Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Ren on June 30, 2020, 03:32:34 PM
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α-D-glucopyranose vs β-D-mannopyranose, how they are not the same thing? I mean, I'm looking at both Fisher's and Haworth's projections and it doesn't make sense to me, what is the difference?
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The difference is in carbon two. The -OH on C2 is glucopyranose is equitorial and the -OH on C2 in mannopyranose is axial.
The anomeric centre is at C1 and the alpha and beta only refer to the orientation of the group attached to C1. If it is axial it is the alpha anomer and if it is equitorial it is the beta anomer.
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They differ at the one stereocenter - axial and equatorial only apply to one of the two possible chair forms.
Technically glucose is (2R,3S,4R,5R)-2,3,4,5,6-Pentahydroxyhexanal while mannose is (2S,3S,4R,5R)-2,3,4,5,6-Pentahydroxyhexanal The difference is at C2 - note that glucose is R and mannose is S. This doesn't change when the cyclic hemiacetal is formed.
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Oh boy. Of course. I found some 3D models and see what you described.
(https://i.postimg.cc/MG5whyNF/alpha-D-Glucose-463-200-3-D-Conformer.png)(https://i.postimg.cc/TwPxGKXY/beta-D-Mannose-463-200-3-D-Conformer.png)
The schemas I have in my book seem not to care much about such tiny differences.
(https://i.postimg.cc/2jFtTkp0/Untitled.png)
Thank you both.
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β-D-Glucose and β-D-mannose are epimers; β-D-glucose and β-D-galactose are epimers; but β-D-galactose and β-D-mannose are not epimers. Does that help? Anomers are a special case, and I would treat anomers distinctly from other isomers. The molecules in your original question were different at two positions, one being the anomeric carbon.