Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Stereochemistry
5.1 Introduction

In a three-dimensional world, our analysis of atoms and molecules must be from that perspective. In this chapter, a detailed examination of the three-dimensional arrangement of atoms in a molecule will be carried out. The study of differences in isomers brought about by differences in location of atoms or groups in molecules in three-dimensional space is called stereochemistry. Geometric and conformational stereoisomers have already been examined in the previous chapter. Geometric isomers differ from each other based on the arrangement of the atoms or groups across a rigid plane, such as a double bond or rigid cyclic ring in the molecule. For example, trans-1,2-dichloroethene is different from cis-1,2-dichloroethene since the chlorine atoms are on different sides of the rigid double bond.

On the other hand, conformational isomers differ from each in that groups or atoms in the molecule are in different locations due to rotation about a single bond, typically a carbon—carbon single bond. Thus, the anti-conformer of 1,2-dichloroethane is different from the gauche conformer. These conformers are different only in the spatial arrangement of the groups around the carbon—carbon bond; and as a result, their relative energies are different.

For some stereoisomers, it is extremely difficult to visualize differences that result in three-dimensional space for molecules brought about by different spatial arrangements of the atoms or groups, unless a model set is used. Stereochemistry is the study of molecules that have the same atom—atom connectivity, but the spatial arrangement among the groups or atoms is different, resulting in different molecules, known as stereoisomers. Just as a pair of shoes looks similar, but the spatial arrangements in three-dimensional space are different. Try putting on the left foot of the shoe on your right foot, it just will not fit since it is different from the right foot of a pair of shoes. Just as the left foot pair of shoes and its right foot can be described as different, we will see in this chapter that the same comparison can be made for some molecules.

Being able to recognize stereoisomers is an essential technique in that it requires students to be able to analyze molecules in the three-dimensional space and carry out various manipulations mentally in order to make important conclusions about molecules. This technique is an important analytical thinking tool since most of today’s highly technical activities are typically carried out while viewing the procedure on a 2-D computer screen. A large percentage of surgeries today are carried out using techniques in which various surgical manipulations are viewed and controlled by viewing three-dimensional procedures on a computer 2-D screen.