Determining when molecules are different

Determining when molecules are different


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.

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