Is there a way to faciliate remembering which are alpha/beta hemiacetals of sugars?

Question

I understood the concept of hemiacetals quite well, but have considerable trouble remembering which are called $\alpha$ or $\beta$ respectively.
Is there some concept/mnemonic/common reaction/something else to faciliate remembering this convention?

The short answer is no, not really. Or, more precisely, not that I know of. Probably because $\alpha$ and $\beta$ depends on whether the sugar is a D or an L. I simply learned that for D, $\alpha$ means down. Then everything else logically follows. — Linear Christmas, Jun 19 '16 at 17:23

EJC · Accepted Answer · 2016-06-19T18:36:46.357

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When you draw the Haworth projection, the alpha anomer is written with the -OH group pointing downwards (see the image), when you write the symbol alpha, you start by writing it downwards. For the beta anomer the -OH group is written upwards and when you write the symbol beta, you start writing it upwards. This mnemonic applies only to D-monosaccharides. For L-monosaccharides the opposite applies.

Actually, a better definition is that the alpha isomer has the -OH group pointing in the direction opposite to the -CH2OH group (on carbon 5), the opposite applies for the beta isomer, therefore for L-monosaccharides the -CH2OH group should point downwards and the alpha isomer will have the -OH group pointing upwards.

edited Jun 19 '16 at 18:36

answered Jun 19 '16 at 18:28

EJC

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I think, this is only true for pyranoses. – Jan Jun 20 '16 at 00:40
CC what I just posted under Ringo’s post. – Jan Jun 20 '16 at 00:49
I think this one is easier to remember, but make sure to read ringos post too. – caconyrn Jun 20 '16 at 16:16

score 3 · Answer 2 · edited Jun 11 '20 at 10:20

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When I was studying for my organic chemistry class last semester, I ran across an ACS published paper that provides several useful mnemonics for interconverting Fischer and Haworth projections of monosaccharides. I believe number four is what you are looking for specifically:

Start the ring with the anomeric carbon on the right, and continue to draw the remaining atoms in the ring in a clockwise direction, ending with the ring oxygen at the top of the drawing.

Hydroxyl groups on the right (Dextro-) are drawn Down; those on the left (Levo-) Levitate up.

The last -$\ce{CH2OH}$ group is drawn beLow for an L-sugar; above for a D-sugar.

The hydroxyl group attached to the anomeric carbon and the last -$\ce{CH2OH}$ group are drown Both on the same side of the ring for the $\beta$-anomer; for the $\alpha$-anomer each is Alone.

edited Jun 11 '20 at 10:20

Community

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answered Jun 20 '16 at 00:16

ringo

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Same comment I gave Marko. I think, this only applies to pyranoses. – Jan Jun 20 '16 at 00:41
@Jan the title of the article says monosaccharides. I don't see why it would only apply to hexoses. – ringo Jun 20 '16 at 00:44
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Ah, there’s the problem. It seems like it’s only strictly true for sugars that have a $\ce{CH2OH}$ as final, extracyclic group. It also doesn’t work for galactofuranose. – Jan Jun 20 '16 at 00:49

Is there a way to faciliate remembering which are alpha/beta hemiacetals of sugars?

2 Answers2