30 Stereoisomers
Stereoisomersare a type of isomer where the order of the atoms in the two molecules is
the same but their arrangement in space is different.
To understand this we need to take a look at the ways that organic molecules can and
cannot move. Again, usingthree-dimensional models is a great tool to visualize this and
almost essential for most people to grasp these concepts clearly.
With cyclo-alkanes, we observe that a group placed on one side of a ring stays on that
same side. Except in very large rings (13+ carbons) the carbons are not free to rotate all
of the way around their axes. This means that a group that is axial will not move into an
equatorial position, and vice versa.
Stereoisomerismis 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:
- DiaStereomerism (including ’cis-trans isomerism’)
- Optical Isomerism (also known as ’enantiomerism’ and ’chirality’)
30.1 Cis-trans Isomerism
Main article:Diastereomers^1
Cis/trans 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: C 2 H 2 Cl 2. Consider the two examples below:
Figure 76 Dichloroethene isomers
The two molecules shown above arecis-1,2-dichloroethene andtrans-1,2-dichloroethene.
1 Chapter 36 on page 151