Stability of geometrical isomers

[Krushnaraj Pandya]

Homework Statement

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

Homework Equations

--

The Attempt at a Solution

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?

 

[TeethWhitener]

Homework Statement

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

Homework Equations

--

The Attempt at a Solution

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?

Hydrogen bonding is the right idea, but why favor 1) over 2)? If the OH groups are trans equatorial, their geometrical relationship is the same as for cis gauche. Also, repulsion will definitely be a factor for the vicinal diols.

 

[Krushnaraj Pandya]

Hydrogen bonding is the right idea, but why favor 1) over 2)? If the OH groups are trans equatorial, their geometrical relationship is the same as for cis gauche. Also, repulsion will definitely be a factor for the vicinal diols.

doesn't trans mean they're farther away therefore reducing the effectiveness of hydrogen bonding? At least relative to the cis configuration

 

[TeethWhitener]

doesn't trans mean they're farther away therefore reducing the effectiveness of hydrogen bonding? At least relative to the cis configuration

This is where a molecular model comes in handy. When one OH is in the equatorial position of the chair conformation, it is equidistant from the vicinal cis and vicinal trans positions.

 

[Krushnaraj Pandya]

Alright, I looked at the model and saw the truth in that. But now both 1) and 2) can be correct- how do we gather that 2 is the correct option?

 

[Krushnaraj Pandya]

Also the fact that I won't have models in the exam puts me at unease...but I'll just have to remember I guess

 

[Krushnaraj Pandya]

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

 

[TeethWhitener]

Alright, I looked at the model and saw the truth in that. But now both 1) and 2) can be correct- how do we gather that 2 is the correct option?

2 is not the correct option. 1 and 2 will be roughly equal in stability. My point was to steer you away from 1 and 2.

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

Yes. Hence in post 2 where I said that hydrogen bonding is key. The distances involved in hydrogen bonding are optimal in the 1,3 diol. The distances are too small (and roughly equal) for the vicinal diols.

 

[Krushnaraj Pandya]

sorry for the late reply, we just have so much coursework that it is proving hard to review previous questions. Please forgive me if I forget to thank you for taking the time to help me, I'll try to reply with a better understanding as soon as I can, thank you :D