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Confused about D/L and absolute configuration

unsung

Full Member
10+ Year Member
Mar 12, 2007
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  1. Resident [Any Field]
    Okay, here's my understanding of chirality now. Correct me if I'm wrong:

    (+) = dextrorotatory = d (small d, not capital d) = rotates light CW (right)
    (-) = levorotatory = l = rotates light CCW (left)


    R = rectus = when assigned priorities, arrow drawn fr 1 to 3 is CW (right)
    S = sinister = when assigned priorities, arrow drawn fr 1 to 3 is CCW (left)


    So +/- (or d,l) designate RELATIVE configuration, because they distinguish between the different ways chiral compounds rotate light. But since a compound could be R and (+) or R and (-), knowing which way a compound rotates light doesn't tell us anything about its actual structure.

    On the other hand, ABSOLUTE configuration gives us info about a molecule's actual structure.

    Is that understanding correct? I'm confused about whether D/L (capital) give info on absolute configuration or relative configuration ?

    My guess is that it gives relative configuration, since D-series sugars could be either (+) or (-). The one exception is glyceraldehyde, for which we know (+)-glyceraldehyde is D glyceraldehyde, so there's a correspondence there.

    But then, what is R/S? Is that relative configuration or absolute configuration ? What gives info on absolute configuration? Also, I've seen problems where a reaction involves an inversion of configuration going from reactant to product, BUT, the reactant and product have the same designation (e.g. "R"). So... doesn't that mean R/S give relative configuration ?

    I'm confused. :confused:
     

    Kaustikos

    Antibiotics 4 Lyfe
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  • Jan 18, 2008
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    1. Fellow [Any Field]
      Okay, here's my understanding of chirality now. Correct me if I'm wrong:

      (+) = dextrorotatory = d (small d, not capital d) = rotates light CW (right)
      (-) = levorotatory = l = rotates light CCW (left)


      R = rectus = when assigned priorities, arrow drawn fr 1 to 3 is CW (right)
      S = sinister = when assigned priorities, arrow drawn fr 1 to 3 is CCW (left)


      So +/- (or d,l) designate RELATIVE configuration, because they distinguish between the different ways chiral compounds rotate light. But since a compound could be R and (+) or R and (-), knowing which way a compound rotates light doesn't tell us anything about its actual structure.

      On the other hand, ABSOLUTE configuration gives us info about a molecule's actual structure.

      Is that understanding correct? I'm confused about whether D/L (capital) give info on absolute configuration or relative configuration ?

      My guess is that it gives relative configuration, since D-series sugars could be either (+) or (-). The one exception is glyceraldehyde, for which we know (+)-glyceraldehyde is D glyceraldehyde, so there's a correspondence there.

      But then, what is R/S? Is that relative configuration or absolute configuration ? What gives info on absolute configuration? Also, I've seen problems where a reaction involves an inversion of configuration going from reactant to product, BUT, the reactant and product have the same designation (e.g. "R"). So... doesn't that mean R/S give relative configuration ?

      I'm confused. :confused:

      The capitaol D/L is a configuration relative to another compound. It tells the spatial configuration relative to, I think, glyceraldehyde. And the D/L for that is based on the priorities like the R/S (which confuses me now, but I'll let that topic be discussed)
      The last part; it seems that you can have an inversion of configuration but still have the same R/S enantionmer. Like, say you have (R) Bromo-Chloro-Iodo-Methane and you attack it with something that inverts its configuration but somehow still manages to make the priorities remain the same (Chlorine is still to the right of Iodine by some magicall way), you still have R, and inversion of configuration?
      I honestly don't even know if that's possible or what they mean. But it would seem plausible.
       
      Okay, here's my understanding of chirality now. Correct me if I'm wrong:

      (+) = dextrorotatory = d (small d, not capital d) = rotates light CW (right)
      (-) = levorotatory = l = rotates light CCW (left)


      R = rectus = when assigned priorities, arrow drawn fr 1 to 3 is CW (right)
      S = sinister = when assigned priorities, arrow drawn fr 1 to 3 is CCW (left)


      So +/- (or d,l) designate RELATIVE configuration, because they distinguish between the different ways chiral compounds rotate light. But since a compound could be R and (+) or R and (-), knowing which way a compound rotates light doesn't tell us anything about its actual structure.

      On the other hand, ABSOLUTE configuration gives us info about a molecule's actual structure.

      Is that understanding correct? I'm confused about whether D/L (capital) give info on absolute configuration or relative configuration ?

      My guess is that it gives relative configuration, since D-series sugars could be either (+) or (-). The one exception is glyceraldehyde, for which we know (+)-glyceraldehyde is D glyceraldehyde, so there's a correspondence there.

      But then, what is R/S? Is that relative configuration or absolute configuration ? What gives info on absolute configuration? Also, I've seen problems where a reaction involves an inversion of configuration going from reactant to product, BUT, the reactant and product have the same designation (e.g. "R"). So... doesn't that mean R/S give relative configuration ?

      I'm confused. :confused:
      Capital D/L is the absolute configuration at the ultimate stereocenter (e.g. a sugar molecule) i.e., R/S configuration using Cahn-Ingold-Prelog convention for chiral carbons.

      Absolute configurations must be done empirically, for example, via measuring % NOEs in NMR experiments.

      NOEs = Nuclear Overhauser Effects = info you don't need for the MCAT.
       
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