Core Concepts
In this organic chemistry tutorial, you will learn what stereoisomers are and how they are classified. You will also learn about chirality, chiral molecules, and how to identify and label the chiral centers.
Topics Covered in Other Articles
What are Stereoisomers?
A stereoisomer is one molecule in a set of other closely related molecules. Stereoisomers are composed of the same atoms; however, their orientations are different and they are not superimposable, meaning they cannot be exactly aligned with each other.
What is a Chiral Center?
Stereoisomers have the same connectivity- that is, they have the same bonds between the same atoms- but a different spatial arrangement. These spatial differences are present at chiral centers, composed of a central atom with four unique constituents (either a single atom, or a group of atoms). Take Molecule A and Molecule B as examples.
In Molecule A, the central carbon has four of the same atom, Hydrogen, attached to it, and is not a chiral center. Molecule B has four separate groups attached to carbon. The carbon is a chiral center, and Molecule B, along with its complementary molecule, Molecule C (seen below), are stereoisomers. So the definition of a chiral center, is an atom with 4 different atoms or groups attached to it.
Labeling Chiral Centers
The orientation of chiral centers can help differentiate between various stereoisomers. These centers can be described as R or S, meaning the molecule can have either “right-handedness” or “left-handedness”.
Here are the steps to determine this:
- Label the groups by priority 1-4, with the highest priority being 1. If two atoms are the same, look to the next connected atom.
- Orient the molecule so that the lowest priority group (labeled 4) faces back.
- Assess: In this orientation, do the numbers increase in a clockwise or counterclockwise fashion. If it is clockwise, the chiral center is R. If it is counter clockwise, the chiral center is S.
Following these rules, the chiral center in Molecule B is S, and Molecule C is R.
Types of Stereoisomers
Stereoisomers can be further classified into enantiomers, or diastereomers. (check out our Enantiomers vs. Diastereomers article for more information) Enantiomers are optical isomer, and diastereomers are geometric isomers.
One relevant difference between the two is that enantiomers have only one chiral center while Diastereomers have two or more. The different arrangements of these centers gives you the different flavors of diastereomers seen in the chart above.