Assigning Absolute Configurations
The entire world, except for biochemists, now describes the arrangement of ligands on a stereogenic center by the IUPAC convention, developed by a committee consisting of Robert Cahn, Sir Christopher Ingold, and Nobel Laureate Vladimir Prelog. The convention comprises the following steps:
- Draw a stereoprojection (dash-wedge style) showing the four ligands on the stereogenic center
- Rank the ligands according to the sequence rules
- Higher atomic number takes precedence over lower
- If a distinction cannot be made at the first attached atom, proceed out the chain until a distinction can be found
- Treat multiply bound atoms as multiple atoms singly bound
- Orient the projection so that the lowest priority ligand is directed away from you
- The remaining three ligands will have one of two orientations:
- The clockwise order of descent is labeled R, for Rectus; the anticlockwise order, S, for Sinister
Some examples:
This scheme can be extended to species lacking a stereogenic (chiral) center, but possessing a chiral axis or a chiral plane.
This page last modified 10:58 AM on Saturday February 24th, 2007.
Webmaster, Department of Chemistry, University of Maine, Orono, ME 04469