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I see and D- and L- glucose are mirror images of each other. and I understand D and L are for chirality. But it is not clear which carbon do D and L refer to. Could anybody let me know? Thanks.

What are the differences between D and L glucopyranose?

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  • $\begingroup$ For some more context, see e.g. chem.libretexts.org/Courses/Purdue/… $\endgroup$ Aug 23 '20 at 20:27
  • $\begingroup$ Tbe discussions in both places are still confusing to me. I don't think they directly answer my question. So D and L do not refer to any specific carbon in glucose? $\endgroup$ Aug 23 '20 at 20:59
  • $\begingroup$ They always refer to the chiral carbon furthest from the carbonyl, so carbon 5 in this case. $\endgroup$ Aug 23 '20 at 21:01
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I think your confusion arises from trying to locate a particular carbon atom, which is D or L. This is not the case.

Points to remember:

The reference for D and L notation is glyceraldehye molecule not a particular carbon atom.

D and L are not equivalent to R and S which are individually applicable to each chiral center in a molecule.

Similarly, D and L cannot tell you the direction of rotation of light before hand.

Capital “D” and “L” notations have no connection with small d and small l, which tell you the direction of rotation of light.

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  • $\begingroup$ So D and L can only be determined by experiment (or complicated computational program). This is no way that I can easily determine whether a molecule is D or L manually? So the answer by @KarstenTheis "carbon 5 in this case" is wrong? Thanks. $\endgroup$ Aug 23 '20 at 23:28
  • $\begingroup$ No, Karsten is also right. Yes, in order to assign D or L, all you need to know is the correct Fisher projection and compare it with glyceraldehyde. $\endgroup$
    – M. Farooq
    Aug 24 '20 at 1:30

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