Achiver Ochem test 3 #80

[Decan]

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I've always been horrible with cyclohexane stereochemistry. I know that big groups like being equitorial, but I dont really understand why the answer to this question is trans 1,4 and cis 1,3.

If someone can help, I would greatly appreciate it. Thanks in advance!

 

[Flipper405]

I've always been horrible with cyclohexane stereochemistry. I know that big groups like being equitorial, but I dont really understand why the answer to this question is trans 1,4 and cis 1,3.

If someone can help, I would greatly appreciate it. Thanks in advance!

I don't have achiever so if you posted the actual question; I might be able to help.

 

[kxb4777]

Try to picture the cyclohexane as a regular hexagon figure and not in the chair formation. Cis would refer to substituents existing on the same side (below or above the ring). Trans refers to substituents on opposite side (one above, one below the ring).

Thus, on a chair formation, cis and trans translate to whether the orientation of the substituent is up or down. Remember, in chair formation, up and down orientations change with each Carbon on the ring. The solution to the problem details which methyls are oriented up and which are down.

Axial and equatorial orientation don't account for cis and trans, but rather the STABILITY of the ring. You're right - bulky groups prefer equatorial because this increases stability.

Hope this helps.

 

[JNew]

Well I don't have the question in front of me but i assume they were asking about the relationship between the sterochem and stability between the 1,3 and 1,4 positions on the cyclohexane. If you draw out a boat config. and put a bulky group in both the 1,3 and 1,4 positions, you will see that in the 1,3 position, in order to put the bulky group equatorial, both bulky groups will be below the H, representing a cis configuration. However, in the 1,4 position, c-4 will have its group equatorial when it is above the H, therefore when one group is above the H and one below, this is the trans position. Therefore, the more stable 1,3 position is cis, and 1,4 is trans.