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Predicting absolute configuration of molecules

  1. Sep 7, 2008 #1
    I have a question about something that has been bothering me for a while...

    In all of my chemistry classes, my professors have always told me that it is impossible to predict which way a chiral molecule will rotate plane-polarized light (i.e., you will see if a molecule is D or L, but, saying you know it's structure, you have no way to tell whether it is R or S). I understand that the R/S configuration is arbitrary, just a way of naming things, but I was wondering if it is actually possible (via computer modeling or something similar) to look at a given chiral molecule's structure and predict its chirality. It seems strange to me that we have no way of knowing this other than experimentation.

    Thanks very much for any information in advance :) Sorry if this is very random :)

    -Mary-
     
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  3. Sep 7, 2008 #2

    GCT

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    It's been a while since I came across the notations of " D " and " L " , however , from what I recall , you can denote the R and S character of a chiral center once you know the substituents as well as the 3D structure around this chiral center.

    It's been a while so perhaps what I claimed doesn't make sense. I remember from my college years , a biology lecture which asked why all biological proteins have one chiral compatability , it turns out that this question had not been resolved.
     
  4. Sep 8, 2008 #3

    chemisttree

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    It is impossible to determine the direction a molecule will rotate plane-polarized light but it is possible to determine chirality since there are rules for that. Whether you are using the D/L nomenclature or the R/S, the rules are either based on a standard substance (glyceraldehyde for carbohydrates, for example) or are arbitrary in nature. The R/S nomenclature determines chirality based on the size or priority of groups attached to the chiral carbon. The D/L nomenclature is determined by the chiral carbon farthest from the aldehyde end of the molecule. If the chiral carbon farthest from the aldehyde end has a hydroxyl on the left side of it's Fischer projection, it's an "L" and if it is on the right, it's a "D".
     
  5. Sep 8, 2008 #4

    Andy Resnick

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    Chemisttree,

    I'm not doubting what you say, but is there a reason why predicting the 'direction a molecule will rotate plane-polarized light' is not possible? Not possible with today's body of theory, or not possible because of some fundamental principle?

    I guess I'm curious as to why so much of chemistry is empirical, even today.
     
  6. Sep 9, 2008 #5

    chemisttree

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    I suppose that there might be some way to calculate the direction and magnitude of the rotation of plane polarized light. I was referring to the D and L designation. "D" is not "d-" and "L" is not "l-". The origin of the symbols d- and l- were dexorotary and levorotary D-fructose is also known as levulose and rotates PP light to the left (or negative). So, it is impossible to determine the rotation direction and magnitude from just the information "D" and "L".
     
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