Cyclohexyl methyl ether

The MeO group has a conformational energy of 0.75 kcal/mol, puzzlingly lower than that of the OH (0.9 kcal/mol), even though the MeO group is larger than the OH one. The C-O bond is single and can rotate around itself as the C-C bonds do. The Me group can thus be situated outwards the ring avoiding its interaction with the syn-diaxial hydrogens at cyclohexane's positions 3 and 5. Following this reasoning -similar to that used for the couple methyl/ethyl-, the MeO and OH should have displayed the same conformational energy. However, they do not. Why? Which of the two is the dissonant one?

The most plausible explanation is that OH is the anomalous one, presenting too large a conformational energy. Cyclohexanol is able to hydrogen bond to neighboring molecules by sharing the oxygen's electron lone pairs with the OH's hydrogen of a close molecule. This fact increases the apparent size of the OH and makes it display a conformational energy larger than expected.

IMPORTANT: Will the conformational energy of cyclohexanol and methoxycyclohexane vary depending on the solvent where the concentrations of the axial and equatorial conformers are measured?

The answer is YES but not that simple!!!. Let's compare the measurements made in two different solvents: hexane and methanol. Hexane is non-polar and unable to form hydrogen bonds. On its part, methanol is highly polar and hydrogen bonding capable. Methanol strongly associates to both cyclohexanol and methoxycyclohexane. In methanol, cyclohexanol stops associating to itself but to methanol but its apparent size wouldn't change much. However, methoxycyclohexane, which cannot self-associate,, starts hydrogen-bonding to methanol, increasing its apparent size. Therefore, the conformational energies of cyclohexanol and methoxycyclohexane in methanol end up being very similar indeed. In hexane the differences would be maintained unless we measure cyclohexanol in very diluted samples. Dilution hampers intermolecular hydrogen bonding and avoids cyclohexanol self-association thus making the OH display its 'normal' size.

What if we measure the conformational equilibria in acetone? Acetone is polar and a hydrogen-bonding acceptor, NOT DONOR. Acetone will hydrogen bond to cyclohexanol but not to methoxycyclohexane. Therefore, both conformational energies, the 'unexpectedly large' of cyclohexanol -it hydrogen bonds to acetone and the OH apparent size will be large- and the 'normal one' of methoxycyclohexane -it cannot associate to acetone- will be maintained in that solvent.