Cis and Trans and Stability

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Alex Nguyen 3I
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Cis and Trans and Stability

Postby Alex Nguyen 3I » Fri Mar 11, 2016 8:47 pm

Can cis and trans for a chair conformation only exist for 2 and only 2 substituents (what if there's 3 or 4)? Also, does the stability of a cis or trans chair change depending on how close or far they are to each other (all in equatorial position)? Are substituents that are furthest apart most stable? Also, is trans more stable than cis because of torsional strain?

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Re: Cis and Trans and Stability

Postby Chem_Mod » Fri Mar 11, 2016 10:38 pm

If there's more than 2, you would have to specify which substituents are cis and/or trans to which other substituents. The stability does change for cis/trans depending on which carbons the substituents are attached to. Basically you want to optimize for the most substituents being equatorial. For instance, trans-1,2-dimethylcyclohexane is more stable than cis-1,2-dimethylcyclohexane, you can get 2 equatorial positions with trans. But trans-1,3-dimethylcyclohexane is less stable than cis-1,3-dimethylcyclohexane, because you can get 2 equatorial positions with cis.

The reason why equatorial is more stable is because you're avoiding strain that's caused by substituents repelling each other when they're in the axial position.

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