Stereoisomerism is one of two branches of isomerism, the other being structural isomerism. Structural isomers are the more common form of isomers and arise from the different possibilities in which a group of atoms can bond. For example, a functional group such as -OH can often bond to various points in a carbon chain. While the resulting alcohols will have the same chemical formula, they differ in structure.
Stereoisomers are a more subtle form of isomer. They occur in two different forms: geometric isomers and enantiomers.
Geometric isomers arise where double bonds are present. Due to a higher electron density in double bonds as compared to single bonds, the atoms joined by a double bond are not free to rotate. Because of this restriction, two forms of the molecule are possible, and these are differentiated with the prefixes cis and trans.

Cl      Cl
 \     /
  C===C
 /     \
H       H
This is cis-1,2-dichloroethene. The molecule which has two similar groups on the same side of the double bond is known as the cis-isomer.
Cl      H
 \     /
  C===C
 /     \
H       Cl
This is trans-1,2-dichloroethene. The similar groups are on opposite sides of the double bond, so this is known as the trans-isomer.

The chemical properties are usually similar between stereoisomers. However, physical properties can be affected. In the above example, the cis-isomer is less symmetrical than the trans-isomer and thus the cis-isomer possesses a dipole. As a result, the boiling point of cis-1,2-dichloroethene is 60 degrees while trans-1,2-dichloroethene boils at 48 degrees.

Log in or registerto write something here or to contact authors.