Naming conventions32.3.1 By optical activity: (+)- and (-)-
An optical isomer can be named by the direction in which it rotates the plane of polarized
light. If an isomer rotates the plane clockwise as seen by a viewer towards whom the light is
traveling, that isomer is labeled (+). Its counterpart is labeled (-). The (+) and (-) isomers
have also been termed d- and l-, respectively (for dextrorotatory and levorotatory). This
labeling is easy to confuse with D- and L- and is therefore not encouraged by IUPAC.
The fact that an enantiomer can rotate polarised light clockwise (d- or+- enantiomer)
does not relate with the relative configuration (D- or L-) of it.
32.3.2 By relative configuration: D- and L-
Fischer, whose research interest was in carbohydrate chemistry, took glyceraldehyde (the
simplest sugar, systematic name 2,3-dihydroxyethanal) as a template chiral molecule and
denoted the two possible configurations with D- and L-, which rotated polarised light clock-
wise and counterclockwise, respectively.
Figure 83 Glycerladehyde, the starting molecule forrelative configurationassignent
All other molecules are assigned the D- or L- configuration if the chiral centre can be
formally obtained from glyceraldehyde by substitution. For this reason the D- or L- naming
scheme is calledrelative configuration.