1, 3 (same as 1, 5) and 1, 4.
http://www.cem.msu.edu/~reusch/VirtTxtJml/sterism2.htm
One aspect of cis and trans isomers that is often overlooked is their relationship to the chair conformations. When we say "planar monocyclic ring" we are actualy talking about the chair conformations of the molecule. Keep in mind that there are many ways to represent one molecule.
