Why is the 2nd Chiral Carbon of Cephalexin R?

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SUMMARY

The second chiral carbon of the cephalexin molecule is designated as R due to the Cahn-Ingold-Prelog priority rules. In this case, the nitrogen (N) and sulfur (S) atoms bonded to the chiral carbon have higher atomic numbers than the oxygen (O) atom, which influences their priority. The carbon bonded to sulfur and nitrogen is assigned second priority, while the hydrogen (H) is assigned fourth priority, confirming the R configuration of the chiral center.

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  • Understanding of Cahn-Ingold-Prelog priority rules
  • Basic knowledge of chiral centers in organic chemistry
  • Familiarity with molecular structure of cephalexin
  • Knowledge of atomic weights of common elements (C, N, O, S)
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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?
 
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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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