Structural Isomers: Definition & Examples

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  • 0:00 What are Structural Isomers?
  • 1:27 Chain Isomers
  • 2:58 Functional Group Isomers
  • 4:07 Positional Isomers
  • 5:03 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Molecular formulas don't always tell us how the atoms are arranged within a molecule. When one formula can lead us to different arrangements of atoms, the results are referred to as structural isomers, examples of which we'll explore in this lesson.

What Are Structural Isomers?

Did you love playing with building blocks when you were young? Remember how you used the same set of blocks to create different shapes and structures by taking them apart and putting them back together again? Well, molecular formulas are a lot like building blocks, and they can provide us with clues about structural isomers.

In theory, molecular formulas tell us how many atoms of each element there are in a molecule. Just like the building blocks, these atoms can bond together in different ways. Structural isomers are molecules with the same molecular formula, but their atoms have different arrangements or bonds.

Let's take a look at a molecular formula: C4 H10. While the molecular formula can't tell us how the atoms are arranged, it can help us find some of the possible arrangements. In this case, there are two different arrangements for this molecular formula: butane and isobutane.

Butane and isobutane have the same number of carbon (C) atoms and hydrogen (H) atoms, so their molecular formulas are the same. However, each one has a different structural formula, which shows how the atoms are arranged. So we can say that butane and isobutane are structural isomers.

Now that we know what structural isomers mean, let's discuss the different types of structural isomers. These include chain isomers, functional group isomers, and positional isomers.

Chain Isomers

Chain isomers are made up of two or more carbon or other compounds with the same molecular formula but different atomic arrangements, or branches. Imagine removing one carbon compound and its attachments from a chain and attaching them to another carbon compound within the chain.

For example, pentane, which has a molecular formula of C5 H12, has three different chain isomers. In the following illustration, you'll see that pentane and isopentane are chain isomers of each other.


When we switch the CH3 group of pentane, which is circled in red, with a hydrogen atom on the carbon chain, we'll have a chain isomer: isopentane.

The following is an example of neopentane, another chain isomer of pentane.


Neopentane results from exchanging the CH3 on the left with the H atom, which is circled in red, as well as the other CH3 group on the right with the H atom, which is circled in blue. By switching the hydrogen and CH3 groups on the pentane chain, we change the way the atoms bond with each other. Here, we can see all three chain isomers of pentane.

Chain isomers not only apply to carbon groups on the chain, but also to different atoms that may be present on the chain. Let's take a look at 2-bromobutane, which has a bromine (Br) group on the chain.


By changing how the carbon, hydrogen, and bromine atoms are attached to the chain, we can come up with the following chain isomers of 1-bromobutane.

Functional Group Isomers

Molecules are classified according to their functional groups, or groups of atoms or bonds with unique chemical and physical qualities. Let's take a look at some functional groups, where R and R' are side groups that consist of carbon, hydrogen, and/or other elements. For example, if a functional group of CH=O (carbonyl) is attached to a molecule, then this molecule is classified as an aldehyde. If a functional group of -OH (hydroxyl) is attached to a molecule, then this molecule is classified as an alcohol.

Functional group isomers have the same molecular formula, but different functional groups on the chain. For instance, ethyl alcohol and dimethyl ether have the same chemical formula, but different functional groups, which are circled in blue.


Ethyl alcohol has a hydroxyl group (R-OH) and a dimethyl ether group (R-O-R'). Other examples of functional group isomers include acetone, which is a ketone, and propanal, an aldehyde. They both have the same chemical formula, but their functional groups are different.

Positional Isomers

Let's go back to one of our building block structures and add a flag. Each time we revisit our structure, we leave the arrangement of the blocks intact and change only the flag. Positional isomers are similar in that everything remains unchanged, except the position of the important functional group, in this case, the flag. Positional isomers have the same carbon skeleton, but only the functional group or a substitute, which is any atom or group of atoms, changes position.

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