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Homework Help: Organic Chem 1- (S) vs. (R)

  1. Nov 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A question on a pretest asks me to name this. I know it's either (1R,2R) or (1S,2S)1,2-Cyclopentanediol because the OH's are facing in opposite directions, but I'm not sure how to figure out if the OH's are S or R

    2. The attempt at a solution
    I somewhat have an understanding of the concept, but I'm not entirely sure how to do it with a ring structure.

    To be more specific, I'm confused about how to set up the determination of S vs. R. I know I can draw the top carbon either one of these two ways:
    I'm just not sure about how to figure out which one is actually what's being depicted
    Last edited: Nov 24, 2009
  2. jcsd
  3. Nov 24, 2009 #2
    Since the carbon atoms are all depicted in the same plane, the c-c bonds also lie within the same plane. That means that of the two remaining bonds on each carbon atom, one is projecting toward the viewer (dark wedge or invisible H) and the other is projecting away from the viewer (dashed wedge or invisible H). This should be sufficient for you to construct a model.


    Remember that in this representation, the vertical bonds are actually projecting away from the viewer, and the horizontal bonds are projecting toward the viewer.

    If you're trying to visualize this rather than using an actual model, think of rolling the molecule away from you until you're looking at it edge-on, with the carbon formerly at the top now pointing away from you. The carbon at the top of the ring is now on the far side of the molecule, the C-C bonds point to the left and right (but the rotation moves them from "down" to "toward me"), the C-H bond points down and away, and the C-O bond points up and away.
    Last edited: Nov 24, 2009
  4. Nov 24, 2009 #3
    So then, rolling it away from me would put the OH on top, and that would mean that it's the (1R,2R)-1,2-cyclopentanediol.

    Thanks alot. :D
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