D-glucopyranose and the anomeric effect

[sibitrum]

Why is beta-D-glucopyrnaose more stable than alpha-D-glucopyranose? Isn't the alpha anomer heavily stabilized by donation of the oxygen lone pair into the anti-bonding orbital of C-O?

Thanks

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[PingPongPro]

Whenever I see ring structures and stability I am conditioned to think of steric hinderance or some sort of ring strain.

In this case, the beta conformation of glucopyranose is preferred because it allows for all the OH groups to be equatorial in the chair conformation. The alpha conformation has 1 OH group in the axial, thus making it less favored.

 

[GreenRabbit]

Also, any stabilization via lone pairs doesn't exist in aqueous solution.

 

[orgohacks]

Why is beta-D-glucopyrnaose more stable than alpha-D-glucopyranose? Isn't the alpha anomer heavily stabilized by donation of the oxygen lone pair into the anti-bonding orbital of C-O?

Thanks

It is, but the donation can only happen if the C-O is axial (i.e. beta). Look up "anomeric effect" in wikipedia. BTW this is far far beyond what you'd be expected to know in the MCAT.

 

[movax]

Why is beta-D-glucopyrnaose more stable than alpha-D-glucopyranose? Isn't the alpha anomer heavily stabilized by donation of the oxygen lone pair into the anti-bonding orbital of C-O?

Thanks

A dirty MCAT "hack" for this type of question would be to identify the substances in question for what they are (six-membered rings), remember that they can go chair / boat / whatever, and then remember that you want your heavy substituents in the lazy, stable equatorial position, not axial.

 

[sibitrum]

It is, but the donation can only happen if the C-O is axial (i.e. beta). Look up "anomeric effect" in wikipedia. BTW this is far far beyond what you'd be expected to know in the MCAT.

Not to belabor the point but, I believe beta is equitorial (sorry for the absurdly large image).