Constitutional Isomers vs. Stereoisomers: Videos & Practice Problems

Isomers are classified into different types based on their molecular formula, connectivity, and shape. Constitutional isomers share the same molecular formula but differ in connectivity, while stereoisomers have the same connectivity but different shapes. Conformers are the most similar, differing only by rotation around a sigma bond. Understanding these distinctions is crucial for analyzing molecular relationships, particularly in organic chemistry, where concepts like index of hydrogen deficiency (IHD) help determine saturation and structural variations.

Determining when molecules are different.

Alright guys. So in the past, we learned how to distinguish between molecules that were completely different, meaning that they did not share a molecular formula at all, or molecules that we called constitutional isomers. And remember, those were molecules that had the same molecular formula, but they were connected differently. Well, it turns out that there are more types of isomers than just that. There are also other types of isomers that have the same molecular formula, that have the same connectivity, but they just simply have a different shape. The name of these types of isomers is called stereo isomers, and that is going to be the topic of this whole chapter. Alright? So what I want to do at the very beginning, just to get started, is to go through these different types of isomers so you guys can visualize these for yourself. Okay?

So basically, we use isomers to describe the relationships between similar molecules. We would not even be talking about the concept of isomers if it were not for the fact that some molecules look like they might be the same, and we want to analyze whether they are the same or different. Well, it turns out that we can order these in terms of the most different to the most similar. And the most different relationship that you can get between two molecules would actually just be different compounds. Okay? So that is what we are going to fill in right here. I have given you guys examples like these before where we determine, okay, I have two molecules, and I want to know what is their relationship. In this case, how would we figure that out? Remember that we used the flowchart that I gave you. And what I said is, okay, first of all, how many non-hydrogen atoms does this have? Well, this one has 5 carbons and this one has 5 carbons. Okay? So so far so good. It seems like okay, these are the same; they might be the same molecular formula. But then we talked about another category or another thing that we need to look at, and that is the IHD or the index of hydrogen deficiency. Okay?

Now remember that the index of hydrogen deficiency had to do with rings, double bonds, and triple bonds. So what would be the IHD of this first molecule? Well, in this case, there is only one ring, so that means this would have an IHD of 1, and remember what IHD of 1 means is that we are missing 2 hydrogens. Now let us look at the second one. The second one does not have any rings. It does not have any double bonds. It does not have any triple bonds. This one would have an IHD of 0. That means that this one is missing no hydrogens. This one is saturated. Okay, so I am just going to put that this one is saturated, whereas the first one is missing 2 Hs compared to the second one.

Now, I just want to let you guys know if you are completely lost by what I just did, you have never seen this before in your life, go back to the topic that is called IHD, and they talk about how to figure out the index of hydrogen deficiency with molecular formula and index of hydrogen deficiency with shape. Alright? So go back and look over that. It is about 20 minutes long, and that will help you guys so much. Alright? And then you might want to go over constitutional isomers as well because that is what we are doing right now. Okay?

So when we look at the relationship between these, what I want to do is I want to use this box to figure out what is shared between the two molecules. So is the molecular formula shared? Actually, no. Because think about it, they have the same amount of carbons but they have different amounts of hydrogens, so they are different compounds. Okay? Is the connectivity the same? No, because they are different atoms, and the shape is also not the same. So basically, if your molecular formula is off, none of these other things can be shared because you already messed up the first step, which is that they do not even have the same atoms. Does that make sense? So these would be different compounds.

Determining when molecules are constitutional isomers.

So now let's go on to this next one. Okay? This next one, what I have is I'm trying to figure out the relationship, I see that this one has 5 carbons. This one has 5 carbons. And then I notice that both of these have the same IHG. This one has an IHG of 1 and this one also has an IHG of 1. What does that mean? What that means is that these both have the same molecular formula. They both have the same amount of carbons and the same amount of hydrogens. So I would put a check mark that the molecular formula is shared. Okay? Cool with that?

Now let's look at the connectivity. Are these connected in exactly the same way? Are all the atoms connected in exactly the same way? The answer is no because in one of these I have a 5-membered ring and in the other, I have a 4-membered ring. What that means is that these have different shapes. Not just shapes, these have different connectivities. For example, this one right here has a tertiary carbon. Okay? And this one over on the 5-member ring only has secondary carbons. Okay? So could these possibly be connected exactly the same? No. They're connected differently. So this is what we would call a constitutional isomer. Okay. And just looking back, remember that a constitutional isomer would be something that has the same molecular formula but different connectivity and obviously a different shape if it's not even connected the same.

Determining when molecules are stereoisomers.

Let's go on to this next example. In this next example, I'm going to go ahead and count my carbons again. It's going to be 5. My carbons are going to be 5. I'll count my IHD, and that's going to be 1 for this first one because I have a double bond. Remember, a double bond counts as 1. I'm going to do IHD over here, and this one's also going to count as 1. So do these have the same molecular formula? Absolutely. They have the same number of carbons, the same number of hydrogens. Are they connected the same? Actually, yes, they are. These have the exact same connectivity as well because what we have here is a double bond in both situations with a methyl group on one side and an ethyl group on the other. Okay? In both situations, it's the same thing. ME, I'm just going to put NE. Alright? In both situations, I have the exact same atoms connected in the same exact way. So, I'm going to also put a check on connectivity. These have the same exact connectivity. Now, the last thing is shape. Do they have the same shape? And the answer is no because, if you'll notice, this first one has a double bond in the orientation of trans. You guys remember cis and trans? Okay. So this one has a trans double bond. Now, if I look at this other one and I draw my fence, I would say that my large groups are on the same side of the fence. So that means they have the same connectivity, but their shape is slightly different. So, the name for this, and you might have guessed it, is stereoisomers. Like I said, this is the topic of this chapter. What I want you guys to be able to do at the end of this chapter is to be able to distinguish with types of stereoisomers, name them, identify them, etcetera.

Determining when molecules are conformers.

Finally, we get to our most similar compounds. Our most similar compounds would be something like this, where I have the atoms are the same, 5 and 5. The Index of Hydrogen Deficiency (IHD) for both of these would be 0. Got that so far. So, these have the same molecular formula. Are they connected the same way? Absolutely. They're both 5 carbon chains. These are both pentanes. Okay? Do they have the same shape? Actually, they do have the same shape. These are all the same thing and the reason is because, check it out, this rotation here happened on a sigma bond. Remember that sigma bonds can freely rotate as much as they want. Okay? So all that happened was that I had 2 H's here and I had a CH₃ here and afterwards that CH₃ flipped up and the two H's flipped down. Is that allowed to happen? Yes. Because this is a sigma bond. Okay? So what do we call this? What's the relationship between these?

These would be what we call conformers. Conformers would just be that they basically have everything exactly the same except that single bonds have rotated. Okay? Notice that stereoisomers are more different than conformers. Conformers are basically the same exact molecule, just a single bond rotated by itself, whereas stereoisomers will always be different molecules. Okay? Because stereoisomers, you can't interconvert between one and the other. One is always going to be stuck as one shape and the other one is always going to be stuck as the other shape.

Here’s what students ask on this topic:

What is the difference between constitutional isomers and stereoisomers?

Constitutional isomers, also known as structural isomers, have the same molecular formula but differ in the connectivity of their atoms. This means that the atoms are connected in different ways, leading to different structures. On the other hand, stereoisomers have the same molecular formula and the same connectivity, but they differ in the spatial arrangement of their atoms. This difference in spatial arrangement can lead to different physical and chemical properties. Examples of stereoisomers include cis-trans isomers and enantiomers.

How do you identify constitutional isomers?

To identify constitutional isomers, follow these steps: 1) Ensure the molecules have the same molecular formula. 2) Check the connectivity of the atoms. If the atoms are connected differently, they are constitutional isomers. For example, butane (C₄H₁₀) and isobutane (C₄H₁₀) have the same molecular formula but different structures. Butane has a straight chain, while isobutane has a branched chain. This difference in connectivity classifies them as constitutional isomers.

What are conformers in organic chemistry?

Conformers, or conformational isomers, are molecules that have the same molecular formula and connectivity but differ by rotation around a single sigma bond. These rotations do not break any bonds and can occur freely, leading to different spatial arrangements. For example, in ethane (C₂H₆), the rotation around the C-C sigma bond can lead to different conformations, such as staggered and eclipsed forms. Conformers are generally considered the same molecule, just in different rotational states.

What is the index of hydrogen deficiency (IHD) and how is it used?

The index of hydrogen deficiency (IHD) is a calculation used to determine the degree of unsaturation in a molecule. It indicates the number of rings, double bonds, and triple bonds present. The formula for IHD is: IHD = (2C + 2 - H + N - X)/2, where C is the number of carbons, H is the number of hydrogens, N is the number of nitrogens, and X is the number of halogens. For example, benzene (C₆H₆) has an IHD of 4, indicating three double bonds and one ring.

How do cis and trans isomers differ?

Cis and trans isomers are types of stereoisomers that differ in the arrangement of substituents around a double bond or a ring. In cis isomers, the substituents are on the same side, while in trans isomers, they are on opposite sides. For example, in 2-butene, the cis isomer has both methyl groups on the same side of the double bond, while the trans isomer has them on opposite sides. This difference in spatial arrangement can lead to different physical and chemical properties.