Lecture 7: Stereochemistry Part 2

Racemic mixture

A 1:1 (or nearly 1:1) mixture of enantiomers of the same structure.

Tartaric acid has two stereocenters. What is the most stereoisomers it can have?

A compound with two stereocenters can be RR, SS, RS, or SR (four stereoisomers) n stereocenters can have 2ⁿ stereoisomers

Meso compound

An achiral substance that also has two or more stereocenters.

Carvone exists as a pair of enantiomers. (R)-(-)-carvone smells like spearmint, whereas (S)-(+)-carvone smells like caraway. Why do they have different smells?

Biological effects, such as smell, may occur when a compound interacts with an enzyme or receptor in the body. An olfactory receptor may bind readily to (R)-(-)-carvone, thereby sending a specific signal (spearmint smell) to the brain. This fit depends upon very specific shape and size of the receptor and the carvone. The (S)-(+)-carvone has a mirror image shape and thus does not fit as well into the same receptor. Lock and key. Cannot fit mirror images.

Diastereomers

One of a set of stereoisomers that are not enantiomers.

Explain why Pasteur was able to resolve racemic acid, but not mesotartaric acid.

Racemic acid is a racemic mixture of (+)-tartaric acid and (-)-tartaric acid. As a mixture of enantiomers, racemic acid can be resolved in to the two enantiomers. Mesotartaric acid is a meso compound, so it cannot exist as enantiomers, and therefore cannot be resolved.

Why must enantiomers be converted to diastereomers before separation processes occur?

Separation by crystallization requires the compounds undergoing separation to have different solubilities. Solubility is a physical property. The only difference in the solubility properties of enantiomers is the direction in which they rotate plane-polarized light. Conversely, diastereomers have different physical properties, such as solubility. Thus, conversion of enantiomers into diastereomers (different solubility) is crucial for separation by crystallization.

Resolution

The process of separating enantiomers. The separation may be achieved by physical or chemical means.

Racemization

To convert a substance into a racemic mixture.