Disubstituted Cyclohexane

[Temperature101]

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Why 1, 2-Diequatorial > 1, 2-Diaxial by 2.8 kcals/mole for Trans-1,2-dimethylcyclohexane?
I thought this should be the reverse since the equatorial position is more stable; therefore, has lower energy. I think the the equatorial position should be less than the axial rather than greater.

 

[DrSmart21]

Try drawing it out..Yes you are correct that equitorial is the most favored position, more stable and thus lower energy..but since it is a 1,2 cyclohexane you need to consider the steric hindrance that the structure has in equitorial v.s axial positions.

 

[Charles Darwin]

^^^^

1,2-diaxial puts the substituents on different sides of the ring; 1,2-diequatorial puts them smack dab next to one another.

 

[LoLCareerGoals]

Yup and methyl <-> methyl strain is much stronger than whatever extra methyl <-> hydrogen strain that might be introduced by 1-2 diaxial.

Edit:
Ok while I agree with you guys intuitively, I am once again wtfed by the book I am using. Page 117 of Organic chemistry by Wade contradicts this.
I can pm a page to you if you know how to post an image.

double edit:
Additionally here is a link discussing the same:
http://classes.yale.edu/01-02/chem220a/studyaids/movies/dimecyclohex_trans.html
I think we might be wrong on this one. OP where did you see that 1, 2-Diequatorial > 1, 2-Diaxial by 2.8???

 

[shaboobly]

just remember you cant put giant substituents right next to each other because of sterics. normally when a molecule is unstable, there is too much sterics somewhere.

equatorial positions are stable only for 1 giant group.. if you try to put 2 giant groups in the same area... well you can't have 2 giant things occupying 1 small area.

 

[CodeRedDew]

I would say that 1,2 diequatorial is definitely more stable that 1,2 diaxial. Where did you see that the diequatorial conformer is higher in energy? Groups are usually more stable in the equatorial position unless the anomeric effect is displayed, which I believe is beyond the scope of the MCAT.


Sent from my iPhone

 

[V5RED]

Yup and methyl <-> methyl strain is much stronger than whatever extra methyl <-> hydrogen strain that might be introduced by 1-2 diaxial.

Edit:
Ok while I agree with you guys intuitively, I am once again wtfed by the book I am using. Page 117 of Organic chemistry by Wade contradicts this.
I can pm a page to you if you know how to post an image.

double edit:
Additionally here is a link discussing the same:
http://classes.yale.edu/01-02/chem220a/studyaids/movies/dimecyclohex_trans.html
I think we might be wrong on this one. OP where did you see that 1, 2-Diequatorial > 1, 2-Diaxial by 2.8???

I like that the source you cite lists the diaxial as being 2.7 higher in energy because this source also lists the diaxial as being the less stable conformer and by about the same amount (2.8).
http://www.nku.edu/~russellk/tutorial/chex/tme12chex.html
I think it is safe to say that the 2 college websites and the Organic chemistry textbook (I have the same book and can confirm that it says the diaxial is less stable) showing that diaxial is less stable than diequatorial are correct and OP either misread his source or his source is incorrect.

 

[LoLCareerGoals]

Yeah yet another example of not relying on your intuition too much in chemistry.
I was so sure of it too, like several other posters.

 

[Temperature101]

I like that the source you cite lists the diaxial as being 2.7 higher in energy because this source also lists the diaxial as being the less stable conformer and by about the same amount (2.8).
http://www.nku.edu/~russellk/tutorial/chex/tme12chex.html
I think it is safe to say that the 2 college websites and the Organic chemistry textbook (I have the same book and can confirm that it says the diaxial is less stable) showing that diaxial is less stable than diequatorial are correct and OP either misread his source or his source is incorrect.

This is from TBR Structure Elucidation chapter page 113...
1, 2-Diequatorial > 1, 2-Diaxial by 2.8 kcals/mole for Trans-1,2-dimethylcyclohexane
My interpretation of this might be wrong but I see it as the Diaxial is more stable than the diequatorial.

 

[imbackasd]

I believe it might be stating 1, 2-Diequatorial IS GREATER STABLE THAN ( > sign applying to stability) 1, 2-Diaxial by 2.8 kcals/mole

 

[V5RED]

I believe it might be stating 1, 2-Diequatorial IS GREATER STABLE THAN ( > sign applying to stability) 1, 2-Diaxial by 2.8 kcals/mole

I just read the page, and this is correct. The > sign refers to which is more favored. This is further indicated by the uneven equilibrium arrows which have the larger arrow pointing toward the equatorial form.

This format is used for all the examples of substitution that compare axial and equatorial and all of them are written equatorial>axial. On the prior page, TBR has "axial is less favorable than equatorial orientation" written.

 

[Temperature101]

I believe it might be stating 1, 2-Diequatorial IS GREATER STABLE THAN ( > sign applying to stability) 1, 2-Diaxial by 2.8 kcals/mole

That is a poor way of writing that...I was scatching my head because I did not understand what this sign (>) meant in term of stability...This makes me get 68% on phase I of this chapter because I scew up in on passage that was about cyclohexane.