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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: narutoverse13 on October 06, 2015, 03:14:03 PM

Title: Which one is the more stable isomer?
Post by: narutoverse13 on October 06, 2015, 03:14:03 PM

Hello!

I've been trying to figure out what the most stable structure of inositol is, but I keep running in to conflicting information. I'm not sure whether the most stable conformation is the myo-inositol form or the scyllo-inositol. All of the stereoisomers can be seen on wikipedia at this link under "Isomers and Structures" : https://en.wikipedia.org/wiki/Inositol . In that section in Wikipedia, it says that the myo-inositol form is the most stable. This does not make sense to me though. If you look at the chair conformations of the myo and scyllo forms of inositol, then that doesn't make sense: http://www.tcichemicals.com/en/us/support-download/tcimail/application/125-12.html . You can see that the myo-form has 5 equatorial and 1 axis, but the scyllo form has all equatorial hydroxyls. Since scyllo form has all equatorials, doesn't that make it the more stable structure?

Title: Re: Which one is the more stable isomer?
Post by: Enthalpy on October 06, 2015, 08:54:02 PM

Hi narutoverse13, welcome here!

I wonder: would the hydrigen bonds be strong enough to decide the best structure?
And: do you consider the molecule alone in vacuum, or in a solid, a solvent...?

Title: Re: Which one is the more stable isomer?
Post by: Babcock_Hall on October 06, 2015, 09:16:02 PM

Quote from: narutoverse13 on October 06, 2015, 03:14:03 PM

Hello!

I've been trying to figure out what the most stable structure of inositol is, but I keep running in to conflicting information. I'm not sure whether the most stable conformation is the myo-inositol form or the scyllo-inositol.

I would have said "configuration," rather than "conformation."  Enthalpy makes some good points about stability.  Is there any experimental information that bears on which is more stable?

Title: Re: Which one is the more stable isomer?
Post by: Enthalpy on October 07, 2015, 06:33:41 AM

Out of curiosity, I've tried to run Am1 on scyllo- and myo-inositol, and if software is any credible, then
- The difference is only 1/4 kcal. If an easy deformation had permitted to go from one to the other, the room temperature would do it.
- Scyllo has indeed a lower heat of formation. This is for one single molecule alone in vacuum, not for a solid, and details like the positions of hydrogens can flip the comparison.
- Oxygens look a tiny bit farther apart with just one axial. Not uncommon, and it sounds rather logic to me.
- A second axial oxygen at para location to the first one and to the other face of the cyclohexane gains 1/2 kcal further.
- Notice that Wiki claims: "myo is more stable in chair", not "myo is more stable than scyllo".
Screenshot attached, log in to see it.

Title: Re: Which one is the more stable isomer?
Post by: narutoverse13 on October 08, 2015, 12:51:03 AM

Thank you! That helps out a lot!