Stereoisomer

Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer.

For example a stereoisomer can occur when a double or triple 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:

H       H              H       Cl 
 \     /                \     /
  C = C                  C = C
 /     \                /     \
Cl     Cl              Cl      H

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 stereoisomers 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 "superposeable" - they are not identical, and cannot take each other's place.

Stereoisomers have different chemical and physical properties and can exhibit dramatically different biological activity. In nature, amino acids are found to be uniformly a single enantiomer (another type of stereoisomerism). Amino acids are the building blocks of proteins, life's molecular machines. This handedness of the building blocks of life explains much of the stereospecific biological activity of stereoisomers.

Types

• Geometric isomerism, aka cis-trans isomerism
• Optical isomerism, which includes enantiomerism and diastereoisomerism

See also

• Chiral chemistry

External Links

• The IUPAC definition of "stereoisomerism"
• The IUPAC definition of "geometric isomerism"