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Isomers play an important role in the biology of life. Would you like to know how to spot a cis-isomer or a trans-isomer? In this lesson, we’ll explore examples and the two broad categories of isomers: constitutional isomers and stereoisomers.

What Is an Isomer?

Imagine you have colored blocks with numbers. You realize that it is possible to rearrange these blocks in multiple ways. You decide this structural rearrangement will involve the construction of one awesome block tower.

Did you notice something while constructing your tower? The blocks didn’t magically change shape, number, or grow in size. However, the arrangement of the blocks changed.This is a great example of isomers! Isomers are compounds whose molecular formulas are the same but chemical structures are different. The larger the molecule and more branched it is, the greater the number of isomers you can produce.

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Different Types of Isomers

Isomers can be divided into two broad categories: constitutional isomers and stereoisomers.

Constitutional Isomers

Constitutional isomers are compounds where the atoms are connected or bonded differently. These isomers can also be referred to as structural isomers.

Let’s look back at our block set example. If there were connections or rods linking one block to another and you just changed those connections, block set (a) would be a constitutional isomer of block set (b) and vice a versa. Just remember, the chemical structure changes, not the formula.


Stereoisomers are compounds that differ because of orientation in space. I know you may be thinking, what does space have to do with chemistry? Think of it this way: Imagine you can lift a chemical structure off a piece of paper and begin rotating the atoms and bonds, twisting the molecule, spinning it in different directions.

This ability refers to the molecular geometry or stereochemistry of a compound. It is the 3D orientation or arrangement of atoms in a compound. There are two types of stereoisomers: enantiomers and diastereomers. Try to visually imagine the spatial arrangement of these molecules.


Enantiomers are a type of stereoisomer where the compounds are a mirror image of each other. These mirror images are non-superimposable.

A great way to understand the concept of a non-superimposable image is to do this exercise with your hands:Take your right hand (with palms facing down) and place it on top of your left hand. What do you notice? The right thumb is facing in a different direction from the left thumb. The right hand is non-superimposable on the left hand because the fingers won’t match up with each other. This ability to be non-superimposable defines a chiral molecule. Chirality is a molecule that has a mirror image, and it is non-superimposable.


Diastereomers are isomers that are not mirror images of each other and are non-superimposable. This is the exact opposite of enantiomers.

There are two types of diastereomers: cis and trans. Cis-isomers contain atoms that are oriented on the same side of a bond. Trans-isomers have atoms oriented on opposite sides of a bond. Let’s look at a few visuals to understand these concepts.Here you see the carbon skeleton drawing of isomers for 3-nitro-2-butanol:

Isomers for 3-nitro-2-butanol

Imagine the dash line to be a mirror or fold.

For molecules A and B, can you see that the OH and H are located in the same position? NH2 and H are also in the same position. This lets you know you are looking at a mirror image. Now if you try to lift A and place it on top of B, you will see that the atoms no longer match up. This is non-superimposable. Thus, A is an enantiomer of B and vice a versa.Let’s take a look at diastereomers.

In C and D, you will notice that NH2 and H are not mirror images. C cannot be a mirror image of D, and if you try placing C on top of D, the atoms H and OH don’t match up. It is non-superimposable.

Thus, C is a diastereomer of D. Also notice that the Hs are on opposite sides in D; this is called a trans-isomer. In C, the Hs are on the same side; this is called a cis-isomer.Always draw carbon skeletons and pay attention to the arrangement of atoms when deciding if a molecule is an enantiomer or diastereomer.

An Example of Isomers

The configuration of atoms in a compound is very important in biology. For example, we have a protein in our eye called rhodopsin. This protein has different isomers.

Rhodopsin has a friend called retinal. Made from vitamin A, retinal also has isomers. When retinal changes from a cis- to trans-isomer, the light energy that enters our eyes is absorbed. Once retinal takes this absorbed energy of light and hands it off to rhodopsin, rhodopsin shuttles it through our vision system. Before you know it, we are able to see! Look at the two isomers; notice the changes between the structure of the retinal isomers.


Lesson Summary

Isomers are compounds that have the same molecular formula but different structures. There are two broad types of isomers: constitutional and stereoisomers.

Constitutional isomers differ in bonding and connectivity. Stereoisomers differ in 3D orientation. Enantiomers and diastereomers are two sub-types of stereoisomers. Enantiomers are isomers that are non-superimposable, mirror images. Diastereomers are not mirror images and are non-superimposable. Diastereomers include both the cis-isomer and the trans-isomer.

Isomers are widely present in the biology of life. Isomerization of rhodopsin and retinal play a key role in our ability to see.

Learning Outcomes

This video should help you to:

  • Define isomer
  • Distinguish between constitutional isomers and stereoisomers
  • Compare and contrast enantiomers and diastereomers
  • Differentiate between cis-isomers and trans-isomers
  • Provide an example of isomers

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