Chapter 7 : Stereochemistry

The Cahn-Ingold-Prelog R/S rules are used for naming enantiomers and diastereomers

NB: The term chirality has superceded the term stereogenic or chiral center.

1. Identify the chirality 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 chirality center (remember the first point of difference rule)
3. Position the lowest priority group away from you as if you were looking along the C-(4) s bond. If you are using a model, grasp the group in your fist.
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)

More about the position of the observer... (i.e. the requirement that you need to put the low priority group away from you)

Example: bromofluoroiodomethane
 

	The chirality center is easy to spot, and the four attached groups are I (purple), Br (maroon), F(green) and H (white), listed in priority order, highest to to lowest. Rotate the JSMOL image on the left 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 (I to Br to F), so this is the R enantiomer.
Highlight chirality center

Subrules:

• Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H
• Same atom attached ? By moving out one unit at a time, locate the first point of difference and apply rules there.

 Can you convey this on a piece of paper?

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© Dr. Ian Hunt, Department of Chemistry