February 23, 2018
What is the difference between the notation (R) and (r) as well as (S) and (s) in stereochemistry?
The R, S and r, s notations are used to provide absolute stereochemistry according to the Cahn-Ingold-Prelog (CIP) priority rules (This rule originates with the Cahn-Ingold-Prelog 'Rule 5').
The notation (R) and (S) are used to provide Standard tetrahedral stereochemistry. However (r) and (s) are used to provide Tetrahedral stereochemistry determined by other sterochemical centers.
Say the lower case r and s refer to stereocenters that depend upon other stereocenters (if the compound has more than one chiral atom - stereo centers)
More specifically, when two substituents on an atom differ only in their absolute configuration (R or S), if the relative priorities of these substituents need to be established, R takes priority over S. When this happens, the descriptor of the stereocenter is a lowercase letter (r or s) instead of the uppercase letter normally used.