Question

Explain the following terms: enantiomers, racemic mixtures.

Short answer

Enantiomers are mirror-image isomers, while racemic mixtures contain equal amounts of both enantiomers, making them optically inactive.

Step-by-step solution

Define Enantiomers

Enantiomers are a type of stereoisomer that exist as pairs. They are molecules that are mirror images of each other but cannot be superimposed onto one another, similar to how left and right hands are related. Enantiomers typically have identical physical and chemical properties except for the direction they rotate plane-polarized light and how they interact with other chiral substances.

Describe Racemic Mixtures

A racemic mixture, or racemate, is a mixture that contains equal amounts of left- and right-handed enantiomers of a chiral molecule. Because the optical activities of the enantiomers cancel each other out, racemic mixtures do not rotate plane-polarized light and are thus considered optically inactive. Racemic mixtures can be formed when synthesizing chiral substances, where both enantiomers are produced equally.

Key concepts

Enantiomers

Two molecules are called enantiomers if they are non-superimposable mirror images of each other. This means that, although they look alike in the mirror, they cannot be perfectly aligned when placed on top of each other. This is similar to how left and right hands are mirror images but don't align when the palms are facing in the same direction. Enantiomers often have identical physical properties such as melting points and solubilities. However, they differ in the way they interact with polarized light.
One enantiomer will rotate plane-polarized light in one direction, while the other will rotate it in the opposite direction. This property is known as optical activity.

• If a compound rotates light clockwise, it is termed dextrorotatory (denoted as "+").
• If it rotates light counterclockwise, it is called levorotatory (denoted as "-").

Enantiomers are also important in biological systems since many biomolecules are chiral. This means that one enantiomer of a drug might be therapeutic, while the other could be inactive or even harmful due to different interactions in the body.

Racemic Mixtures

A racemic mixture is a combination of equal quantities of two enantiomers. In this mixture, the optical activities of each enantiomer neutralize each other, making the entire mixture optically inactive. This occurs because one enantiomer rotates plane-polarized light one way, while the other rotates it by the same amount in the opposite direction.
As a result, racemic mixtures do not rotate light at all. Racemic mixtures can be commonly encountered in reactions that produce chiral molecules, where there is no intrinsic preference for forming one enantiomer over the other.
The process by which a racemic mixture is formed is called racemization. Separating the enantiomers from a racemic mixture can sometimes be challenging but is often necessary in fields like pharmaceuticals where the activity of one > enantiomer is desirable. Techniques like chiral resolution are employed to achieve such separation.

Chirality

Chirality is a key concept in stereochemistry and refers to the geometric property where a molecule is not superimposable on its mirror image. Think of chirality like your hands: they are mirror images, but you can't place your left hand over your right and have all fingers align perfectly. This lack of identical supremacy is what gives rise to the concept of enantiomers.

• Chiral molecules have no plane of symmetry, meaning they can exist in two distinct forms, often labeled as left-handed and right-handed forms.
• Chirality is what enables the unique interactions substances have in chemical and biological contexts.

A molecule is termed "chiral" if it lacks internal symmetry, while one that possesses such symmetry is said to be "achiral." Chirality is significant in many scientific fields, particularly in organic chemistry and the pharmaceutical industry, where the 3D arrangement affects how drugs interact with the body. Recognizing and understanding chirality allows scientists and engineers to design molecules with specific desirable properties.