D vs L configuration

GomerPyle

Full Member
10+ Year Member
7+ Year Member
Dec 27, 2007
711
94
Boston
  1. Medical Student
    Hi Everybody,

    Does "D" (dextrorotation) ALWAYS mean + aka clockwise rotation of plane polarized light and "L" always rotates counterclockwise? In EK 1001 Ochem, it says that you can't tell which way L-Fructose rotates plane polarized light even though it is an L-glyceraldehyde. Confused on when you know when it rotates light and when you cannot tell.

    Thanks!
     

    milski

    1K member
    5+ Year Member
    Dec 30, 2009
    2,641
    527
    Where the rain grows
    1. Pre-Medical
      No, it does not. D/L and R/S are just naming conventions - they are based on arbitrary ordering of the atoms. d/l and +/- are physical properties and describe which way the light will rotate. The physical properties (+/-) cannot be deduced from the naming convention. The only thing that can be said for sure is that if D is - then L is + and vice versa - if D is +, then L is -.
       

      GomerPyle

      Full Member
      10+ Year Member
      7+ Year Member
      Dec 27, 2007
      711
      94
      Boston
      1. Medical Student
        No, it does not. D/L and R/S are just naming conventions - they are based on arbitrary ordering of the atoms. d/l and +/- are physical properties and describe which way the light will rotate. The physical properties (+/-) cannot be deduced from the naming convention. The only thing that can be said for sure is that if D is - then L is + and vice versa - if D is +, then L is -.

        Okay...so D/L is different from d/l? and d = + and l = -?
         

        Laureatebarrel

        Full Member
        7+ Year Member
        Jul 31, 2011
        345
        123
        1. Medical Student (Accepted)
          Dear Gomer, D/L and d/l is different. D/L is often used for sugar and protein and it denotes this: If a sugar has a D configuration, it means that the penultimate (the next to last) carbon has the same configuration as D-glyceraldehylde, and if it has L configuration it has the penultimate carbon the same configuration as L-glyceraldehylde. Now, a d - dextrorotarory means that it rotates polarized light in clockwise direction and a l-levorotarory rotates polarized light in counter-clockwise direction.
          Now, a D-glyceraldehylde will rotate light in clockwise direction which means that D-glycer has d configuration and L-glycer will have l configuration. The tricky part is that you cannot say a D-fructose will also have d configuration without experimenting since it has other chiral centers also. But if you know that D-fructose let say has + rotation, then L-fructose which is the enantiomer will have the opposite but same magnitude rotation. Hope that helps.
           
          • Like
          Reactions: 1 user
          About the Ads

          GomerPyle

          Full Member
          10+ Year Member
          7+ Year Member
          Dec 27, 2007
          711
          94
          Boston
          1. Medical Student
            Dear Gomer, D/L and d/l is different. D/L is often used for sugar and protein and it denotes this: If a sugar has a D configuration, it means that the penultimate (the next to last) carbon has the same configuration as D-glyceraldehylde, and if it has L configuration it has the penultimate carbon the same configuration as L-glyceraldehylde. Now, a d - dextrorotarory means that it rotates polarized light in clockwise direction and a l-levorotarory rotates polarized light in counter-clockwise direction.
            Now, a D-glyceraldehylde will rotate light in clockwise direction which means that D-glycer has d configuration and L-glycer will have l configuration. The tricky part is that you cannot say a D-fructose will also have d configuration without experimenting since it has other chiral centers also. But if you know that D-fructose let say has + rotation, then L-fructose which is the enantiomer will have the opposite but same magnitude rotation. Hope that helps.

            Awesome. Thanks! :)
             
            This thread is more than 8 years old.

            Your message may be considered spam for the following reasons:

            1. Your new thread title is very short, and likely is unhelpful.
            2. Your reply is very short and likely does not add anything to the thread.
            3. Your reply is very long and likely does not add anything to the thread.
            4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
            5. Your message is mostly quotes or spoilers.
            6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
            7. This thread is locked.