[krishnaraj]

Among the following, which should be the most stable compound?
1)Cis-cyclohexane-1,2-diol
2)Trans-cyclohexane-1,2-diol
3)Cis-cyclohexane-1,3-diol
4)Trans-cyclohexane-1,3-diol

My thought process is-cis isomers with adjacent OH groups lead to Hydrogen bonding so 1) should be the most stable, I'm not sure if repulsion will be the dominating factor and change the answer- what is the correct way to think about this?

[mjc123]

Think about what conformational isomers are possible.

[krishnaraj]

isn't the main difference among the option geometrical isomerism? How would conformational isomers be relevant here?

[mjc123]

As I said, think about what conformers are possible. Draw them. What makes one more stable than another?

[Babcock_Hall]

@OP, Are cyclohexane rings flat, or does the ring pucker?

[krishnaraj]

cyclohexane has boat, chair, half-chair forms- the stability is mainly dependent on minimum repulsive interactions in these forms. Therefore the chair form is more stable than half chair or boat forms. But this doesn't tell us anything about the stability of the options since they are all the same except in the arrangement of two substituents. Now these substituents happen to show hydrogen bonding when placed next to each other in cis form- so should be more stable.

[Babcock_Hall]

In the absence of hydrogen bonding, not all chair forms of derivatives of cyclohexane are equal in energy.  Moreover, you seem to be assuming that the cis-form will experience hydrogen bonding whereas the trans-form will not.  Can you justify that assumption?

[krishnaraj]

I have never encountered different chair forms- I thought only one form was called 'chair'. Till now I was under the impression that trans meant totally on opposite sides but with some help I looked at a model and realized that equatorial position is equidistant from vicinal trans and cis positions so both 1) and 2) can be correct.
How do we justify that 2) is the better option then since the distance seems almost the same in both

[krishnaraj]

Alright, confusingly -I found that the answer key says the most stable form is cis-cyclohexane-1,3-diol

[Babcock_Hall]

I would tentatively assume that hydrogen bonding was not intended to be part of this question.  In other words, the question may have been formulated to consider only sterics of the substituents on the cyclohexane ring.  Have you encountered axial versus equatorial substituents?  Have you encountered 1,3-diaxial strain?  Without those concepts, it would not be easy to explain (although a model might help).  If you can make a model of a derivative of cyclohexane, you can see that there are two chair forms.  When they interconvert, the axial substituents become equatorial and vice versa.