D and L nomenclature

[echoyjeff222]

I wanted to make sure I understood the terminiology. Is this the right classification?

D and L forms of the same numbered carbohydrate are enantiomers of one another and have opposite optical rotations from one another

D and L forms of sugars (in general) differ at the penultimate carbon in stereochemistry


I didn't learn about carbs in orgo, so I'm trying to understand the terminology. The enantiomer rule applies ONLY to carbohydrates of the same # of carbons, correct? Thanks!

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[NextStepTutor_1]

I wanted to make sure I understood the terminiology. Is this the right classification?

D and L forms of the same numbered carbohydrate are enantiomers of one another and have opposite optical rotations from one another

D and L forms of sugars (in general) differ at the penultimate carbon in stereochemistry


I didn't learn about carbs in orgo, so I'm trying to understand the terminology. The enantiomer rule applies ONLY to carbohydrates of the same # of carbons, correct? Thanks!

D- and L- sugars with the same carbohydrate name are enantiomers. For example, D-(+)-glucose and L-(-)-glucose are enantiomers and so all of their chiral carbons have changed chirality.

Because they are enantiomers they have equal but opposite optical rotations (one has +# and the other has -#). In terms of stereochemistry, all chiral carbons will change chirality, not just the penultimate, because that is part of the definition of enantiomers.
But at a quick glance to distinguish whether a carbohydrate (in it's straight chain form, or Fischer projection) is D- or L- look at the penultimate carbon. If the -OH group is on the right the carb is D-, on the left the carb is L-.

Enantiomers are stereoisomers (same structure, same connectivity, different spatial arrangement) that differ in the chirality of all chiral carbons (ex. one enantiomer has all R chiral carbons, the other has all S). So yes, two carbohydrates with different numbers of carbons in them cannot be stereoisomers and thus cannot be enantiomers.

Hope this helps!


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[echoyjeff222]

"But at a quick glance to distinguish whether a carbohydrate (in it's straight chain form, or Fischer projection) is D- or L- look at the penultimate carbon. If the -OH group is on the right the carb is D-, on the left the carb is L-. "

This can also be used for comparing molecules of different lengths, right?

Thanks for the response!

 

[NextStepTutor_1]

Yes, as long as you are looking at the carbohydrates written as a Fischer projection.