Nomenclature of Absolute Configuration: (R,S) System - Naming
Once the substituents of the chirality center have been ranked according to the CIP sequencing rules, the absolute configuration can be unequivocally determined.
This is now explained with 2-butanol as an example:
|Carbon C2 is the chirality center of 2-butanol. The sequence of CIP rank for the substituents of C2 is: hydroxy group (A) (atomic number of oxygen is 8), ethyl group (B) (atomic number of carbon is 6 and a further carbon in the second sphere), methyl group (C) (atomic number of carbon is 6 but only hydrogens in the second sphere), and finally the hydrogen (D) (atomic number of hydrogen is 1).|
|Subsequently, one looks at the molecule in the direction of the bond between the chirality center (C2) and the substituent of the lowest CIP priority (H (D)) from the opposite side of this substituent. For this purpose, the molecular model is orientated such that the substituent of lowest CIP priority is covered with the chirality center. Then, the three substituents of higher priority form a triangle (A,B,C) around the chirality center. Try to orientate the interactive molecular models below with your mouse so that you can see the triangle (A,B,C).|
If the order of the substituents (A), (B), and (C) (from (A) to (C)) is clockwise, the absloute configuration is (R) (from the Latin rectus, right). If the order is counterclockwise, the absolute configuration is (S) (from the Latin sinister, left). (R) and (S) are called CIP descriptors.