Racemic Mixture: Definition & Example

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  • 0:01 Vocabulary of Stereochemistry
  • 1:02 Definition of Racemic Mixture
  • 1:57 Biological Significance
  • 2:31 Racemic Mixtures in…
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Nancy Devino
In this lesson, you'll first review some important ideas about stereochemistry, learn the definition of a racemic mixture, and explore the biological significance of racemic mixtures. From there, you'll be able to test your knowledge with a quiz.

Vocabulary of Stereochemistry

There are some important terms that you'll need to know in order to master the concepts in this lesson. A chiral molecule possesses the property of handedness and is non-superimposable on its mirror image. Each member of a pair of enantiomers is chiral. For example, (R)-2-chlorobutane and (S)-2-chlorobutane are each chiral, and they are enantiomers.

Another important term to know is optically active. A single enantiomer - the R or the S configuration of the molecules you were just looking at - is optically active because it rotates plane-polarized light. One member of an enantiomeric pair will rotate light a certain number of degrees in one direction. Its mirror image will rotate light the same number of degrees, but in the opposite direction. In other words, they have equal but opposite specific rotation when placed in a polarimeter. Keep this in mind as you learn about the optical activity of a racemic mixture.

Definition of Racemic Mixture

A racemic mixture is a 50:50 mix of two enantiomers. No matter how many molecules are in a mixture, it is racemic if there are equal numbers of the two enantiomers. In the pharmaceutical industry, and sometimes the organic chemistry laboratory, you may hear the term enantiomeric excess. This means you have more of one enantiomer than the other. Since it's not a 50:50 mixture, a sample or a reaction that has enantiomeric excess cannot be a racemic mixture.

One way to know that you have a racemic mixture is by measuring its optical activity in a polarimeter. Because two enantiomers have equal and opposite specific rotations, a racemic mixture has a specific rotation of zero. A compound or a mixture whose specific rotation is zero is optically inactive. The bottom line: a racemic mixture will always be optically inactive.

Biological Significance

Why should we be concerned about racemic mixtures, when most biologically active compounds occur in nature as single enantiomers? Think about the amino acids - those found in most proteins in living systems are L-amino acids. There are some other fascinating examples where two enantiomers have very different properties because their 3D shape causes them to be recognized differently by receptors or enzymes. In the case of carvone, the R configuration smells like spearmint and the S configuration smells like caraway seeds.

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