A popular misconception of organic chemistry is that it is a discipline of rote
and tedious memorization. But in actuality, the modern study of organic
chemistry is engaging because it poses particular intellectual challenges. In
this chapter we will be introducing the concept of stereochemistry. It is only
in the recent fifty years that we have discovered the implications of
stereochemistry in biological reactions. As a result of this understanding, new
directions in research and medicine have been opened.
Stereochemistry is a characteristic of molecules that goes beyond the
connectivities of atoms to describe how the atoms are arranged in space.
Previously in our discussion of
Isomerism, we spoke
of cis-trans isomerism, one type of stereochemistry that pertains to the
arrangement of atoms around a double bond. But
stereoisomers don't only arise from rigid
double
bonds; molecules can be stereoisomers even if they are chiral, a term
referring to substances that are not superimposable on (i.e. identical to) their
mirror images. Organic molecules exhibit chirality most frequently when they
contain one or more carbons that are attached to four different groups. These
carbons are called stereocenters, because they give rise to stereoisomerism.
Stereochemical considerations of molecules are important to how they behave in
chemical environments. The implications of stereochemical distinctions between
molecules are far-reaching; the biological environments where molecules interact
are often stereochemically discriminating. Substances that are chiral but
otherwise identical can play very different roles as they interact with other
chiral substances. For example, one version of a molecule may function as a
therapeutic drug, while its stereoisomer might be toxic to the human body.
Stereoisomers are divided into two classes. Enantiomers are stereoisomers
that are non-superimposable mirror images. Enantiomers differ at the
configuration of every stereocenter. They can be understood in terms of
handedness, like gloves for the right or left hands. Molecules that are not
mirror images but differ in spatial arrangements of atoms are diastereomers.
In this section we will discuss how to identify, draw, and name enantiomers and
diastereomers. These concepts will be important in future chapters, when we
want to understand how chiral molecules interact in chemical reactions.