Stereoisomerism
Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer.
The two main types of stereoisomerism are:
cis-trans isomerism
optical isomerism
Cis-trans Isomerism
For example a geometrical isomerism occurs when a double bond is present, because the pi bond involved prevents that bond from being "twisted" the same way that a single bond can be. A good example is 1,2-dichloroethene: C2H2Cl2. Consider the two examples below:
The two molecules shown above are cis-1,2-dichloroethene and trans-1,2-dichloroethene. This is more specifically an example of cis-trans isomerism. These two molecules are geometrical isomers because the two carbon atoms cannot be rotated relative to each other, due to the rigidity caused by the pi bond between them. Therefore, they are not "superimposeable" - they are not identical, and cannot take each other's place. Geometrical isomers have different chemical and physical properties and can exhibit dramatically different biological activity.
Optical Isomerism
Optical isomers are stereoisomers formed when asymmetric centers are present, for example, a carbon with four different groups bonded to it. Enantiomers are two optical isomers that are reflections of each other. Every stereocenter in one has the opposite configuration in the other. Compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. In nature, most biological compounds, such as amino acids, occur as single enantiomers. As a result, different enantiomers of a compound may have substantially different biological effects. When a molecule has more than one source of asymmetry, two optical isomers may be neither perfect reflections of each other nor superimposeable: some but not all stereocenters are inverted. These molecules are called they are diastereomers, not enantiomers. Diastereomers seldom have the same physical properties.
