||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has superceded the term stereogenic or chiral center.
- Identify the chirality centers
(most commonly an sp3 C with 4 different groups attached)
- 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)
- 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.
- For the other 3 groups, determine
the direction of high to low priority (1 to 3)
- If this is clockwise, then the
center is R (Latin: rectus = right)
- If this is counter clockwise,
then it is S (Latin: sinister = left)
||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 JMOL 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, so this is the R enantiomer.
Can you convey this on a piece
- 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
|| © Dr. Ian Hunt,
Department of Chemistry, University of Calgary