Asymmetry and Configuration
Structure and Isomerism
Full IUPAC listing of Cahn-Ingold-Prelog rules found here.
Move the models below to convince yourself if each is has symmetry (symmetrical) or lacks symmetry (asymmetric)
2 different types of atom attached
symmetric (bigger version here)
Morphine - asymmetrical (bigger version here)
3 different types of atom attached
symmetric (bigger version here)
4 different types of atom attached
asymmetric (bigger version here)
In general, when an sp3 carbon has four different atoms or groups are attached it is asymmetric. This leads to the possibility of an isomer that is the mirror image but not superimposable. These are enantiomers, examples of which are shown here with the stereoisomeric 2-butanols. Move the models so they are facing each other through a mirror; they do not match if we try to put one on top of the other so they are not the same molecule.
The idea of non-superimposability is illustrated here; however we try to overlay the two molecules they will not match:
If we have two molecules, enantiomers, that only differ by their projection of groups into space, we need a naming system to differentiate them. This will be the Cahn-Ingold-Prelog (CIP) rules for designating configuration as either R or S.
