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Homework Help: Ochem, name R or S for this? Why?

  1. Feb 26, 2013 #1

    Why is the 2nd chiral carbon of cephalexin molecule R??? I thought the N has first priority, then the Carbon with the double bond to the O would be second (b/c has no H), and the Carbon bonded to the S would be third priority b/c it has a hydrogen, and of course, H is fourth... But then it would be S if that was the case... Why is it R???
  2. jcsd
  3. Feb 28, 2013 #2
    Never mind. Well, not that anyone was going to answer anyway.

    But the chiral carbon in question is bonded to an S and an N. S and N have a greater atomic weight than O. So the H doesn't matter.

    The Carbon bonded to the S and N is second priority.
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