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- [Voiceover] Let's say we
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
one, two, three, and four.

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Notice we have a double
bond starting at carbon two.

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

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

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"But" because we have four
carbons and "ene" because

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we have a double bond
presence in the molecule.

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What about naming the
molecule on the right?

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We number our carbons
one, two, three, and four.

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

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

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However these are two
different molecules and

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the reason why is because
there's no free rotation

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around a double bond.

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Single bonds have free rotation
but double bonds don't.

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So you couldn't rotate the
molecule on the left to

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look like the molecule on the right.

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Therefore they must be
isomers of each other

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and we need a way to
distinguish between our isomers.

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And so one way to do that is
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
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,
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
at those methyl groups,

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these two methyl groups
are on opposite sides

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of the double bond.

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

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those two methyl groups
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
in italics, attempt to anyway.

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So we have cis-2-butene
and trans-2-butene.

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These are different molecules
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
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
an ethyl group attached

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to our double bond

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

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Those two ethyl groups
are on the same side of

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our double bond so this
must be the cis isomer.

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On the right we have this ethyl
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
cinnamaldehyde molecule.

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We're looking for two
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
methyl group or an ethyl group

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so if we look at our double
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
attached to this carbon.

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And those two hydrogens are
on opposite sides of our

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double bonds.

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

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Right, these two hydrogens
are on opposite side

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so we're talking about trans here.

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Those hydrogens are
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
groups to use our cis/trans

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and here we have an ethyl
group, and here we have

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an ethyl group.

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All right over here we
have a methyl group and an

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isopropyl group.

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But the two methyl groups
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
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
a cis configuration.

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Let's compare the drawing
on the left to the

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drawing on the right.

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The first time you look
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
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
the same molecule.

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If you picked up this
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
out is to look at this carbon.

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And you can see you have
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
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
groups weren't bonded

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to the same carbon.

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Those two ethyl groups are
bonded to different carbons.

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So this ethyl group is bonded
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
cis/trans terminologies.

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So we looked at our
double bond and we said

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those two ethyl groups
are on the same side of

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our double bond,

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so this represents a cis
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
bonded to the same carbon.