Constitutional Isomers

Core Concepts:

Learn how to identify constitutional isomers as well as write them from a molecular formula using the Index of Hydrogen Deficiency (IHD).

Topics Covered in Other Articles:

What Is a Constitutional Isomer?

constitutional isomers structure example

Isomers are organic compounds that have the same molecular formula but have different 3D arrangement of atoms. We’ll be focusing on constitutional isomers which are the isomers that only differ in how they are structurally connected. They have the same molecular formula but have different structures. Think of it as if you and your friend are wearing the exact same articles of clothing, but you’re wearing them in a completely different order!

Why are constitutional isomers important?

How a compound is connected has a variety of effects on the characteristics of a compound. In can affect the stability of the compound (it’s likeliness to interact with other compounds), the naming of the compound, and even the melting and boiling point of the compound.

How do you identify constitutional isomers?

The first step to identifying constitutional isomers is to create a molecular formula for the compound. Here we can see that both of these structures have 7 Carbon atoms:

We can also count the number of hydrogens present as well. We know that carbon has to have 4 bonds to it. So, if each line counts as a bond, we can take 4 minus the number of bonds at each carbon and count those as hydrogens. We can also see that there is both a fluorine (F) and chlorine (Cl) on each of these structures.

labeling atoms in constitutional isomers for molecular formula

Thus, we have 7 carbons, 14 hydrogens, 1 chlorine (Cl), and 1 fluorine (F) on each structure. Our molecular formula for both structures will be C₇H₁₄FCl. Even though these two structures have the same molecular formula, we can see that they look completely different! The carbons aren’t in a chain and the fluorine and chlorine are connected to different carbon atoms than before. This tells us that we have constitutional isomers.

How do you draw constitutional isomers from a molecular formula?

We can draw a compound and its constitutional isomers from the molecular formula. I find is easiest to first make all of the single bond structures, in order to rule out making any mistakes. However, make sure to not make the same structures (as shown in the red box below).

From the molecular formula we see that we have 5 Carbon atoms, so I started off with making a chain of 5 carbons. The next step is to remove one carbon from the chain and make it into a substituent (branch) at every carbon possible. After that, keep removing carbons until you arrive at the smallest carbon chain; 2 carbons!

Index of Hydrogen Deficiency (IHD)

After making the different variations of the carbon chains (without creating duplicating structures), I then check to see if I can create any rings (or membered structures). You do this by checking the IHD (index of hydrogen deficiency) of a structure. Some chemists may also call IHD the “degrees of unsaturation,” which you can learn more about in this article.

IHD is something that you’ll use in spectroscopy but it’s helpful to know here in order to know what kind of double bonds and rings are possible. To calculate the IHD you multiply the number of carbons atoms by 2, add the number of nitrogen atoms, add 2, then subtract the number of hydrogen and halogen atoms. Since our molecular formula doesn’t have any nitrogen atoms or halogen atoms, we don’t need to worry about this for now. Below is a chart that summarizes what the IHD number means and how to calculate it.

Here we have an IHD of 0. This means we will not have any rings or double bonds for this structure! That means for this structure, we have created all of the constitutional isomers for C₅H₁₂!

Lecture Video explaining Constitutional Isomers

For more help, please enjoy our animated lecture video explaining constitutional isomers and ending with a cool real world application!

Practice Problems:

Below are some more practice problems with images at the bottom of that article with the answers!

Construct all of the constitutional isomers for the given molecular formulas:

C₃H₈O
C₃H₇N

Answers:

From the IHD we can only create single line structures. So I created a 3 carbon chain with the oxygen in every possible substituent while keeping the number of hydrogens at 8. The oxygen can also be substituted into the carbon chain!

Here our IHD is 1; this means that we can have either a double bond OR a ring. I first created the longest carbon chain possible with nitrogen as a branch. Notice that I didn’t substitute the nitrogen in the branch because that would have meant we had too many hydrogens. I then created the longest possible carbon chain with nitrogen in every branch and added a double where in all spaces that kept the number of hydrogens at 7. Here we can substitute hydrogen in the chain because we can use a double bond to reduce the number of hydrogens. After that I then created all the ring structure possible with putting the nitrogen as a branch and substituted into the ring!