Stereochemistry

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Absolute stereochemistry is calculated for tetrahedral atoms and double bonds according to the Cahn-Ingold-Prelog (CIP) priority rules. The CIP rules are designed to order ligands by their priority and determine a descriptor based on the orientation of the ordered ligands in space.

A ligand is an entity attached to a stereocenter. For example, a tetrahedral carbon has four ligands corresponding to its four substituents. When a tetrahedral carbon is in a ring, it still has four ligands: the two ligands outside the ring, a third consisting of the ring "unpeeled" clockwise, and a fourth consisting of the ring "unpeeled" counterclockwise.

Five rules are used to determine the priority of ligands are summarized below in simplified form. For more detailed information, see the references. They are checked sequentially as follows:

• Rule 1-Higher atomic number precedes lower
• Rule 2-Higher atomic mass precedes lower
• Rule 3-cis precedes trans
• Rule 4-Like pairs of descriptors precede unlike pairs
• Rule 5-R precedes S

ChemDraw checks differences up to 15 atoms distant from the stereocenter.