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
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
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.
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.