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does the highest energy barrier = most stable?
The one at the top of the hill (highest energy content) is always the LEAST stable. The one at the bottom of the hill (lowest energy content) is always the MOST stable.
 
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Kneecoal

Kneecoal

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mer, sorry, i should be more specific - a problem in topscore asked which has the highest energy barrier to a chair-chair interconversion - the answer was cyclohexane with two methyl groups opposite each other in the equatorial position. so my line of thinking is that that's a very stable structure, so it would have the highest energy barrier to interconverting because it doesn't want to change?
 
May 15, 2009
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mer, sorry, i should be more specific - a problem in topscore asked which has the highest energy barrier to a chair-chair interconversion - the answer was cyclohexane with two methyl groups opposite each other in the equatorial position. so my line of thinking is that that's a very stable structure, so it would have the highest energy barrier to interconverting because it doesn't want to change?
Just wondering: Are the methyl groups on the same carbon?

The rule of thumb is that when you have methyl groups on the equatorial position the structure is highly stable. What does highly stable mean?
It means it's located at the bottom of the hill. Therefore, in order to go from one side of the hill to the other (interconversion from one conformation to the other) it must climb the highest energy barrier. So, yes! You're line of thought is correct.
 

joonkimdds

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I was confused too but this is the way I understand it now.
You know that noble gas are octet and very stable so they have super high ionization energy.

Equatorial position is also very stable so making it unstable will require a lot of energy.

And I believe that the question asks which one will require higher energy to change the shape from the most stable to the not so stable position. But at first, I thought they were asking which is most unstable because that would mean which has the highest energy.
 

americanpierg

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mer, sorry, i should be more specific - a problem in topscore asked which has the highest energy barrier to a chair-chair interconversion - the answer was cyclohexane with two methyl groups opposite each other in the equatorial position. so my line of thinking is that that's a very stable structure, so it would have the highest energy barrier to interconverting because it doesn't want to change?
Topscore made a mistake in their explanation. They said that the highest energy conformation for cyclohexanes are when the largest substituents are in the equatorial positions, which is wrong. I think they meant the highest energy barrier to interconversion is when the cyclohexane is already most stable, ie largest substituents are in the equatorial positions.