bobeanie95

2+ Year Member
Apr 19, 2016
224
137
Status
Pre-Medical
The question is: the stereochemical designators alpha and beta distinguish between:

Answer: Epimers at the anomeric carbon.

I thought the answer was epimers at non-anomeric carbons.

From my understanding, D and L distinguish between epimers at the anomeric carbon, meanwhile alpha and beta simply denote the orientation of the hydroxyl group that experienced nucleophilic attack at the carbonyl carbon. Could someone clear this up?
 

theonlytycrane

5+ Year Member
Mar 23, 2014
1,956
1,687
Status
Medical Student
D/L distinguish between epimers at the last chiral carbon in fischer form (carbon 5 in glucose).

The alpha / beta distinction is specific to the -OH group orientation on the anomeric carbon, which is another name for the carbonyl carbon that gets attacked as you mentioned.
 

Alpha-ketogluterate

2+ Year Member
Jun 25, 2017
18
3
D/L distinguish between epimers at the last chiral carbon in fischer form (carbon 5 in glucose).

The alpha / beta distinction is specific to the -OH group orientation on the anomeric carbon, which is another name for the carbonyl carbon that gets attacked as you mentioned.

Hey, sorry for reviving this old thread, you may not even remember it anymore. But you mentioned D and L distinguish between "epimers". Are you sure about this? D and L desginate enantiomers, that means they distinguish a difference at EVERY carbon, and also by default would NOT be epimers since more than 1 carbon is diff in chirality. Yes this does come to be a change in the turn of the arm when drawn in the hawthorne projection, but in Fischer, D and L, much like with Amino Acids, distinguish between enantiomers not epimers. Or so I believe.

Anomers are a substet of epimers which are a subset of diastreomers.

Now another question about this particular AAMC question, the question is asking about Alpha and Beta, but the answer reads the following:
"The answer to this item is C since the D versus L designation distinguishes between molecules with multiple chiral centers, but differ only in the configuration of the site known as the anomeric carbon atom."

I have 2 issues with this explanation. First Alpha vs Beta has naught to do with D and L, so why is this even mentioned? And secondly as I explained above, D and L are enantiomers not epimers, and even if one was to consider them in the hawthorne projection where it "seems" only one carbon is changing, it is certainly not the anomeric one... The anomeric carbon is distinguished by Alpha and Beta.

Again to clarify, I do not have a problem with the question or the proposed answer, they are fine. It is the explanation which is very puzzling to me.