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Stability of geometrical isomers

  • #1
Krushnaraj Pandya
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71

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?
 

Answers and Replies

  • #2
TeethWhitener
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1,677
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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.
 
  • #3
Krushnaraj Pandya
Gold Member
697
71
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
 
  • #4
TeethWhitener
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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.
 
  • #5
Krushnaraj Pandya
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697
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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?
 
  • #6
Krushnaraj Pandya
Gold Member
697
71
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
 
  • #7
Krushnaraj Pandya
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697
71
Alright, confusingly -I found that the answer key says the most stable form is cis-cyclohexane-1,3-diol
 
  • #8
TeethWhitener
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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.
 
  • #9
Krushnaraj Pandya
Gold Member
697
71
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
 

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