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Labeling Enantiomers

Article ID: 141
Last updated: 25 Jan, 2013
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Check out this interactive stereochemistry tutorial on the ChemEdDL

The Cahn-Ingold-Prelog R/S rules are used for labeling enantiomers.

Note: The terms stereogenic, chiral, or asymmetric can be used interchangeably below.

1. Identify the stereogenic centers (most commonly an sp3 C with 4 different groups attached)
2. Assign the priority to each group (high = 1, low = 4) based on atomic number of the atom attached to the stereogenic center
3. Position the lowest priority group away from you (as if you were looking along the C-(4) sigma bond). If you are using a model this is relatively simple, grasp the lowest priority group in your fist and move on to step 4.  If working on paper, make sure the lowest priority group is a dashed line (away).  If the lowest priority group is a wedge or straight line, then switch the lowest priority group with the group on the wedge and reverse the R or S after finishing steps 5 and 6 (see example pictures below).
4. For the other 3 groups, determine the direction of high to low priority (1 to 3)
5. If this is clockwise, then the center is R (Latin: rectus = right)
6. If this is counter clockwise, then it is S (Latin: sinister = left)

Example: chlorofluoroiodomethane (Click here to see the structure in Chemspider, click on "3D" below the picture and click on the model to move)

The stereogenic center is easy to spot, and the four attached groups are I (purple), Cl (bright green), F(dull green) and H (white), listed in priority order, highest to to lowest. Rotate the image so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority. It decreases clockwise, so this is the R enantiomer.

Notes:

  • Isotopes: H vs D? Still use atomic number so D > H
  • Same atom attached? By moving out one unit at a time, locate the first point of difference and apply rules there.



 

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Stereogenic or Chiral Center

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Department of Chemistry & Chemical Biology