0
$\begingroup$

Why is cis-1,2-cyclohexadiol less polar than trans-1,2-cyclohexadiol? I know it has something to do with stability and chair conformations, but I'm not sure how it relates to polarity. I know cis-1,2-cyclohexadiol has a more stable chair conformation than trans.

$\endgroup$
  • $\begingroup$ Does the cis compound have an internal hydrogen bond which decreases the polarity of the molecule? $\endgroup$ – user1945827 Apr 13 '17 at 7:31
  • $\begingroup$ Could you indicate what the difference in dipole is, and perhaps add pics of the structures? $\endgroup$ – porphyrin Apr 13 '17 at 12:38
1
$\begingroup$

If I assume you refer to 1,2-cylohexanediols, the two isomers to consider were (cis), and (trans)

enter image description here

than there are three conformers to consider. For the (cis)-configuration, there is only the (axial, equatorial) orientation of the two O-substitutents:

enter image description here

This contrasts to the case of the (trans)-configuration, where the two substitutents may be either in axial, or (likely preferred by thermodynamics) equatorial orientation:

enter image description here

From the later picture, taking into account i) the relative orientation of the two hydroxyl groups towards each other as well in respect to the cyclohexane moiety and ii) the (assumed) conformational preference for this form over the conformer with two axial oriented hydroxyl groups, I would assume the two individual vectorial contributions along $\ce{C -> O}$ are better lined up than in the instance of the (cis)-configuration to yield a larger (global) dipolar moment.

$\endgroup$
  • $\begingroup$ The dihedral angle of the cis- structure is -28.2° while that of the trans- is 77.6° when drawn in Avogadro and geometry optimised using the molecular mechanics software installed. Wouldn't vector algebra suggest that the cis- structure has a greater dipole moment? Hence my question about internal hydrogen bonds mitigating the polarity of C-O bond. $\endgroup$ – user1945827 Apr 14 '17 at 7:35
  • $\begingroup$ I found some values on the web which are 2.33 D and 2.39 D for cis and trans, so probably identical within error (not quoted with data). I calculated (v. simply using bond dipoles from textbook and cyclohexane geometry) and got 1.99D cis and 2.1D trans (so also similar and surprisingly accurate considering model used) but only 0.16D for trans a-a isomer in your bottom figure. So it would seem trans o-e is most stable as expt values are similar. $\endgroup$ – porphyrin Apr 15 '17 at 10:01
  • $\begingroup$ @porphyrin I refrained from bringing all three structures into Avogadro. Beside it is currently not accessible for me, I am not aware if applying MMFF94 (as maybe more suitable than UFF) and its implementation in Avogadro are tools allowing to discern the situtation between the three instances (especially trans-configuration as in axial, axial vs. equatorial, equatorial orientation) and hence addresses the question "correctly" within its allowances by theory. The small differences (experimentally, and by calculation) you fond suggest me, here "no" is more likely than anticipated by mine. $\endgroup$ – Buttonwood Apr 15 '17 at 15:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.