Trans vs. Cis

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Trans vs. Cis

Postby cgarcia » Fri Mar 14, 2014 11:49 am

I understand that a trans a,a chair conformation is less stable than a trans e,e chair conformation and that a cis a,e chair conformation is just as stable as a cis e,a chair conformation, but out of curiosity are the cis chair conformations (both a,e and e,a) in between those chair conformations in regards to stability?

In other words, is the statement trans a,a > cis (a,e or e,a) > trans e,e in terms of stability correct?

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Joined: Sat Nov 23, 2013 3:00 am

Re: Trans vs. Cis

Postby irisc23 » Fri Mar 14, 2014 3:08 pm

Yes that is correct for most cases. There are a few caveats for using such a broad generalization though: 1) If you have substituents of different sizes then cis a,e is not equal in energy to cis e,a. The one that puts the larger substituent in the equatorial position is more stable. 2) If you have two substituents connected to consecutively bonded carbons (e.g. C1 and C2), then trans e,e may not be the most stable. You can verify this by building a model with the modeling kit, or more effectively, drawing a space-filling model on Chem3D. The two adjacent equatorial substituents will be competing for space with each other, since they are large regions of electron density confined in a small space. In some cases it would even be preferable to have the 1,3,5-diaxial interactions for one of the substituents (the smaller one of course) than to have the steric interactions between the two equatorial regions. However this was not discussed in Chem 14B and will not be tested on the final (I've asked Dr. Lavelle in office hours)

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