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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?

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    $\begingroup$ 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. $\endgroup$ – Linear Christmas Jun 19 '16 at 17:23
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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.enter image description here

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  • $\begingroup$ I think, this is only true for pyranoses. $\endgroup$ – Jan Jun 20 '16 at 0:40
  • $\begingroup$ CC what I just posted under Ringo’s post. $\endgroup$ – Jan Jun 20 '16 at 0:49
  • $\begingroup$ I think this one is easier to remember, but make sure to read ringos post too. $\endgroup$ – caconyrn Jun 20 '16 at 16:16
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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:

  1. 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.

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

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

  4. 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.

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  • $\begingroup$ Same comment I gave Marko. I think, this only applies to pyranoses. $\endgroup$ – Jan Jun 20 '16 at 0:41
  • $\begingroup$ @Jan the title of the article says monosaccharides. I don't see why it would only apply to hexoses. $\endgroup$ – ringo Jun 20 '16 at 0:44
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    $\begingroup$ 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. $\endgroup$ – Jan Jun 20 '16 at 0:49

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