Chemistry Reference
In-Depth Information
4
Stereochemistry
In Chapter 3 the reasons why drugs behave as weak acids or weak bases
were discussed and strategies were developed to exploit differences in
physicochemical properties to separate components of a mixture. In
this chapter, the three-dimensional shapes of molecules will be introduced
and, in particular, the unusual geometry that arises around a carbon atom
with four different substituents attached to it - an
asymmetric carbon atom
.
The study of the three-dimensional shape of molecules is absolutely
fundamental to a student's understanding of complex topics such as
biochemistry, medicinal chemistry and drug design.
Chemical compounds that have the same molecular formula but
different structural formulas are said to be
isomers
of each other. These
constitutional isomers (or structural isomers) differ in their bonding
sequence, i.e. their atoms are connected to each other in different ways.
Stereoisomers have the same bonding sequence, but they differ in the orien-
tation of their atoms in space. Stereoisomerism can be further divided into
optical isomerism (
enantiomerism
) and
geometrical isomerism
(
cis
-
trans
isomerism). The relationships between the different types of isomerism are
shown in Figure 4.1.
All isomers
Constitutional isomers
(structural isomers)
Stereoisomers
Diastereoisomers (diastereomers)
Enantiomers
Cis-trans
isomers
(geometric isomers)
Other diastereoisomers (diastereomers)
(2 or more chiral carbons)
Figure 4.1
Different types of isomerism.
