Introduction: Most of the drugs substance single enantiomer is active. In such cases the inactive enantiomer is considered as an impurity, e. g. If Dextro form is active then in this case levo form is considered as an impurity.
An enantiomer can be named by the direction in which it rotates the plane of polarized light. An optical isomer can be named by the spatial configuration of its atoms. Clockwise rotation of the light traveling toward the viewer is labelled (+) or R (in Latin Rectus for right) also termed as d-isomer i.e. dextrorotatory enantiomer. Its mirror-image is labelled (−) or S (in Latin Sinister for left) also termed as l-isomer i.e. levorotatory enantiomer.
The R / S system is an important nomenclature system for representing enantiomers. This method labels each chiral centre R or S according to a system by which its substituents are each assigned a priority, according to the Cahn–Ingold–Prelog priority rules (CIP), based on atomic number.
