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- [Voiceover] Let's say we were asked to name the molecule on the top left. We would start by numbering our carbons. So this would be carbon one, two, three, and four. Notice we have a double bond starting at carbon two. So the name of this molecule would be 2-butene. Two because we have our double bond starting at carbon two. "But" because we have four carbons and "ene" because we have a double bond presence in the molecule. What about naming the molecule on the right? We number our carbons one, two, three, and four. And once again we have a double bond starting at carbon two. So the name of this molecule would be 2-butene. However these are two different molecules and the reason why is because there's no free rotation around a double bond. Single bonds have free rotation but double bonds don't. So you couldn't rotate the molecule on the left to look like the molecule on the right. Therefore they must be isomers of each other and we need a way to distinguish between our isomers. And so one way to do that is to use cis/trans terminology. So if we look at the molecule on the left, we can see we have two methyl groups. And those two methyl groups are on the same side of our double bond. So if I draw a line in here, it's easier to see those two methyl groups are on the same sides. And we call that the cis isomer. So we put cis in front of our name here. I'm attempting to write it in italics. So this would be cis-2-butene. On the right when we look at those methyl groups, these two methyl groups are on opposite sides of the double bond. So I draw a line in here to make it easier to see those two methyl groups are on opposite sides. And we call that trans. So this is trans isomer. I'm going to write trans here in italics, attempt to anyway. So we have cis-2-butene and trans-2-butene. These are different molecules with different properties. If you want to use cis/trans terminology, you're looking for two identical groups and you are comparing them. So let's look at these next two examples here and figure out which one is cis and which one is trans. We're looking for identical groups. So over here we have an ethyl group attached to our double bond and on the right we have an ethyl group to our double bond. Those two ethyl groups are on the same side of our double bond so this must be the cis isomer. On the right we have this ethyl group and this ethyl group on opposite sides of our double bond. So that must be the trans isomer. All right, let's do some more examples. I'll go down to here. On the left we have cinnamaldehyde molecule. We're looking for two identical groups so we can use cis or trans. You can also use hydrogens, right. You don't have to use a methyl group or an ethyl group so if we look at our double bond we know there's a hydrogen attached to this carbon and we know there's a hydrogen attached to this carbon. And those two hydrogens are on opposite sides of our double bonds. And I'm drawing a line here to make it easier to see. Right, these two hydrogens are on opposite side so we're talking about trans here. Those hydrogens are across from each other. What about the tetra-substituted alkene on the right? We need two identical groups to use our cis/trans and here we have an ethyl group, and here we have an ethyl group. All right over here we have a methyl group and an isopropyl group. But the two methyl groups are on the same side of our double bond. So I draw a line in here and we see that these two groups are on the same side, therefore we're talking about cis here. So this double bond has a cis configuration. Let's compare the drawing on the left to the drawing on the right. The first time you look at these two drawings you might think these are two isomers, and I could use cis/trans terminology to distinguish between them. However, you can't because these are just two ways to represent the same molecule. If you picked up this molecule on the left and you flipped it up, you would get the drawing on the right. So they're not isomers of each other. This is the same molecule. And a fast way to figure that out is to look at this carbon. And you can see you have two identical groups bonded to that carbon. So you can't use cis/trans terminology. That's different from the example we did a minute ago. We had two identical groups, right these two ethyl groups here. However those two ethyl groups weren't bonded to the same carbon. Those two ethyl groups are bonded to different carbons. So this ethyl group is bonded to this carbon, and this ethyl group is bonded to this carbon. So we were able to use cis/trans terminologies. So we looked at our double bond and we said those two ethyl groups are on the same side of our double bond, so this represents a cis configuration of the double bond. So we can't do that up here because while we do have two identical groups, those identical groups are bonded to the same carbon.