0:00:02.320,0:00:04.450
- [Voiceover] Let's say we[br]were asked to name the molecule

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on the top left.

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We would start by numbering our carbons.

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So this would be carbon[br]one, two, three, and four.

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Notice we have a double[br]bond starting at carbon two.

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So the name of this[br]molecule would be 2-butene.

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Two because we have our double[br]bond starting at carbon two.

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"But" because we have four[br]carbons and "ene" because

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we have a double bond[br]presence in the molecule.

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What about naming the[br]molecule on the right?

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We number our carbons[br]one, two, three, and four.

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And once again we have a double[br]bond starting at carbon two.

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So the name of this[br]molecule would be 2-butene.

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However these are two[br]different molecules and

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the reason why is because[br]there's no free rotation

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around a double bond.

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Single bonds have free rotation[br]but double bonds don't.

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So you couldn't rotate the[br]molecule on the left to

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look like the molecule on the right.

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Therefore they must be[br]isomers of each other

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and we need a way to[br]distinguish between our isomers.

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And so one way to do that is[br]to use cis/trans terminology.

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So if we look at the molecule on the left,

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we can see we have two methyl groups.

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And those two methyl[br]groups are on the same side

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of our double bond.

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So if I draw a line in here,[br]it's easier to see those two

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methyl groups are on the same sides.

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And we call that the cis isomer.

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So we put cis in front of our name here.

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I'm attempting to write it in italics.

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So this would be cis-2-butene.

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On the right when we look[br]at those methyl groups,

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these two methyl groups[br]are on opposite sides

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of the double bond.

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So I draw a line in here[br]to make it easier to see

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those two methyl groups[br]are on opposite sides.

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And we call that trans.

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So this is trans isomer.

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I'm going to write trans here[br]in italics, attempt to anyway.

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So we have cis-2-butene[br]and trans-2-butene.

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These are different molecules[br]with different properties.

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If you want to use cis/trans terminology,

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you're looking for two identical groups

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and you are comparing them.

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So let's look at these next two[br]examples here and figure out

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which one is cis and which one is trans.

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We're looking for identical groups.

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So over here we have[br]an ethyl group attached

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to our double bond

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and on the right we have an[br]ethyl group to our double bond.

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Those two ethyl groups[br]are on the same side of

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our double bond so this[br]must be the cis isomer.

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On the right we have this ethyl[br]group and this ethyl group

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on opposite sides of our double bond.

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So that must be the trans isomer.

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All right, let's do some more examples.

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I'll go down to here.

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On the left we have[br]cinnamaldehyde molecule.

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We're looking for two[br]identical groups so we can use

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cis or trans.

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You can also use hydrogens, right.

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You don't have to use a[br]methyl group or an ethyl group

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so if we look at our double[br]bond we know there's a

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hydrogen attached to this carbon

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and we know there's a hydrogen[br]attached to this carbon.

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And those two hydrogens are[br]on opposite sides of our

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double bonds.

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And I'm drawing a line here[br]to make it easier to see.

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Right, these two hydrogens[br]are on opposite side

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so we're talking about trans here.

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Those hydrogens are[br]across from each other.

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What about the tetra-substituted alkene

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on the right?

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We need two identical[br]groups to use our cis/trans

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and here we have an ethyl[br]group, and here we have

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an ethyl group.

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All right over here we[br]have a methyl group and an

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isopropyl group.

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But the two methyl groups[br]are on the same side of

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our double bond.

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So I draw a line in here and[br]we see that these two groups

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are on the same side,

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therefore we're talking about cis here.

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So this double bond has[br]a cis configuration.

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Let's compare the drawing[br]on the left to the

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drawing on the right.

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The first time you look[br]at these two drawings

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you might think these are two isomers,

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and I could use cis/trans[br]terminology to distinguish

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between them.

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However, you can't because these are

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just two ways to represent[br]the same molecule.

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If you picked up this[br]molecule on the left and

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you flipped it up,

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you would get the drawing on the right.

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So they're not isomers of each other.

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This is the same molecule.

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And a fast way to figure that[br]out is to look at this carbon.

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And you can see you have[br]two identical groups bonded

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to that carbon.

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So you can't use cis/trans terminology.

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That's different from the[br]example we did a minute ago.

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We had two identical groups,

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right these two ethyl groups here.

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However those two ethyl[br]groups weren't bonded

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to the same carbon.

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Those two ethyl groups are[br]bonded to different carbons.

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So this ethyl group is bonded[br]to this carbon, and this

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ethyl group is bonded to this carbon.

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So we were able to use[br]cis/trans terminologies.

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So we looked at our[br]double bond and we said

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those two ethyl groups[br]are on the same side of

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our double bond,

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so this represents a cis[br]configuration of the double bond.

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So we can't do that up here because

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while we do have two identical groups,

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those identical groups are[br]bonded to the same carbon.