Constitutional Isomers - Online Tutor, Practice Problems & Exam Prep

Constitutional isomers are compounds with the same molecular formula but different connectivity of atoms. To determine their relationship, first check if all non-hydrogen atoms and the Index of Hydrogen Deficiency (IHD) are identical. If they differ, the compounds are different. If they match, assess the connectivity by identifying landmark atoms. If the connections are the same, the compounds are identical; if not, they are constitutional isomers. Understanding these distinctions is crucial for analyzing molecular structures and their properties.

What is a constitutional isomer?

So what are constitutional isomers? They're compounds that have the same exact molecular formulas, meaning that both of the compounds will have all the same atoms in them. But what is different about them is their connectivity. So go ahead and write that down. Connectivity. What does that mean? Connectivity has to do with the way the atoms are bonded to each other. So for example, if my first molecule has atoms linked to or bonded to, my second molecule might have the same atoms, but they're arranged differently. So maybe one atom is connected to another and then to another; that would be an example of a constitutional isomer.

For this course, what you're going to have to be able to do is you're going to have to be able to look at two different compounds and tell what that relationship is. Are they the same compound, meaning that everything is exactly the same in terms of the molecular formula and the connectivity, or are they completely different compounds, meaning that they have different atoms entirely? And then there's this other category, which is are they constitutional isomers, which means that they have the same atoms, but they're connected differently. It can be really tricky to differentiate which one is which. And that's why I've made this nice little flowchart for you guys to follow. All right?

Using the flowchart to determine isometric relationships.

So imagine that you're given this question right here. This is an exam. How are the following 2 compounds related? Are they identical, constitutional isomers, or different compounds? Where do you even begin? Well, it turns out that this question is, I think, a little bit beyond your level. All of you guys are probably a little bit stumped by this question. You might think that you know the right answer, and you might be right, but most likely, you didn't get to that answer in a systematic way. So what I want to teach you guys is the systematic way to get these problems right every single time. All right. So we're going to come back up to that question. What I actually want to do is go through these steps right here, and these steps will guide you through constitutional isomer questions.

Notice that it uses Index of Hydrogen Deficiency (IHD). That's why I had to teach you the IHD first because this is the easiest way to tell the difference between different types of compounds. So, my first question that I have to ask myself makes sense. It's just, are all the atoms the same? Because remember, I told you that in order to be a constitutional isomer, all the atoms have to be exactly the same. So the way that we tell, a lot of students what they do is they start counting every single atom in the entire molecule and they compare it to the other one, and then obviously, they see if they're the same or not. But what I would recommend is don't count the hydrogens. Count only non-hydrogen atoms and then count the IHD in both compounds. The reason why is that IHD is actually just a measurement of hydrogens. Remember that IHD would tell you how many hydrogens are missing? And IHD happens to be much easier to calculate the number of hydrogens. You know why? Because if you have to calculate the number of hydrogens, that means you're looking at a bond line structure. You're going to have to first put all the hydrogens in there, and then you might have so many hydrogens that you might miscount, and that happens all the time with students. So, what I prefer is instead of counting each hydrogen out at a time, just use the IHD instead. The IHD is a placeholder for the number of hydrogens. Now, check this out. If those two numbers are not exactly the same, so if I have a different amount of non-hydrogen atoms or a different amount of IHD in both of the atoms, in both of the molecules, then these are going to be different compounds. Why are they different compounds? Well, because if they don't have the same atoms, then they're just automatically different. A compound can only be the same as another compound if it has the same exact atoms inside of it. So that would be the answer for this question up here if we found that they had different amounts of carbons, which is a non-hydrogen atom, or different amounts of IHD. But let's keep going and see what happens. If they happen to line up, meaning that the non-hydrogen atoms and the IHD are exactly the same in both, then we go to step 2.

What is step 2? Well, step 2 makes sense as well. Now, I'm going to ask myself, are all the atoms connected exactly the same way? Now, it turns out that most of the time, this is going to be a very easy question to answer. Most of the time you're going to see your 2 compounds, and they're going to look very different. For example, one of them is a square and one of them is a 4-carbon chain. Would that be the same thing? Would it be the same thing if I had a square on one side and a 4-carbon chain on the other side? Would you say that they're connected the same way? Absolutely not. They look completely different. The only way that they can be connected the same is if every atom is connected to the same atom on the other molecule. So most of the time, common sense is just going to tell me yes or no. Either they look the same or they don't. But sometimes, you get a situation like up here where these two molecules above me, they look like they are kind of similar, but maybe they're rotated. I don't know. So, for this one, we're going to want to use a systematic method. What I always say is to look for what I call a landmark atom. Now, this is not a word that you're going to find in your book, so don't look for "landmark atom." It isn't anything. It's just a word that I use to say look for something that stands out. So, you would look for something that stands out, everything that you can compare in both molecules. So, for example, maybe if your compound has an oxygen in it, then you would look at where the oxygen is in the first one and where the oxygen is in the second one and see, are they in the same place. If your compound has something else like a ring in it, maybe you look at the ring and you say, is the ring in the same place in both? If they are not exactly the same, then that means that you have constitutional isomers. Why? Because that means that we got to step 2, which means all the atoms and the IHD were the same. But then, step 2, what we said is that they are not exactly the same in the way they're connected. So that's constitutional isomers. That's the definition of an isomer. What if they are the same, though? So, let's say that means that now all my atoms are the same and they're connected exactly the same. Then those are just going to be identical compounds. Does that make sense? And those are my 3 options.

Isomeric Relationships

Want you guys to do, we're going to do the first one as a worked example and what I want you guys to do is work with me, think through this. Where do we start with these 2 molecules and maybe let's just call this question, "What would we start to compare them?" First of all, it's saying how are they related? Are they identical? Are they constitutional or are they different? I know right away there's one answer choice that we can eliminate. What do you guys think? Which one can we eliminate right away? Identical. These are not identical compounds. One of them has a ring and one of them is a chain. There's no way that those are the same thing. Wouldn’t you agree? So we're just going to scratch out identical. Let's not even think about that.

Now what I want to do is I want to go through my flowchart. Okay. So the first thing I do is I count up non-hydrogen atoms and IHD (InIndex of Hydrogen Deficiency). Let's start off with non-hydrogen atoms. What I have here is carbon, oxygen, and fluorine. Would you guys agree? I also have hydrogens present, but remember I'm going to ignore those for now. How many carbons do I have in the first molecule? Good. I have 5. 1, 2, 3, 4, 5. How many oxygens do we have? 1. How many fluorines do we have? 1. Makes sense so far?

Now I'm going to do the same thing for the other one. Carbons, oxygens, fluorines. How many carbons do I have? Also, 5. How many oxygens do I have? 1, and fluorine is 1. So far, it looks like all the atoms are the same. So it looks like maybe these are constitutional isomers. But let's also count IHD (InIndex of Hydrogen Deficiency). Remember I have to count IHD. So the IHD of this first one, just remember when you have a structure, how do you count IHD? Remember that double bonds and rings equal 1 and triple bonds equal 2.

So in this case, what's my IHD? It's 1. Now let's take the IHD of the second compound. What's the IHD of the second compound? 0. Are these IHDs the same? It's 0 because there's no ring, no double bond, no triple bond. So are these IHDs the same? No. What that means is that remember what it means for an IHD? It means that you're missing 2 H’s. It means you're missing 2 H’s. So do these two compounds have the same number of hydrogens? No. One has fewer than the other. So what are these going to be? Different compounds. Because I got stuck right up here, if not exactly the same, they are different compounds. That's exactly what happened. They were not the same, so I had different compounds. Does that make sense?

Now, the reason that I went through this whole trouble of teaching you IHD is because, as our molecules get bigger, that's going to be even more and more helpful. Instead of having to count out every single hydrogen, we just use IHD, and it's so much easier.

Isomeric Relationships

Right. So first of all, there's another one that we could immediately cross out and again, that was identical compounds. The first one has a triple bond. The second one is a square. There's just no way that these things are the same. So now let's talk about carbons and nitrogens. How many carbons does this one have? 4. How many carbons does this one have? 4. How many nitrogens does this one have? 1. How many nitrogens does this one have? 1. Are there any other non-hydrogen atoms? No. I just have carbon, nitrogen, and hydrogen. So I'm done with that part. Now I just have to do the IHD. So what is the IHD? Wow. What is the IHD of the first molecule? It's 2. Why is it 2? Because remember that double bonds count as 1, triple bonds count as 2. I just did 3. Triple bonds count as 2. So that means that it's missing 4 hydrogens. Let's see if the other one is the same. My IHD over here is, I have a ring and a double bond that's also going to be equal to 2. That means that this one is also missing 4 hydrogens, which means that these have the same atoms. So I'm just going to put here same atoms. So now that I just found out that these have the same atoms, are they connected the same? No. We just said that they are not connected the same. One has a square. One has a triple bond. They're way different, so the answer has to be constitutional isomers. I could tell that a lot of you guys got that. So very good. Hopefully, that makes sense to you guys. This is just the formula, the general structure that you use to solve these problems. I hope that this systematic way will help because it can get confusing the way that these compounds look very similar.

Isomeric Relationships

All right. So this one was tricky. But what I want to show you guys is, first of all, let's count up the Index of Hydrogen Deficiency (IHD) and carbons. So carbons for this one were 10 and carbons for this one were 10. Now if we calculate the IHD, the IHD for the first one is 0. There are no double bonds, no rings, no triple bonds. The IHD for the second one is also 0. So what that means is that these have the same atoms. Now all I have to do is figure out if they are also connected the same way. In this case, I said this one's tricky because I need to find a landmark atom. I need to find an atom that I can compare in both of these and see if it's connected the same way. What I would choose here is I would choose the atom that has 4 bonds on it because notice that I have only one carbon that has 4 bonds in this entire thing. If I can locate that one on both of these, then I could compare what are the 4 things that are attached. So over here, if I'm looking for the atom that has 4 bonds, it would be right here. So now what I do is I'm going to look at the 4 things that are attached and see if those same four things are attached to the blue carbon on the other side. So notice that this blue carbon has 2 methyls coming off of it. Does the other blue carbon have 2 I'm sorry, and that's called a methyl group, but we'll talk about that more later. You don't need to know that's called a methyl group. But for right now, you do need to know that that's a CH3. So I'm just going to make this a little bit bigger. CH3, CH3. So we have 2 CH3s coming off of that. Do we have the same thing coming off of the other one? Yes. CH3, CH3. Cool. So then what else? Well, we also have a carbon chain that has 3 carbons 1, 2, 3, and it's connected in the middle. Now let's look on the other side to see if we can find the same thing. Actually, yes, we have one that has 3 carbons and it's connected in the middle. So, so far, this is looking like it's the same compound. But let's just look at this last one. So this last one looks like it's 4 carbons. 1, 2, 3, 4. And it has that pattern where basically it's connected to a CH2 and then a CH and then 2 CH3s. So basically, I have this pattern of CH2, CH, and then CH3CH3. Do you guys see that? I'm just using the condensed formula. So now let's see if I get the same thing on the other side. On the other side, I have a CH2 connected to a CH and then connected to CH3CH3. It's the same exact thing on both sides. It's just now I just drew the condensed structure, the condensed formula going the other way. All right. So it turns out that these molecules are exactly the same. They were just rotated in different ways. Now, I don't want you to try to visualize, oh, if I flip this, it would be that. I just want you to know, hey, they have the same groups on all sides, so these would be identical compounds because they have the same atoms and they have what we call the same connectivity.

Here’s what students ask on this topic:

What are constitutional isomers in organic chemistry?

Constitutional isomers are compounds that have the same molecular formula but differ in the connectivity of their atoms. This means that while they contain the same number and types of atoms, the way these atoms are bonded to each other is different. For example, two compounds might both have the formula C₄H₁₀, but one could be a straight chain (butane) and the other a branched chain (isobutane). Understanding constitutional isomers is crucial for analyzing molecular structures and their properties.

How do you determine if two compounds are constitutional isomers?

To determine if two compounds are constitutional isomers, follow these steps: First, check if all non-hydrogen atoms and the Index of Hydrogen Deficiency (IHD) are identical. If they differ, the compounds are different. If they match, assess the connectivity by identifying landmark atoms. If the connections are the same, the compounds are identical; if not, they are constitutional isomers. This systematic approach helps ensure accurate identification.

What is the Index of Hydrogen Deficiency (IHD) and how is it used in identifying constitutional isomers?

The Index of Hydrogen Deficiency (IHD) is a measure of the number of hydrogen pairs missing from a molecule compared to a fully saturated hydrocarbon. It helps determine the degree of unsaturation (double bonds, rings, etc.). To use IHD in identifying constitutional isomers, calculate the IHD for both compounds. If the IHD values and the non-hydrogen atoms are identical, proceed to check the connectivity of atoms. If they differ, the compounds are not constitutional isomers.

Can you provide an example of constitutional isomers?

Sure! Consider the molecular formula C₄H₁₀. Two constitutional isomers of this formula are butane and isobutane. Butane has a straight-chain structure: CH₃-CH₂-CH₂-CH₃. Isobutane, on the other hand, has a branched structure: (CH₃)₂CH-CH₃. Both have the same molecular formula but different connectivity of their carbon atoms, making them constitutional isomers.

Why is it important to understand constitutional isomers in organic chemistry?

Understanding constitutional isomers is important because they have different physical and chemical properties despite having the same molecular formula. This knowledge is crucial for predicting reactivity, understanding mechanisms, and designing synthesis pathways in organic chemistry. It also aids in the identification and characterization of compounds in research and industry.