In this tutorial, you will learn about two types of stereoisomers: enantiomers and diastereomers. This tutorial will explain how to identify them, and explain their similarities and differences.
A Brief Note on Stereoisomers
Both enantiomers and diastereomers are types of stereoisomers. Stereoisomers are sets of molecules that have the same chemical formula, and the same connectivity, but differ in how their atoms are arranged in space. To review stereoisomers in a general sense, chiral centers, and how to classify them, please see the article, “Stereoisomers and Chiral Centers.”
Enantiomers and diastereomers are also referred to as optical isomers, because of the way they interact with light. Each molecule in a pair of enantiomers or diastereomers bends polarized light opposing directions.
What are Enantiomers?
Enantiomers are stereoisomers, so, they are molecules with the same connectivity, but different spatial orientation. They differ in their arrangement at positions called chiral centers, made of one central atom connected to four unique atoms, or groups of atoms. Molecules that are enantiomers must have at least one chiral center. Molecule A, along with its complementary molecule, Molecule B, are enantiomers. The central carbon in both cases acts as a chiral center.
How Do We Identify Enantiomers?
The simplest way to identify an enantiomer is to recognize that two molecules are mirror images of each other. Enantiomers must be mirror images. The molecules in the image above reflect over the bold line, which represents a mirror plane. Enantiomers are also non-superimposable, meaning that the two mirrored molecules cannot perfectly overlap. When the structure of Molecule A and Molecule B overlap, the Me and H group of each structure do not match up.
Another quick way to distinguish non-chiral compounds from chiral ones, like enantiomers, is to count the number of unique atoms branching from the compound’s center. This method works because of our requirement for enantiomers and diastereomers to have a chiral center (a chiral center has, “one central atom connected to four unique atoms, or groups of atoms.”) If a molecule has four unique atoms, it is possibly a chiral molecule, but if it has less than four distinct atoms (i.e.. two or more of the same atoms) it is automatically ruled out.
Additionally, you can determine if a molecule is a chiral compound, by looking for symmetry. If a plane of symmetry can be found in a molecule, its mirror image will be superimposable it will not be chiral. Molecules that meet these qualifications are called meso compounds.
Since enantiomers have the same make-up and only differ in their stereochemistry, they have the same physical properties such as melting and boiling point. However, they bend polarized light differently, either clockwise (+) or counterclockwise (-). Using + or – to describe this phenomenon is common in the field of chemistry. Another way to discern these structures is by labeling their chiral centers either R or S.
What are Diastereomers?
Diastereomers, unlike enantiomers, are not mirror images of each other. However, they are still non-superimposable.
In this example, Molecule D and Molecule E are not mirror images, and have differently assigned chiral centers that make them non-superimposable. Molecule D has a chiral center (blue) labeled S, and Molecule E’s corresponding center (green) is labeled R. Molecule D has a second stereocenter (red) labeled R, and the corresponding S stereocenter (orange) is found on Molecule E.
Physical Properties of Diastereomers
While enantiomers share physical properties, diastereomers do not. Their differing arrangement has an effect on their physical qualities such as melting or boiling point.
Enantiomers and diastereomers are both types of stereoisomers and optical isomers, and they are both are non-superimposable. While they share these similarities, enantiomers and diastereomers have important distinctions from one another, like their physical properties. The table below is summarizes what these compounds have in common, and what differences they have between them.
|Same physical properties, bends polarized light differently||Different physical properties|
|Mirror images||Not mirror images|
See which one of the compounds below is an enantiomer, diastereomer, or neither.
The compounds above are neither optical isomer. If you consider groups originating from the central atom, there are only three unique atoms. This does not fit the definition for a chiral center.
The compounds above are enantiomers. The molecules are mirror images that cannot be overlapped.