Core Concepts-Racemic Mixtures
In this tutorial, you will learn about what makes a racemic mixture and how they are optically inactive. In addition, you will learn some of the various ways to separate racemic mixtures.
Topics Covered in Other Articles
Stereoisomers and Chiral Centers
IUPAC Naming of Organic Compounds
What is a Racemic Mixture?
A racemic mixture or racemate is a mixture that contains an equal amount of two enantiomers. As light passes through the mixture, the enantiomers rotate the light equally but in opposite directions; this causes the mixture to be optically inactive. Louis Pasteur was the first to identify two types of isomers in tartaric acid. When he identified the presence of both enantiomers within the mixture, he named it a racemic mixture.
Optical Activity of Racemic Mixtures
Optical activity occurs in chiral molecules and refers to the ability of the molecule to rotate the plane-polarized light passed through them.
The polarizer acts as a filter, ensuring that the light moving through it goes in one direction. The light produced by the polarizer is called plane-polarized light. As the light passes through the racemate, the dextrorotary enantiomer rotates the plane-polarized light clockwise, and the levorotary enantiomer rotates the plane-polarized light equally counterclockwise. The overall rotation level is equal to zero, making racemic mixtures optically inactive.
Because enantiomers are so similar in chemical structure, the only way we can distinguish between them is by the direction they rotate plane-polarized light. We use this knowledge to name the two compounds. If a compound rotates the plane-polarized light counterclockwise, it is called dextrorotation. The prefix for a dextrorotary compound is either a (+) or a d-. In contrast, if a compound rotates the plane-polarized light clockwise, it is called levorotation. On the other hand, the prefix for a levorotary compound is either a (-) or an l-.
Racemic mixtures are often denoted by (±)- or less often by dl-. These symbols show that equal amounts of the dextrorotary (+) and levorotatory (-) enantiomers are present in the mixture. If the ratio of enantiomers is not 1:1 or unknown, they are prefixed by (+)/(-) or d/l-. (Note that dextrorotary and levorotary are different from R and S!)
Resolution of Racemic Mixtures
Resolution is the process of separating a racemic mixture into its enantiomers. Enantiomers are difficult to separate because of their similar physical and chemical properties. Despite this, we have a few methods that get the job done.
Louis Pasteur used this method in his discovery of racemic mixtures. If the enantiomers are solids, they are separated based on their differences in shape. This method is fairly simple but very tedious.
Formation of Diastereomers
The first step is to react the racemic mixture with chiral resolving agents. The product of this reaction will be a mixture of diastereomers. Because diastereomers have different properties from each other, they will be easier to separate by ordinary techniques (e.g., fractional crystallization). After separation, the diastereomers are treated with a resolving agent to convert back to their respective enantiomers.
Enzymes act as catalysts and are stereospecific chiral protein molecules. They will react with only one enantiomer in a racemic mixture due to their chirality. The enantiomer that binds to an enzyme will undergo a reaction while the other enantiomer remains unchanged. The unchanged enantiomer is removed from the mixture by ordinary separation methods (e.g., distillation). After the process is complete, the other half of the mixture is lost.
Racemic mixtures can be separated into enantiomers using chiral column chromatography or gas chromatography. A chiral stationary phase will have stronger interactions with one enantiomer, isolating the other enantiomer from the mixture.
Recapping Racemic Mixtures
A racemic mixture contains an equal amount of two enantiomers. The mixture is optically inactive because each of the enantiomers rotates plane-polarized light in equal but opposite directions. The prefix for a dextrorotary compound of a racemate is a (+), and the levorotary compound is a (-), while the entire mixture’s prefix is a (±). There are various ways to separate a racemic mixture, including mechanical separation, forming diastereomers, using enzymes, and chromatography.