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Topic: stability of cis- and tran- disubstituted cyclohexane?  (Read 7933 times)

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Jean

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stability of cis- and tran- disubstituted cyclohexane?
« on: November 16, 2004, 07:03:58 AM »
I am wondering how to determine the stability of cis- and trans- disubstituted cyclohexane.
Should I calculate the average energy of the ring-flipped isomers and then compare the average energy between them?
Thanks.

Jean

Offline maxyoung

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Re:stability of cis- and tran- disubstituted cyclohexane?
« Reply #1 on: November 16, 2004, 10:18:54 AM »
for 1,2 disubstituted cyclohexane, trans(most stable conformation with two e bonds) is more stable than cis(most stable conformation with one e bond and one a bond). Just compare the stability of their most stable conformations.

Jean

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Re:stability of cis- and tran- disubstituted cyclohexane?
« Reply #2 on: November 16, 2004, 11:20:07 AM »
sorry, what is a bond and what is e bond. why don't they compare the energy?
Thanks any way ::) ???

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Re:stability of cis- and tran- disubstituted cyclohexane?
« Reply #3 on: November 16, 2004, 11:48:03 AM »
I assume "a" and "e" refer to axial and equatorial groups.

Your initial idea of comparing the energies of the various ring flipped conformations of the two compounds should give you a good idea.  The major interactions to look at are the 1,3-diaxial interactions and the 1,2-gauche interactions.

My off-the-cuff calculations for 1,2-dimethyl cyclohexane suggest an energy difference of about 1.8 kcal/mol between the most stable conformations of the cis and trans isomers (trans being more stable).

Jean

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Re:stability of cis- and tran- disubstituted cyclohexane?
« Reply #4 on: November 16, 2004, 01:02:50 PM »
 :) thank you. I got it.
The average energy depends mostly on the most stable one. We can calculate the K of the these two isomers. If the energies of these two isomers are big, the most sbale one always occupied >90%. We just need to calculate the energy of the most stable one and compare them.
 :P

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