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What is the difference between D and L configuration, and + and −?

My textbook says they are two different things. It also says that the correct way to name glucose is D(+)-glucose.

Could someone please explain what D, L and +, − represent, and why they are different?

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The D-L system corresponds to the configuration of the molecule: spatial arrangement of its atoms around the chirality center.

While (+) and (-) notation corresponds to the optical activity of the substance, whether it rotates the plane of polarized light clockwise (+) or counterclockwise (-).

D-L system tells us about the relative configuration of the molecule, compared to the enantiomers of glyceraldehyde as the standard compound. Compounds with the same relative configuration as (+)-glyceraldehyde are assigned the D prefix, and those with the relative configuration of (-)-glyceraldehyde are given the L prefix.

It's kind of another way to tell the configuration of molecules beside the Cahn–Ingold–Prelog convention (R/S system), with little difference. (D-L system labels the whole molecule, while R/S system labels the absolute configuration of each chirality center.)

In short, the D-L system doesn't have direct connection to (+)/(-) notation. It only relates the stereochemistry of the compound with that of glyceraldehyde, but says nothing about its optical activity. We may have compound with same relative configuration as (+)-glyceraldehyde (thus, it's given the D prefix), yet it rotates the polarized light counterclockwise (-), such as D-(-)-ribose.

And also, don't confuse the D-L system with d- and l- naming. d- and l- is the exact same with (+) and (-) notation.


Additional explanation

D-L system (also called Fischer–Rosanoff convention) is mainly used for naming α-amino acids and sugars. It compares the relative configurations of molecules to the enantiomers of glyceraldehyde. This convention is still in common use today.

Rosanoff in 1906 selected the enantiomeric glyceraldehydes as the point of reference[1]; any sugar derivable by chain lengthening from what is now known as (+)-glyceraldehyde (or named D-glyceraldehyde) belongs to the D series. In other words, we used a D to designate the sugars that degrade to (+)-glyceraldehyde and an L for those that degrade to (-)-glyceraldehyde.

In assigning the D and L configurations of sugars, we could direcly look for the OH group of the bottom asymmetric carbon in the Fischer projection. If it's located on the right, we designate it with D, and vice versa, since they would have the same relative configurations with glyceraldehyde for the bottom asymmetric carbon.

Reference

[1]: IUPAC and IUBMB. Joint Commission On Biochemical Nomenclature. Nomenclature of Carbohydrates. 1996, 7.

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    $\begingroup$ That was very clear and informative. Thank you. Is it right to infer from your explanation that there is no way to find out whether a molecule turns light left or right just by looking at it? $\endgroup$ – Mahathi Vempati Jan 26 '16 at 14:49
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    $\begingroup$ @user93868 Yes, it is. The direction which it rotates the polarized light, (+) or (-), is something that we measure in the polarimeter. It's directly the physical property of that compound. We can't easily tell the direction just by looking at it. $\endgroup$ – Dean Jan 26 '16 at 15:01
  • $\begingroup$ does the lower case d- and l- that you mention refer to dextrorotatory and levorotatory? $\endgroup$ – LiamH Jan 27 '16 at 14:06
  • $\begingroup$ @LiamH Yes, they are, respectively. $\endgroup$ – Dean Jan 27 '16 at 14:12

protected by orthocresol Apr 27 '17 at 10:00

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