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I drew the two chair conformers of Beta D-glucose: enter image description here

Which one is more stable? I think it's the one with all of the OH groups in the equatorial position because of less steric hindrance, but I also know that the anomeric effect makes the axial one favored as well. So I'm unsure which is more stable.

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3 Answers 3

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I also know that the anomeric effect makes the axial one favored as well.

The anomeric effect does stabilize an axial group, but only at the anomeric position. Other axial groups are not similarly stabilized. Thus $\alpha$-D-glucose is more stable than you think it should be based on steric considerations alone.

Knowing this, which is the more stable?

PS - very pretty chairs.

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isn't the beta d-glucose the one with the equitorial bonds? –  user176105 Oct 28 '12 at 22:23
    
Both of your structures are beta-D-glucose, just different conformations. alpha-D-glucose is the anomer, and would have the OH at the anomeric position different. Otherwise, the two are the same. –  Ben Norris Oct 28 '12 at 22:41
    
right. so i'm going to say the one on the left is more stable because the anomeric effic is still higher in energy than having all groups equitorial –  user176105 Oct 28 '12 at 22:55
    
And I would agree. –  Ben Norris Oct 29 '12 at 12:33

When glucose forms a ring (think Haworth conformation), carbon one can have its hydroxyl group in one of two conformations: axial (pointing straight down) or equatorial (pointing slightly up). When it is axial, the glucose is alpha. When it is equatorial, it is beta. This doesn't have as much impact on free, monomeric glucose because it can switch back and forth rapidly (though beta is preferred because it is more stable).

When glucose is polymerized into polysaccharides, though, it matters a lot. If alpha glucose is polymerized you get starch, which is soluble and digestible by humans. If you polymerize beta glucose, however, you get tough, undigestable cellulose.

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I would like to point out that $\beta $-glucan is a $\beta $- linked polysaccharide of dextrose. It has (low) water solubility, unlike cellulose. Because we lack an enzyme to digest $\beta $- linked polysaccharides, it is also difficult to digest. –  LDC3 Aug 9 at 14:09

i think in alpha d glucose the position of OH group and OH of CH2OH are trans so having trans is more stable than having cis.

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Welcome to Chemistry.SE. We like to see answers that are based in experimental results, data, literature, or accepted theory. Could you rephrase your answer so that it does not sound like an opinion? –  Ben Norris Apr 18 '13 at 20:20

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