A question requires you to assign the R/S configuration for 3‐chloro‐2‐hydroxybutanoic acid:

3‐chloro‐2‐hydroxybutanoic acid

In the answer, while assigning the configuration to carbon #1, apparently the group below is given the third priority, lower than the COOH group above.

Using the CIP rules, doesn't the Cl atom in the bulky group give it a higher priority than the COOH group?

  • $\begingroup$ In second carbon(C2), Cl is first priority group due to its higher atomic number than C1 attached to C2, secomd priority group is C1 and third is CH3. This giving anticlockwise rotation, but H is in front of the observer(on horizontal line), so the overall rotation is clockwise. Thus it is R configuration. $\endgroup$
    – Manu
    Jun 24 '20 at 14:10
  • 1
    $\begingroup$ You can also think the other way, instead of seeing the Fischer projection from above the paper, you can see it from below the paper, then horizontal line groups are away from you and vertical line groups are toward you. Then the anticlockwise rotation seems to be clockwise from below the plane of paper. $\endgroup$
    – Manu
    Jun 24 '20 at 14:12
  • $\begingroup$ Sorry, the picture's not clear; I was talking about the other carbon. I'll edit it. $\endgroup$
    – harry
    Jun 24 '20 at 14:27
  • $\begingroup$ I undid andselisk's edit. I meant carbon #1. $\endgroup$
    – harry
    Jun 24 '20 at 14:58
  • 2
    $\begingroup$ In carbon#1, OH is first priority group , C#2 is second priority group due to Cl attached to it, COOH is third priority group. So the rotation (move finger from first to third priority groups) is clockwise but as H is towards the observer so the rotation is anticlockwise so the configuration is S. $\endgroup$
    – Manu
    Jun 24 '20 at 15:05

The compound mentioned is (2S,3R)-3-chloro-2-hydroxybutanoic acid. For Carbon atom #2, the CIP priority order is

  1. $\ce{Cl}$
  2. Carbon #1
  3. $\ce{CH3}$
  4. $\ce{H}$

The methyl group is lower priority because Carbon #1 has a hydroxyl group attached to it, which is higher priority than Hydrogen. This implies that the orientation at carbon atom #2 is R.

For more in-depth information, you might want to check out Wikipedia

  • $\begingroup$ Sorry, the diagram was unclear when you posted; I meant the other carbon. $\endgroup$
    – harry
    Jun 24 '20 at 14:34
  • $\begingroup$ I meant carbon #1, and no one edit that, please! $\endgroup$
    – harry
    Jun 24 '20 at 14:59
  • $\begingroup$ My first answer was for Carbon #1, after which you edited to carbon #2. Anyways, I think you have enough hints now to give it a try yourself. $\endgroup$ Jun 24 '20 at 15:52

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