December 07, 2022, 12:38:25 AM
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Topic: Does this question give about cyclohexane conformations give enough information?  (Read 1361 times)

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Offline Felagund

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Hello all. This is my first time posting on this forum so please let me know if I make any mistakes. I received the following question in organic chemistry, and I don't think it provides sufficient information for an answer. "Draw 1,2-dimethylcyclohexane and its ring flip isomer (conformational isomer). Circle the most stable conformation." I think that it would be necessary to know whether the chemical in question was cis or trans 1,2-dimethylcyclohexane in order to determine the stability of its conformations. If it were trans, the conformation that put both methyl groups in the equatorial position would be more stable, but if it were cis, both conformations would be of equal stability. My teacher wrote trans in his answer sheet, so I think he might have added it to the problem in class, but I wanted to make sure I wasn't missing anything. Thank you!

Offline spirochete

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Yes, you have the right idea. If it doesn't say cis or trans, the question is technically ambiguous.  It could be phrased to say "draw the most stable stereoisomer of 1,2-dimethylcyclohexane in its most stable conformation". Then you would have to think about how the trans one is more stable in this case.

You are completely correct. The person who wrote the question might say "well, you knew what I was trying to say." But your issue with the premise of the question is reasonable. If a molecule has diastereomers, at the very least the question should state what diastereomer it is referring to. Cis and trans rings with the same connectivity are examples of diastereomers.

Offline Babcock_Hall

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I think that the question could be worded more clearly, and if I were grading it, I would accept all reasonable answers.  I could, however, make the argument that one chair conformation of the trans-isomer (the one with both methyl groups equatorial) is the most stable of all possibilities.

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