Enantiomers and Diastereomers

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.