Notes for Chapter 5

Chem 243

Stereochemistry

So now the real fun begins! What looks, at first, like a very subtle aspect of the geometry of carbon atoms, leads to concepts that are critical to the existence of life itself.

The tetrahedral nature of the carbon sp3 orbitals (and those of other atoms like nitrogen and phosphorus as well), lead to the yet another type of isomerism, stereoisomers.

Stereoisomers are molecules which have the same atoms, connected in the same order, but are not identical. By identical is meant that one stereoisomer cannot be superposed onto the other with all atoms coinciding in space. You really need to use molecular models to appreciate this concept effectively.

The two types of stereoisomers with which we will be concerned are enantiomers and diastereomers.

Enantiomers are nonsuperposable mirror images of each other. Diastereomers are nonsuperposable non mirror images of each other. But in each case, the connectivity of the atoms is identical! Well, if we consider the 3-dimensional connectivity they are not really connected identically, so give the organic chemists a little semantic leeway in this matter.

Again, we look at the energy of each isomer, calculated by quantum mechanics. Take two enantiomers, they are mirror images of each other, they are not superposable, so if you calculate their energies they should have identical values. This is the case. Since many physical and chemical properties can be derived from these energy values, their properties should be identical, e.g., boiling point, melting point,, and they are. Well, not always! (Doesn't it really bug you that whenever we introduce some new concept, with some rules, there is often some perverse exception to that rule? That's the universe we live in, don't blame organic chemistry!)

We have this concept called chirality (handedness) which allows us to describe these molecules. Two enantiomers will have identical properties in achiral conditions. But if we place them into chiral environments, which means that the environments are mirror images of each other, the two enantiomers can have different properties. In this chapter we will study a number of situations in which this occurs.

Diastereomers, on the other hand, are not mirror images of each other and so when we calculate their energies, there is no reason they should be identical except by coincidence. Therefore, they are expected to have different properties, and they do. Sometimes these differences are not great, but they are usually not identical.

Of course, being organic chemistry, there are symbolic names for the various types of stereoisomers, and stereogenic centers. You need to learn these systems.

Essentially, you will need to be able to identify chiral molecules as well as stereogenic centers in molecules. Be able to interpret Fischer projections and assign R or S configurations to the stereogenic centers in the diagram. Know the difference between enantiomers and diastereomers and be able to distinguish these relationships from the Fischer projections. You should also be able to draw a Fischer projection for a specifically named molecule. Understand the differences between constitutional isomers, stereoisomers for open chain and cyclic compounds. Also be able to recognize meso commpounds and answer questions concerning optical rotation.