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Topic: Any easy way to tell if a sugar/carb is D or L alpha or beta?  (Read 22632 times)

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Offline 1101

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It is my understanding that the only way to tell if a sugar or carbohydrate (and i tend to get those mixed up...) is to draw its Fischer projection and look to see if the OH group on the anomeric carbon is on the left (L) or right (D). Then to tell if it is alpha or beta you have to convert the fischer into a Haworth projection and see if the anomeric OH group is above the ring (alpha if its L, beta if its D) or below the ring (alpha if its D, beta if its L).  I was hoping someone out there had a simpler way of distinguishing D,L,alpha, and beta. Or for that matter any other way (maybe i'll find it simpler).

Offline Dan

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Re: Any easy way to tell if a sugar/carb is D or L alpha or beta?
« Reply #1 on: April 01, 2010, 04:33:03 AM »
It is my understanding that the only way to tell if a sugar or carbohydrate (and i tend to get those mixed up...) is to draw its Fischer projection and look to see if the OH group on the anomeric carbon is on the left (L) or right (D).

Unfortunately not, no. To tell whether a sugar is D or L you need to look at the last stereogenic centre in the Fischer projection (not the anomeric), then left is L, right is D

Quote
Then to tell if it is alpha or beta you have to convert the fischer into a Haworth projection and see if the anomeric OH group is above the ring (alpha if its L, beta if its D) or below the ring (alpha if its D, beta if its L).

This is not reliable as it depends on what conformation/orientation you draw the ring. The technical definition of alpha is when the anomeric centre has the same configuration as the last stereogenic centre when drawn in the Fischer projection, and beta is when those two centres have different configuration in the Fischer projection.

Luckily, there are some shortcuts to quickly identify alpha/beta. I wrote a model answer for my students a while back which is publicly available, it can be found here - the first 3 pages will be of particular interest:

https://www.dropbox.com/s/ewwebmetzavpbcx/DL%20anomers.pdf


Edit: Link fixed
« Last Edit: June 23, 2012, 10:01:14 PM by Dan »
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