 So, let's talk about some more structural isomers. We'll talk about the structural isomers of this molecule here that we just built, 2-butene. So, 2-butene actually has two structural isomers, okay? So I'm going to erase all the other structural isomers of C4H8, and honestly, on the last video, I might should have drawn this other structural isomer, too, before I turned it off. In fact, there's another structural isomer, 2-butene, that's like that, okay? And we'll talk about why these two things are structural isomers. And so, C4H8 is this thing, okay? This is 2-butene, and this is 2-butene, right? Because we go 1, 2, 3, 4, 1, 2, 3, 4. So they're both 2-butene, right? If you want to know why I'm calling it butene, it's because, I forgot to tell you this, guys, but it's because of this. So, 1-carbon is methyl, 2-carbon is ethyl, like ethanol, 2-carbons. 3-carbons is propyl, like propane, 4-carbons is butene. And the way to remember this is something silly, mice eat peanut butter. So that's the way to remember your first four, and catch mice, too. The rest are propane, hexane, heptane, so they're normal numbers. These ones have, like, like, like pentane is the next one, right? That's bottom, right? Hexane's the next one, that's 6. Hex is 6. Hept is 7. Oct is 8. Non is 9. Deck is 10. Okay? So it's only the first four that's got weird names, okay? So that's how to remember them. So that's why this thing is called butene. Why do we have 4-carbons? Okay? Anyways, let's go back to this structural isomer. Structural isomer business. So let's build these two butenes. And I'll show you why they're structural isomers. Did you guys see that? So you guys can see the double bond, hopefully, right? See? Notice, when I try to turn it to the other structure, try to turn it to the Charlie Brown structure, I can't twist, okay? So I can twist around single bonds just fine, okay? But around double bonds and around triple bonds, and we'll see that in a second. You can't twist. There's no rotation, okay? So in fact, what do I have to do in order to take this isomer of 2 butene to make this isomer? I'm going to have to break a bond and reform a bond, right? Now it looks like the Charlie Brown, right? Okay, so notice I had to break that bond and reform it, okay? Why? Because I can't do like a single bond and twist around it, okay? They're not conformational isomers. And in fact, they have different names, as you would imagine, because they're two different molecules, okay? So since they're two different molecules, they've got different names. So the way to remember this is, so you can call them E and Z, okay? So this one here is called Z, okay? This is Z2 butene, like this, Z, because they're on the same side, okay? So remember that, okay? So there's two things that you remember now, okay? And the other one's E, so when they're on opposite sides, it's E, okay? Another way to call these, you might hear, is cis and trans, okay? So trans is not on the same side, not on the same side, right? So they're opposite of each other. So you can also hear these called trans double bonds, okay? So you'll hear them called E double bonds or trans double bonds, if they're on opposite sides, okay? They're on the same side, right? They're going to be either Z or cis double bonds, okay? So those are also structural isomers of each other. Are there any questions on that stuff? No questions? Okay, good job, guys. Remember the alkene is a functional group, okay? So we're going to learn functional groups next time, okay? Oh, well, the next time's the exam, so study for that instead. Okay, good luck, guys.