Racemic mixture

Carbocation attack to racemic product

Racemic mixture refers to a mixture of equal amounts of two enantiomers of a chiral molecule. In chemistry, a chiral molecule is one that cannot be superimposed on its mirror image, much like how left and right hands are mirror images but not identical. The two enantiomers in a racemic mixture have identical physical properties except for the direction in which they rotate plane-polarized light and their reactions in a chiral environment. A racemic mixture is optically inactive, meaning it does not rotate the plane of polarized light, due to the equal and opposite effects of the two enantiomers.

History

The concept of racemization, leading to the formation of racemic mixtures, was first introduced by Louis Pasteur in the 19th century. Pasteur discovered that certain compounds could exist in two forms that were mirror images of each other, laying the groundwork for the field of stereochemistry.

Preparation

Racemic mixtures can be prepared by several methods:

• Synthesizing a chiral compound in the absence of any chiral influences, which usually results in a 50:50 mixture of enantiomers.
• Mixing equal amounts of the two pure enantiomers.
• Racemization of a single enantiomer, where the enantiomer is converted into an equal mixture of both enantiomers under certain conditions.

Separation

The process of separating a racemic mixture into its individual enantiomers is known as resolution. This can be achieved through various methods, including:

• Using a chiral resolving agent that forms diastereomers with the enantiomers, which can then be separated due to their different physical properties.
• Chromatography using a chiral stationary phase.
• Enzymatic resolution, where an enzyme selectively reacts with one enantiomer.

Importance

Racemic mixtures are significant in various fields, particularly in the pharmaceutical industry. The biological activity of a drug can vary significantly between its enantiomers. For example, one enantiomer may be therapeutically active, while the other is less active or may even cause adverse effects. Therefore, the resolution of racemic mixtures to obtain the active enantiomer is a critical step in the development of many pharmaceuticals.

Examples

• Thalidomide is a notorious example where one enantiomer was effective as a sedative and anti-nauseant, while the other caused severe birth defects.
• Ibuprofen is sold as a racemic mixture, though only one of its enantiomers is active as a pain reliever.

See Also

• Chirality (chemistry)
• Enantiomer
• Stereochemistry
• Pharmaceutical drug

External links

• Chemguide: Racemic Mixtures
• National Center for Biotechnology Information: The Importance of Being Earnest: Enantiomers in Clinical Practice

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