 Okay in this video we are going to talk about a new kind of isomer. We're not going to get to it until the next slide. There needs to be a little bit of background information that you need. One thing that I will point out to you, this is supposed to be a carbon. These are supposed to be hydrogens, although it's a little bit weird. This is, let's pretend that this is methane, because methane is CH4, that's one carbon for hydrogens. What I'm trying to point out to you with this picture is that these molecules have a three-dimensional shape. When you have a carbon and four things pointing off of it, the shape of the molecule is almost always like a pyramid. There is a fancy name for this, the shape of this molecule, if you want to, if you really care, it's called tetrahedral. I don't really care that you know that, but I want you to know that that molecule has a three-dimensional shape. If you have two carbons connected to each other, and then there's three things sticking onto this carbon, there's three things sticking onto the other carbon, shape is going to look something like this. The point I want to make here is that if you have a single bond, single covalent bond between the carbons, then these atoms over here and these atoms over here, they can rotate around like a propeller relative to each other. So these atoms, they can spin around, that's supposed to be an arrow of them, arrows of them spinning, and these can spin around too, and they can just spin around relative to each other. I think there may be another video or a video of my son when he was younger showing you this. So here is a video of my son just turning these atoms attached to the carbon on the left with respect to the atoms attached to the carbon on the right. Here he's just showing them to you, he's going to flip them around, and then he's going to show you that they actually turn. You can rotate around that bond. And this happens in real life too. If you had this molecule in real life, what would be happening, a fair amount of the time, is that these atoms over here would be rotating with respect to these atoms over here. So you can do it in this toy, and it actually does happen in real life with the real molecules. If you have a very similar kind of molecule, but this time there's a double covalent bond in between the two carbons, you can see the double covalent bond here with these two springs there, supposed to be a double covalent bond. You can't rotate anymore. These two atoms are stuck in place relative to the other two atoms here. You cannot, you can't rotate across that double bond. So these chlorines here are stuck pointing in the same direction, and these hydrogens are stuck pointing in the same direction as well. And that's only true if you have a carbon-carbon triple bond like that. If you have a carbon-carbon single bond, that's not true. All of these guys can rotate like a propeller, and these ones can rotate like a propeller too. Here's my son showing you the atom, the molecule with the carbon-carbon double bond, and showing you that you can twist, but the atoms snap back into place. They don't rotate. Twist and snap back. The fact that you can't rotate anymore when you have a carbon-carbon double bond gives us a new kind of isomer. And here it is, almost. So if I showed you this molecule and I said what's its name, you would say, oh well it's four carbons in a row. So that's kind of like butane, but it has a double bond between the carbons. So it's not going to be called butane. It's going to be called butene, and we have to number the carbon. So that's one, two, three, four. And the double bond starts at carbon number two, so this molecule is called two butene. And that's pretty good, but it turns out that even this drawing is ambiguous, because there are two different versions of two butene that exist. Remember you can't rotate across this carbon-carbon double bond. You could have this CH3 and this CH3 pointing off of the carbon-carbon double bond, and you could have them pointing in the same direction, like they are here. And you would call this two butene, or you could have the CH3s pointing in opposite directions coming off of this double bond. And you would call this two butene as well. But this molecule over here and this molecule over here, they're not the same thing, because in one case the CH3s are pointing in the same direction, and the Hs are pointing in the same direction. And in the other case, the things are pointing in opposite directions, the Hs are pointing in opposite directions. So they're not the same thing. And you can't rotate these around because you have this carbon-carbon double bond. So this molecule on the left and this molecule on the right, they are isomers of each other. They have the same simple formula, but the atoms are attached in different ways. And this is a special kind of isomer. When the hydrogens come off of the carbon-carbon double bond and point in the same direction, you say this is the cis isomer. When the hydrogens come off of the carbon-carbon double bond and point in opposite directions, you say that this is the trans isomer. And trans means across from, so the Hs are across from each other. So this is the trans isomer. And so there are many of these kinds of isomers, cis and trans isomers, in biology and chemistry. You might be saying, well, I don't really believe you. Those two molecules are actually the same thing. They aren't. They have completely different behaviors. This one melts at negative 139 degrees Celsius. This one melts at a completely different temperature. So they have different behaviors. They are different things. Those two things are real, cis and trans isomers. You may have heard of trans fats. Basically, what that means is with trans fats, the fat molecule has a carbon-carbon double bond and the Hs coming off of that carbon-carbon double bond in the fat molecule point in the opposite directions. And that makes it a trans fat versus a cis fat. I haven't the foggiest idea of which one is good for you and which one is not. But there's a little bit more biology that maybe you learned. Maybe you didn't. Maybe you knew that already. All right. What do I want you to know? I want you to be able to recognize the difference between a cis and a trans isomer. You can ignore this bullet point here. You should also be able to look at cis and trans isomers and figure out which one is cis and which one is trans. So you can pause the video and look at these and figure out which one is the cis isomer and which one is the trans. Unpausing the video. Well, this one has a carbon-carbon double bond, hydrogen's pointing in the same direction. So this is the cis isomer. This one has a carbon-carbon double bond, hydrogen's pointing in opposite directions. That one is the trans isomer. And that is the end of the video on cis and trans isomers. See you in the next one.