Constitutional Isomers vs. Stereoisomers

Isomers are terms used to describe relationships between similar molecules.

Concept: Determining when molecules are different. 

Video Transcript

All right, guys, so in the past we learned how to distinguish between molecules that were completely different, meaning that they didn't share molecular formula at all or molecules that we called constitutional isomers. Remember those were molecules that had the same molecular formula, but they were connected differently.
Well, it turns out that there's more types of isomers than just that. There's 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 names of these types of isomers is called stereoisomers and that's going to be the topic of this whole chapter.
So what I want to do at the very beginning just to get started is go through these different types of isomers so you guys can visualize these for yourself.
Basically, we use isomers to describe the relationships between similar molecules. We wouldn't even be talking about the concept of isomers if it weren't for the fact that some molecules look like they might be the same and you want to analyze are they the same or are they different. It turns out that we can order these in terms of the most different to the most similar.
The most different relationship that you can get between two molecules would actually just be different compounds. That's what we're going to fill in right here. I've given you guys examples like these before where we determine I have two molecules and I want to know what's their relationship. In this case, how would we figure that out?
Remember that we used the flow chart that I gave you. What I said is first of all, how many non-hydrogen atoms does this have? Well, this one has 5 carbons and this one has 5 carbons. So far, so good. It seems like these might be the same molecular formula.
But then we talked about another category or another thing that we need to look at and that's the IHD or the index of hydrogen deficiency. 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's only one ring so that means this would have an IHD of 1. And remember what IHD of 1 means is that we're missing 2 hydrogens.
Now let's look at the second one. The second one doesn't have any rings. It doesn't have any double bonds. It doesn't 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. I'm just going to put that this one is saturated whereas the first one is missing 2 H's compared to the second one.
Now I just want to let you guys know if you're completely lost by what I just did, you've never seen this before in your life, go back to the topic that is called IHD and I talked about how to figure out index of hydrogen deficiency with molecular formula and index of hydrogen deficiency with a shape. Go back and look over that. It's about 20 minutes long and that will help you guys so much. You might want to go over constitutional isomers as well because that's what we're doing right now.
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're different compounds.
Is the connectivity the same? No, because they're different atoms and the shape is also not the same.
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 don't even have the same atoms. Does that make sense? So these would be different compounds. 

If you are confused by how to solve for IHD, refer to my Index of Hydrogen Deficiency topic. 

Concept: Determining when molecules are constitutional isomers. 

Video Transcript

So now let's go on to this next one. 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 IHD. This one has an IHD of 1 and this one also has an IHD 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. 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 the other one 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. This one over on the 5-membered ring only has secondary carbons. So could these possibly be connected exactly the same? No, they're connected differently, so this is what we would call a constitutional isomer.
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. 

*These are also known as structural isomers

Concept: Determining when molecules are stereoisomers.


Heads up: The terms enantiomers, diastereomers, and meso compounds are all used to describe specific types of stereoisomers- we’ll get to those later in this chapter.  

Concept: Determining when molecules are conformers.


Since conformers are simply rotations along single bonds, these would also count as identical molecules; they can easily twist back into position with each other.  

Constitutional Isomers vs. Stereoisomers Additional Practice Problems