# Why is the A-value of methoxy group lower than that of the hydroxyl group?

I am studying conformational analysis of monosubstituted cyclohexane. "A-value" refers to the energy difference between conformers with substitutent in equatorial (more favored) and axial positions. A higher A-value means that the axial conformer will more readily convert to equatorial.

According to Wikipedia, methoxy has A-value of $$0.6 \text{ kcal/mol}$$ while that of hydroxyl is $$0.87 \text{ kcal/mol}$$. Why does methoxy, a bulkier group, have a lower A-value than hydroxyl?

I think that in both cases the group attached to the oxygen faces away from the axial hydrogens, so the size of those groups shouldn't matter very much. That is to say, the A-value of methoxy may be the same as or at the most only slightly larger than that of hydroxyl. This explanation does not suffice as its A-value is in fact smaller.

A possible reason could be that the $$\text{C-O}$$ bond is shorter in methoxy than in hydroxyl. However, I don't understand why this should be true. Hints would be appreciated.

## 1 Answer

I'm hazarding an educated guess here and going to say that the O-C bond is longer than the O-H bond and that there are more interaction between the axial substituents of the H from the hydroxyl group compared to the O-C-H of the longer distant from the other substituent methoxy group.