Enantiomers – Definition and Properties

Enantiomers

Enantiomers – Definition: Enantiomers are molecules that are mirror images of each other. They are structurally identical, but the orientation of their atoms is reversed. This means that their physical and chemical properties are also reversed. Enantiomers are not superimposable, because their molecules are not symmetrical.

Stereochemistry

• The term stereochemistry refers to the three-dimensional arrangement of atoms in molecules. The prefix stereo- comes from the Greek word for “solid.” Stereochemistry is the study of the way in which molecules are put together and the way in which their atoms are oriented in space.
• In stereochemistry, we consider molecules to be three-dimensional objects. The atoms in a molecule are not all located in the same plane. Some atoms may be located in the front plane, some in the back plane, and some in the side planes. The three-dimensional arrangement of atoms in a molecule is called its stereochemistry.
• The atoms in a molecule are not all located in the same plane. Some atoms may be located in the front plane, some in the back plane, and some in the side planes. The three-dimensional arrangement of atoms in a molecule is called its stereochemistry.

There are two types of stereochemistry:

1) Optical stereochemistry
2) Geometric stereochemistry

1) Optical stereochemistry is the study of the way in which molecules interact with light.

2) Geometric stereochemistry is the study of the way in which molecules interact with one another.

Optically Active

An optically active molecule is one that can rotate the plane of polarized light. This occurs because the molecule has a rotational symmetry that allows it to flip the direction of the electric field vector of the light as it passes through the molecule.

Racemic Mixture

A racemic mixture is a 50/50 mixture of two enantiomers. Enantiomers are molecules that are mirror images of each other, but they are not identical. They have different chemical and physical properties.

Chiral Centre

The chiral centre is carbon-1.

Superimposable

Refers to two shapes that can be superimposed on each other, like pieces of a puzzle.

Properties of Enantiomers

Enantiomers are a type of stereoisomers that are non-superimposable mirror images of each other. They have the same chemical formula and connectivity of atoms but differ in their spatial arrangement. Here are some properties of enantiomers:

1. Identical Physical Properties (except for optical activity): Enantiomers have the same boiling point, melting point, density, and solubility in achiral solvents. They have identical physical properties because these properties are determined by factors such as intermolecular forces and molecular weight, which remain the same for enantiomers.
2. Different Optical Activity: Enantiomers exhibit different optical activity because they rotate the plane of polarized light in opposite directions. One enantiomer will rotate light in a clockwise direction (dextrorotatory, designated as “+”) while the other enantiomer will rotate light in a counterclockwise direction (levorotatory, designated as “-“).
3. Different Biological Activity: Enantiomers often have different biological effects and interactions with chiral molecules in living organisms. This is particularly important in pharmaceuticals, as one enantiomer can be therapeutically active, while the other may be inactive or even have adverse effects. For example, the drug Thalidomide exists as a racemic mixture of enantiomers, and while one enantiomer has therapeutic properties, the other caused severe birth defects.
4. Different Interactions with Chiral Environments: Enantiomers can interact differently with other chiral molecules or environments. For instance, enzymes in living organisms are often chiral, and enantiomers can have different affinities or activities with specific enzyme binding sites.
5. Different Chemical Reactions with Chiral Reagents: Enantiomers can react differently with other chiral molecules or reagents. This is particularly relevant in asymmetric synthesis, where the desired enantiomer is selectively produced using chiral catalysts or reagents.

It’s important to note that enantiomers have identical chemical properties in an achiral environment, as they have the same functional groups and bonding patterns. However, their interactions with chiral environments or chiral reagents can lead to significant differences in their behavior and effects.