Stereoisomers and Chiral Centers

Core Concepts

In this organic chemistry tutorial, you will firstly 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. 

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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 Chirality?

Chirality is an important concept in chemistry that deals with the symmetry or lack of symmetry in certain molecules. To understand chirality, let’s start with the idea of symmetry itself.

Symmetry is a property that describes how an object looks the same when certain operations, such as rotation, reflection, or translation, are applied to it. For example, if you have a square, it looks the same when you rotate it by 90 degrees or when you flip it horizontally or vertically.

Now, let’s talk about molecules. Molecules are made up of atoms, and atoms can be arranged in different ways to form different molecules. When we talk about chirality, we’re specifically interested in molecules that have a central atom bonded to different groups or atoms.

Imagine you have a central atom with four different groups attached to it, represented as four different colored balls. To determine if the molecule is chiral or not, we need to see if there is a plane of symmetry. A plane of symmetry is an imaginary line that divides the molecule into two equal halves, and each half is the mirror image of the other.

If the molecule has a plane of symmetry, it means that it is achiral, which basically means it is symmetrical and does not exhibit chirality. Just like a square, it looks the same after certain operations.

However, if the molecule does not have a plane of symmetry, it means it is chiral. This means that the molecule is not superimposable on its mirror image. It’s like having a right hand and a left hand—they look similar, but they are not exactly the same and cannot be superimposed on each other.

Chiral molecules are important because they often have different properties from their mirror images. For example, when it comes to pharmaceuticals, one mirror image of a chiral molecule might be effective as a drug, while the other mirror image could be inactive or even harmful. This is why it’s crucial to understand chirality in fields like drug development.

In summary, chirality refers to the property of molecules that lack a plane of symmetry and have mirror images that are not superimposable. It’s like having a left and a right hand—they have similarities, but they are not the same.

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. 

chiral centers and chirality

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.

chiral molecules

Labeling Chiral Centers

The orientation of chiral centers can generally 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:

  1. 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.
  2. Orient the molecule so that the lowest priority group (labeled 4) faces back.
  3. 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.

stereoisomers examples

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. 

Stereoisomers – Further Reading