Name: Racemic Mixtures and the Resolution of Enantiomers
Uploaded: 2023-04-30T18:28:34
Description: Learn by watching this video about Racemic Mixtures and the Resolution of Enantiomers at JoVE.com

A racemic mixture, also referred to as a racemate, is an equimolar mixture of enantiomers of a molecule. Racemic mixtures are denoted with the (&#177;) prefix, as they consist of an enantiomeric pair, and the corresponding optical rotation descriptor applies to the whole solution rather than a specific stereoisomer.
Recall that enantiomers can exhibit different properties when they interact with chiral media. For instance, enantiomers of the carvone molecule interact differently with the chiral olfactory receptors in our body. While (R)-carvone smells like spearmint, (S)-carvone smells of caraway or dill.
Similarly, enantiomers in a racemic mixture can react differently with other chiral molecules, such that the addition of a chiral molecule to a racemic mixture results in the formation of two or more different products.
For example, when N-propylglucosamine, a chiral glucose derivative, is added to a racemic mixture of the drug naproxen, two different salts are formed. These salts are diastereomers of each other and exhibit different physical properties.
When the reactants are stirred in an aqueous solution, the salt of (S)-naproxen crystallizes out of the solution, while the salt of (R)-naproxen remains dissolved in the solution. Accordingly, these salts can be easily separated and converted back to obtain enantiomerically pure (R)- and (S)-naproxen.
This process of separating the enantiomers is termed &#8216;enantiomeric resolution&#8217;. Here, N-propylglucosamine is referred to as a chiral resolving agent owing to its ability to react differently with the enantiomers of naproxen in the racemic mixture.
Some molecules, such as tartaric acid, can undergo spontaneous enantiomeric resolution without the addition of any chiral resolving agent. Here, upon cooling the racemic mixture, each enantiomer selectively crystallizes with itself. This results in the separation of the enantiomers into different crystals.
In general, the resolution of racemic mixtures into separate enantiomers requires the application of special techniques such as chiral chromatography.

racemic mixtures and the resolution of enantiomers

A racemic mixture, or racemate, is an equimolar mixture of enantiomers of a molecule that can be separated using their unique interaction with chiral molecules or media. Racemic mixtures are denoted by the (&#177;)- prefix. This &#8216;optical rotation descriptor&#8217; applies to the whole solution of a racemic mixture rather than a specific stereoisomer. Enantiomers typically have the same physical and chemical properties. Hence, they are not easily separable. However, enantiomers can exhibit different properties when they interact with chiral media. For instance, enantiomers of a molecule can interact differently with the chiral olfactory receptors in our body so that each enantiomer smells unique.
Racemic mixtures can therefore be separated using a unique process called enantiomeric resolution. Here, the enantiomers react with a chiral resolving agent to produce diastereomers. These diastereomers can be easily separated and reconverted to obtain the enantiomerically pure molecules. Depending on the molecular structure, the interaction between the chiral resolving agents and enantiomers can be either covalent, ionic, or a weak intermolecular interaction involving van der Waals forces. However, some racemic mixtures can undergo spontaneous resolution without the presence of a chiral resolving agent. For example, the racemic mixture of an N-trifluoroacetylated &#945;-amino alcohol in carbon tetrachloride undergoes spontaneous resolution.
The resolution processes can be classified in general into chiral and kinetic resolution. In a chiral resolution, the interaction of the racemic mixture with a pure enantiomer forms diastereomers. These are then separated owing to the difference in their physical properties. In the kinetic resolution, the separation of enantiomers is based on the difference in reaction rates of enantiomers with a chiral catalyst. Unlike the former, kinetic resolution takes advantage of the difference in the chemical properties of the starting materials in the racemic mixture.

Learn by watching this video about Racemic Mixtures and the Resolution of Enantiomers at JoVE.com

Racemic Mixtures and the Resolution of Enantiomers

A racemic mixture, or racemate, is an equimolar mixture of enantiomers of a molecule that can be separated using their unique interaction with chiral ...