Which is the more stable structure?

[narutoverse13]

Hello!

I've been trying to figure out what the most stable structure of inositol is, but I keep running into 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?

 

[Ygggdrasil]

Myo- and scyllo- inositol cannot interconvert, so there's not much point in comparing their stability. In fact, the wikipedia article writes:

In its most stable conformational geometry, the myo-inositol isomer assumes the chair conformation, which puts the maximum number of hydroxyls to the equatorial position

It is not saying that myo-inositol is more stable than the other stereoisomers. Rather it's just saying that myo-inositol will adopt the confromation that places five of the hydroxyls in the equatorial position (rather than the conformation that places the five of the hydroxyls in the axial position).

 

[narutoverse13]

Myo- and scyllo- inositol cannot interconvert, so there's not much point in comparing their stability. In fact, the wikipedia article writes:It is not saying that myo-inositol is more stable than the other stereoisomers. Rather it's just saying that myo-inositol will adopt the confromation that places five of the hydroxyls in the equatorial position (rather than the conformation that places the five of the hydroxyls in the axial position).

Myo- and scyllo- inositol cannot interconvert, so there's not much point in comparing their stability. In fact, the wikipedia article writes:It is not saying that myo-inositol is more stable than the other stereoisomers. Rather it's just saying that myo-inositol will adopt the confromation that places five of the hydroxyls in the equatorial position (rather than the conformation that places the five of the hydroxyls in the axial position).

When you say, "myo-inositol will adopt the confromation that places five of the hydroxyls in the equatorial position", does that mean that the most stable isomer will have 5 axial and 1 equatorial hydrogens? Or should it be that the more stable stereoisomer has 6 hydroxyls that are all equatorial? My homework asks to draw all the isomers of the inositols (which I assume is 9 by the number of isomers on wikipedia) and choose the most stable one. Without thinking about interconversion, I just need to know which one of the 9 stereoisomers is the most stable. When they're all converted to chair forms (second link), the scyllo- inositol has all of the hydroxyl groups placed on equatorial positions, making me think that the scyllo-inositol is the most stable. Would I be correct?

 

[Ygggdrasil]

The equilibrium that the wikipedia article was referring to is regard to the two different chair conformations that myo-inositol can adopt through "flipping" the ring:

One conformer places five hydroxyls in the equatorial position and therefore is favored over the conformation that places five hydroxyls in the axial position.

Anyway, I would agree with your answer that scyllo-inositol would be the most stable stereoisomer of inositol because it can adopt a configuration where all hydroxyls are equatorial.