Some molecules may occur with identical atoms in the same sequence but with different spatial arrangements. These are referred to as stereoisomers. A stereoisomer that is nonsuperimposable on its mirror image is chiral, and an atom with 4 different substituents is a chiral center; the 2 mirror images are enantiomers.10 An equal mixture of the 2 enantiomers is racemic. Generally only 1 enantiomer is biologically active, as in the case of ibuprofen. In some cases, one enantiomer may be biologically beneficial while the other enantiomer is harmful. For example, one enantiomer of thalidomide is a beneficial drug whereas the other enantiomer causes birth defects. The enantiomers may be designated by their optical activity, typically by using a polarimeter. The enantiomer that rotates plane-polarized light to the left is levorotatory.10 This is shown in the name with the prefix (−), which has replaced the now-obsolete designation l-. The enantiomer that rotates plane-polarized light to the right is dextrorotatory, shown in the name with the prefix (+), replacing the now-obsolete designation d-. A racemic mixture has the prefix (±), which replaces the now-obsolete designation dl-.
Molecules may also be designated by their spatial configuration. The enantiomers can be labeled with the prefix d- or l- depending on the arrangement of the substituents of the chiral center in relation to those of glyceraldehyde; this system is typically used to label biomolecules such as amino acids and carbohydrates. Conversely, the enantiomers may be labeled with the prefixes (R)- or (S)-, which refer to the priority given to the orientation of groups of atoms that surround the chiral center according to the Cahn-Ingold-Prelog priority rules, with (R)- if the rank by atomic number is clockwise and (S)- if it is counterclockwise; these designations are especially useful for describing compounds with multiple asymmetric centers. Note that the d-/l- and (R)-/(S)- naming systems are unrelated.10
For racemic mixtures, rac- or race- is added to the name of the compound (eg, racepinephrine). For the levorotatory form, the (S)- enantiomer may use the lev- or levo- prefix (eg, levamisole, levdobutamine), whereas the (R)- enantiomer may use the ar- prefix. For the dextrorotatory form, the (S)- enantiomer may use the es- prefix (eg, escitalopram), whereas the (R)- isomer uses the dex- or dextro- prefix (eg, dexibuprofin, dextroamphetamine, dexamisole).10