37 Diastereomers
Diastereomersare stereoisomers that are not enantiomers (mirror images) of each other.
Due to their different shape, diastereomers can have different physical and chemical prop-
erties. This is perhaps especially true of diastereomers involved in biological systems.
According to IUPAC the term ”geometric isomerism” is an obsolete synonym of ”cis-trans
isomerism” and its use is strongly discouraged. Sometimes the term ”geometric isomerism”
has been used as a synonym of stereoisomerism, i.e. optical isomers being considered to be
geometric isomers. This, however, is not consistent with current standard chemical nomen-
clature. The exact term for stereoisomers that are not optical isomers is diastereomers.
A special kind of diastereomer is an epimer. Epimers are diastereomers that differ at one
of several asymmetric carbon atoms. There is also something called an anomer, a special
type of epimer. An anomer differs at a new asymmetric carbon atom when a ring is formed
(in carbohydrate chemistry).
37.1 Cis-trans Isomerism
Stereoisomerismcan occur 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 102 Dichloroethene isomers
The two molecules shown above arecis-1,2-dichloroethene andtrans-1,2-dichloroethene.
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. Cis/trans isomers have different chemical and physical properties and can
exhibit dramatically different biological activity.
Cis-trans isomerism(Often calledgeometric isomerismalthough this term refers to
all stereoisomers) is a form of stereoisomerism and describes the orientation of functional
groups at the ends of a bond around which no rotation is possible. Both alkenes and
cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond
restricts movement and rotation, as does the looped structure of cycloalkanes.
Rotation is possible around the double bond of an alkene but it requires between 60 and
70 kcal of energy. Without the addition of this energy, groups that start on one side of the
double bond stay there. This is the basis of cis/trans isomerism.