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Stereochemistry Information

Rules for Designating Stereochemistry Configurations

Catalyst uses the Cahn-Ingold-Prelog (CIP) conventions to denote the stereochemistry of a chiral atom, as described in the following references:

R.S. Cahn, C.K. Ingold & V. Prelog, Angew. Chem. Int. Engl. Ed. 5(1966), 385.
V. Prelog & G. Helmchen, Angew. Chem. Int. Engl. Ed. 21(1982), 567.
Below is an example of how the chirality of an atom is determined using the CIP conventions.
First, orient the molecule so that the lowest ranking group connected to the chiral carbon points away-hydrogen in the example below. Next, identify in which rotation direction you must go from the highest-ranking substituent (Cl) to the next highest ranking substituent (N). If the direction is clockwise, it is an R configuration. If the direction is counterclockwise, it is an S configuration.
For information on pseudochirality, select the NEXT PAGE button.

Pseudochirality

Sometimes you will see a carbon designated as (r) or (s) chirality. This indicates pseudochirality. A pseudochiral carbon is a carbon that has four substituents A, B, C and D, where B, C and D are different, and the only difference between A and B is their chirality.

In these cases, the Cahn-Ingold-Prelog rules apply, with R substituents having a higher order than S substituents.

So in the example molecules below, the (r) designation for the molecule on the left arises because (in viewing the chiral center with the hydrogen pointing away) going from highest order substituent (the R carbon) to lowest order substituent around the pseudochiral center is a clockwise direction. Conversely, for the molecule on the right, (s) pseudochirality is designated since the highest-to-lowest order of substituents is now counterclockwise.

Examples of (r) and (s) pseudochirality

Last updated April 17, 1996 at 08:45pm PDT.

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