This is to give an overview of the evolving chirality nomenclature system commonly employed to distinguish enantiomers of a chiral drug. In the beginning, enantiomers were distinguished based on their ability to rotate the plane of plane-polarized light. The enantiomer that rotates the plane-polarized light to the right is named "dextro-rotatory", abbreviated as "dextro" or "d" and the counterpart as "levo" or "l". A racemic mixture is denoted as "(±)", "rac", or "dl". Now the d/l system of naming based on optical rotation is falling into disuse.
Later, the Fischer convention was introduced to specify the configuration of a stereogenic center and uses the symbols D and L. The use of capital letters is to differentiate from the "d" / "l" notation (optical descriptor) described earlier. In this system, the enantiomers are named with reference to D- and L-glyceraldehyde which is taken as the standard for comparison. The structure of the chiral molecule should be represented in the Fischer projection formula. If the hydroxyl group attached to the highest chiral carbon is on the right-hand side it is referred to as D-series and if on the left-hand side it is called L-series. This nomenclature system has also become obsolete. But D-/L-system of naming is still employed to designate the configuration of amino acids and sugars. In general the D/L system of nomenclature is superseded by the Cahn-Ingold-Prelog (CIP) rule to describe the configuration of a stereogenic/chiral center.
In the CIP or R/S convention, or sequence rule, the configuration, spatial arrangements of ligands/substituents around a chiral center, is labeled as either "R" or "S". This convention is now almost worldwide in use and become a part of the IUPAC (International Union of Pure and Applied Chemistry) rules of nomenclature. In this approach: identify the chiral center, label the four atoms directly attached to the stereogenic center in question, assign priorities according to the sequence rule ( from 1 to 4), rotate the molecule until the lowest priority (number 4) substituent is away from the observer/viewer, draw a curve from number 1 to number 2 to number 3 substituent. If the curve is clockwise, the chiral center is of R-absolute configuration, "R" (Latin, rectus = right). If the curve is counterclockwise, the chiral center is of S-absolute configuration, "S" (Latin, sinister = left). Refer to figure, the Cahn-Ingold-Prelog rule.
An overview of the nomenclature system is presented in the table below.