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Why are D and L-glucose enantiomers???! I thought D/L refers to the "configuration" of the last chiral carbon on the open chain Fischer projection. In the D form, the higher priority group is on the right in the Fischer projection; in the L form the higher priority group is on the left. So why are D and L glucose enantiomers? Shouldn't they be diastereomers??!

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  • $\begingroup$ Diastereomers aren't mirror images. $\endgroup$ – JM97 Oct 30 '16 at 14:36
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Simply by inspecting the two figures of the molecules, you can see that they are in fact enantiomers—non-superimposable mirror images and were they diastereomers instead of enantomers, they wouldn't both be named "Glucose". The D-/L- system is simply an antiquated naming system for enantiomers based upon the relation of the structures to glyceraldehyde. D-glucose refers to the enantiomer of glucose in which the C5 stereocentre has the same configuration as (R)-glyceraldehyde and L-glucose that of (S)-glyceraldehyde. Because this notation refers specifically to enantiomers, it is assumed that the configuration at all stereocentres is inverted and the configuration at the last stereocentre was chosen to assign unambiguous names.

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  • $\begingroup$ Okay, thanks! I didn't know that D/L applied specifically to enantiomers .... I was just told to look at the penultimate carbon. $\endgroup$ – Dissenter Oct 22 '14 at 18:15
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For D-glucose, it has the configuration (2R,3S,4R,5R). For L-glucose, it has the configuration (2S,3R,4S,5S).

As a general rule, for an enantiomer, all S will change to R and vice versa at each respective carbon centres. All the other configurations are diastereomers.

For example, (R,R) and (S,S) are enantiomers, (R,S) and (S,R) are enantiomers, but (R,R) and (R,S) are diastereomers.

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