Chair conformation stability - 1,3-diaxial Interactions

[fittycent]

Ok, so I'm watching Chad's videos and it's obvious to me why 1,3-diaxial interactions occur - sterics.

I don't understand why equatorial positions are more stable.

I don't understand why there aren't any equatorial interactions that would result in a less stable chair conformation. For example, on the image I linked, if you had two large groups on the equatorial positions for C1 and C2, how do those not have significant (if not more significant than 1,3-diaxial) interactions? It just seems like they're pointed in the general direction that would make them as close (if not closer) than those same groups if they were in the 1,3 or 1,5 axial positions.

https://d2vlcm61l7u1fs.cloudfront.net/media/0d3/0d3b78fc-8a18-4805-a6c6-8c5b248b83f9/phpP39b9J.png (ignore the accompanying text, I was just trying to find a numbered chair conformation to use as a reference here)

Am I just visualizing this wrong or something? Sorry if this is a dumb question, it's been over five years since I took organic. Any help is much appreciated. Thanks!

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[w4nt2baDDS]

One way to think of this problem is in terms of cis or trans. 1,3 or 1,5 axial side groups are in cis with each other, they are on the same side of the ring which causes steric strain. 1,2 equatorial r-groups are in trans with one another, pointing towards opposite sides of the ring (one points downward and the other points upward). This alone extremely diminishes the steric strain . Also, 1 axial and 2 equatorial are in cis confirmation with each other and that would be less stable than 1,2 equatorial groups. However, from the imagine provided it would appear that 1 axial and 2 equatorial molecules would be further apart from each other than 1,2 equatorial groups -- which isn't the case and thus the image can be a bit misleading.

Your proposed question is difficult though because the idea of having less hindrance on adjacent trans molecules than separated 1,3 cis molecules isn't always that intuitive.

I haven't worked o-chem problems in 6 months, but that is my reasoning behind things.

 

[faceman]

This is not usually covered in depth in undergraduate orgo, but I learned from a master in chem that another main reason for instability in axial positions is the increased gauche interactions with all surrounding locked groups. Draw Newmans and you can see it easily. In axial positions there are more gauche interactions leading to increased steric interference.

Also, the axial positions are just closer to each other in terms of molecular distance and therefore have stronger Van der Waals Repulsion. These repulsive forces are far lower in Equatorial positions b.c. the molecular orbitals don't come nearly as close.

http://research.cm.utexas.edu/nbauld/teach/cyclohex.html

 

[DAT Destroyer]

Ok, so I'm watching Chad's videos and it's obvious to me why 1,3-diaxial interactions occur - sterics.

I don't understand why equatorial positions are more stable.

I don't understand why there aren't any equatorial interactions that would result in a less stable chair conformation. For example, on the image I linked, if you had two large groups on the equatorial positions for C1 and C2, how do those not have significant (if not more significant than 1,3-diaxial) interactions? It just seems like they're pointed in the general direction that would make them as close (if not closer) than those same groups if they were in the 1,3 or 1,5 axial positions.

https://d2vlcm61l7u1fs.cloudfront.net/media/0d3/0d3b78fc-8a18-4805-a6c6-8c5b248b83f9/phpP39b9J.png (ignore the accompanying text, I was just trying to find a numbered chair conformation to use as a reference here)

Am I just visualizing this wrong or something? Sorry if this is a dumb question, it's been over five years since I took organic. Any help is much appreciated. Thanks!

You posed a very good question and the best way to approach this is to get a set of models. This will convince you that when both groups are equatorial they are 60 degrees apart in a gauche relationship. By placing groups equatorial, we reduce the 1,3 - diaxial interactions.

Hope this helps..

Dr. Jim Romano