Specific Rotation - Epimers and Anomers

[rpdnathan]

I know that enatiomers of a compound exhibit same specific rotation (if they are optically active) - however, I am not clear how to whether epimers of carbohydrates have the same specific rotation.

BR tells me that D-mannose and D- glucose have the same specific rotation (although they are diasteromers). This doesn't make sense to me. Please clarify.

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[ponyo]

I know that enatiomers of a compound exhibit same specific rotation (if they are optically active) - however, I am not clear how to whether epimers of carbohydrates have the same specific rotation.

BR tells me that D-mannose and D- glucose have the same specific rotation (although they are diasteromers). This doesn't make sense to me. Please clarify.

I think that's wrong. Enantiomers and only enantiomers are optical rotation pairs. D-glucose's enantiomer is L-glucose, not D-mannose...

I'm pretty sure the only epimers that are also enantiomers are L and D glyceraldehyde.

 

[Johnnywithnoh]

It is because it is asking the specific rotation of the osazone. Notice that the osazone changes the 2nd carbon, so it wouldn't matter what the chirality is for the second carbon. Thus D. is wrong because the C-2 epimer after osazone generates the same osazone formation, leading to the same specific rotation. The passage says that the rotation should be different, so it can't be D.

BR is correct in saying that glucose and mannose have the same specific rotation AFTER osazone, since the chirality is lost at carbon 2

hope it helped

 

[BerkReviewTeach]

I know that enatiomers of a compound exhibit same specific rotation (if they are optically active) - however, I am not clear how to whether epimers of carbohydrates have the same specific rotation.

BR tells me that D-mannose and D- glucose have the same specific rotation (although they are diasteromers). This doesn't make sense to me. Please clarify.

Enantiomers are mirror images that have exact opposite optical rotations. +X degrees and -X degrees. But epimers are diastereomers of one another when the carbon is four or more, so epimers do not share the magnitude of rotation. Except in the three-carbon case of glyceraldhyde (as ponyo pointed out--good job ponyo).

What D-glucose and D-mannose share is identical structure from carbon 3 down (being that they are C-2 epimers). As Johnnywithnoh points out, C-2 epimers lead to the same osazone (a chemical test to determine if sugars are C-2 epimers). His post is so good that I can't add much more than a "Good job Johnny; excellent post!"