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In this video, we're going to
look at pairs of molecules and

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see if they relate to each other
in any obvious way or

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maybe less than obvious way.

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So these first two right here,
they actually look like a

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completely different
molecules.

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So your gut impulse might
be to say that these are

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completely different
molecules.

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And it wouldn't be completely
off, but we look a little bit

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closer, you see that this guy
on the left has one, two,

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three, four carbons, and so does
this guy on the right.

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It has one, two, three,
four carbons.

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This guy on the left
has two, four, six,

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seven, eight hydrogens.

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This guy on the right has two,
four, six, eight hydrogens.

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And they both have one oxygen.

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So both of the molecular
formulas for both of these

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things are four carbons, eight
hydrogens, and one oxygen.

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They're both C4H8O.

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So they have the same
molecular formula.

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They're made up of the same
thing, so these are going to

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be isomers.

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They're going to be isomers,
and they're a

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special type of isomers.

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In this situation, we don't
have the same bonds.

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We're made up of the same
things, but the bonds, what is

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connected to what
is different.

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So we call this a constitutional
isomer.

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So we are essentially made up of
the same things, but we are

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actually two different molecule,
actually, two very

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different molecules here.

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Now let's look at this
next guy over here.

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So if we look at this molecule,
it does look like

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this carbon is chiral.

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It is an asymmetric carbon.

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It is bonded to four different
groups: fluorine, bromine,

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hydrogen, and then
a methyl group.

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And so's this one.

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And they're both made up
of the same things.

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You have the carbon-- and not
only are they made up of the

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same things, but the bonding
is the same.

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So carbon to a fluorine, carbon
to a fluorine, carbon

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to a bromine, carbon to a
bromine, carbon to hydrogen in

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both of then carbon to the
methyl group in both.

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But they don't look
quite the same.

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Are they mirror images?

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Well, no.

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This guy's mirror image would
have the fluorine popping out

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here, the hydrogen going back
here, and then would have the

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bromine pointing out here.

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Let's see if I can somehow get
from this guy to that guy.

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Let me flip this guy first. So
let me-- a good thing to do

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would be to just flip to see
the fastest way I could

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potentially get there.

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Let me just flip it like this.

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So I'm going to flip out of
the page, you can imagine.

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I'm going to flip
it like this.

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So I'm going to take this methyl
group and then put it

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on the right-hand side.

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And you can imagine, I'm going
to turn it so it would come

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out of the page and
then go back down.

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So if I did that, what
would it look like?

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I would have the carbon,
this carbon here.

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I would have the methyl group
on that side now.

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And then since I flipped it
over, the bromine was in the

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plane of the page.

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It'll still be in the plane of
the page, but since I flipped

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it over, the hydrogen, which was
in the back, will now be

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in the front.

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The hydrogen will now be in
the front and the fluorine

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will now be in back because
I flipped it over.

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So the fluorine is
now in the back.

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Now, how does this
compare to that?

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Let's see if I can somehow
get there.

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Well, if I take this fluorine
and I rotate it to where the

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hydrogen is, and I take the
hydrogen and rotate it to

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where-- that's all going to
happen at once-- to where the

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bromine is, and I take the
bromine and rotate it to where

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the fluorine is, I get that.

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So I can flip it and then I can
rotate it around this bond

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axis right there, and I would
get to that molecule there.

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So even though they look pretty
different, with the

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flip and a rotation, you
actually see that these are

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the same a molecule.

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Next one.

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So let's see, what
do we have here?

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Let me switch colors.

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So over here, this part
of both of these

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molecules look the same.

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You have the carbons
on both of them.

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This carbon looks like
a chiral center.

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It's bonded to one, two,
three different groups.

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You might say, oh, it's two
carbons, but this is a methyl

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group, and then this side has
all this business over it, so

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this is definitely
a chiral carbon.

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And over, here same thing.

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It's a chiral carbon.

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And this has the same thing.

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It's bonded to four
different things.

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So each of these molecules has
two chiral carbons, and it

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looks like they're made
up of the same things.

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And not only are they made up
of the same things, but the

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bonds are made in
the same way.

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So this carbon is bonded to a
hydrogen and a fluorine, and

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the two other carbons,
same thing, a

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hydrogen and a fluorine.

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Carbon, it looks like
it's a hydrogen.

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It's bonded to a hydrogen and a
chlorine, so it's made up of

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the same constituents
and they're

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bonded in the same way.

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So these look like--
but the bonding is

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a little bit different.

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Over here on this one on the
left, the hydrogen goes in the

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back, and over here, the
hydrogen's in the front.

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And over here, the chlorine's
in back, and over here, the

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chlorine's in front.

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So these look like
sterioisomers.

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You saw earlier in this video,
you saw structural isomers,

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made up of the same
things but the

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connections are all different.

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Stereoisomers, they're made
up of the same thing, the

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connections are the same, but
the three-dimensional

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configuration is a little
bit different.

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For example, here on this
carbon, it's connected to the

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same things as this carbon, but
over here, the fluorine's

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out front, and over here--
out here, the

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fluorine's out front.

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Over here, the fluorine's
backwards.

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And same thing for the
chlorine here.

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It's back here and
it's front here.

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Now, let's see if they're
related in a more nuanced way.

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You could imagine putting
a mirror behind.

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I guess the best way to
visualize it, imagine putting

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a mirror behind this molecule.

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If you put a mirror behind this
molecule, what would its

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reflection look like?

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So if you put a mirror behind
it, in the image of the

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mirror, this hydrogen would now,
since the mirror's behind

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this whole molecule, this
hydrogen is actually closer to

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the mirror.

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So then the mirror image, you
would have a hydrogen that's

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pointed out, and then you would
have the carbon, and

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then you would have the fluorine
being further away.

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And same thing in the
mirror image here.

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You would have the chlorine
coming closer since this

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chlorine is further back, closer
to the mirror, and then

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you would have the hydrogen
pointing outwards like that.

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And then, obviously, the rest
of the molecule would look

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exactly the same.

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And so this mirror image that I
just thought about in white

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is exactly what this molecule
is: hydrogen pointing out in

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front, hydrogen pointing
out in front.

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You might say, wait, this
hydrogen is on the right, this

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one's on the left.

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It doesn't matter.

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This is actually saying that
the hydrogen's pointing out

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front, the fluorine is pointing
out back, hydrogen up

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front, fluorine back, chlorine
out front, hydrogen back,

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chlorine out front,
hydrogen back.

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So these are actually mirror
images, but they're not the

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easy mirror images that we've
done in the past where the

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mirror was just like that
in between the two.

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This one is a mirror image where
you place the mirror

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either on top of or behind
one of the molecules.

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So this is a class of
stereoisomers, and we've

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brought up this word before.

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We call this enantiomers.

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So if each of these are an
enantiomers, I'll say they are

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enantiomers of each other.

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They're steroisomers.

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They're made up of the same
molecules, so that they have

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the same constituents.

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They also have the same
connections, and not only do

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they have the same connections,
that so far gets

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us a steroisomer, but they are a
special kind of stereoisomer

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called an enantiomer, where they
are actual mirror images

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of each other.

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Now, what is this one
over here in blue?

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Just like the last one, it looks
like it's made up of the

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same things.

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You have these carbons, these
carbons, these carbons and

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hydrogens up there.

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Same thing over there.

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You have a hydrogen, bromine,
hydrogen and a bromine,

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hydrogen, chlorine, hydrogen,
chlorine, hydrogen, chlorine,

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hydrogen, chlorine.

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So it's made up of
the same things.

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They're connected in the same
way, so they're definitely

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stereoisomers.

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Well, we have to make sure
they're not-- well, let's make

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sure they're not the same
molecule first. Here,

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hydrogen's in the front.

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There, hydrogen's in the back.

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Here, hydrogen is in the back.

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Here, hydrogen is
in the front.

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So they're not the
same molecule.

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They have a different
three-dimensional

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configuration, although their
bond connections are the same,

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so these are stereoisomers.

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Let's see if they're
enantiomers.

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So if we look at it like this,
you put a mirror here, you

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wouldn't get this
guy over here.

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Then you would have a chlorine
out front and a hydrogen.

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So you won't get it if you
get a mirror over there.

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But if we do the same exercise
that we did in the last pair,

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if you put a mirror behind this
guy, and I'm just going

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to focus on the stuff that's
just forward and back, because

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that's what's relevant
if the mirror is

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sitting behind the molecule.

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So if the mirror's sitting
behind the molecule, this

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bromine is actually closer to
the mirror than that hydrogen.

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So the bromine will now be out
front and then the hydrogen

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00:09:20,680 --> 00:09:21,380
will be in back.

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This hydrogen will
be in the back.

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I'm trying to do kind of a
mirror image if it's hard to

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conceptualize.

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And then that would
all look the same.

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And then this chlorine will
now be out front, and this

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hydrogen will now be in the back
in our mirror image, if

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you can visualize it.

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And then we have another one.

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And this chlorine is closer to
the mirror that it's kind of

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been sitting on top of.

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So in the mirror image, it would
be pointing out, and

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then this hydrogen would
be pointing back.

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Now let's see, is our mirror
image the same as this?

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So the mirror image, our bromine
is pointing in the

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front, hydrogen in
the back there.

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Then we have hydrogen in-- then
in our mirror image, we

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have the hydrogen in back,
chlorine in front.

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Same there.

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So far, it's looking like
a mirror image.

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And then in this last carbon
over here, chlorine in front,

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hydrogen in back.

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But here, we have chlorine in
the back, hydrogen in front.

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So this part, you could
think of it this way.

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This is the mirror image of
this, this is the mirror image

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of this part, but this is not
the mirror image of that part.

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So when you have a stereoisomer
that is not a

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mirror, when you have two
stereoisomers that aren't

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mirror images of each other,
we call them diastereomers.

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I always have trouble
saying that.

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Let me write it.

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These are diastereomers, which
is essentially saying it's a

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stereoisomer that is
not an enantiomer.

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That's all it means: a
stereoisomer, not an

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00:10:50,470 --> 00:10:51,090
enantiomer.

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00:10:51,090 --> 00:10:52,830
A stereoisomer's either going
to be an enantiomer or a

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diastereomer.

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00:10:54,960 --> 00:10:57,040
Now, let's do this last one.

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Let's see we have two-- we have
this cyclohexane ring,

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and they have a bromo on the
number one and the number two

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group, depending how
you think about it.

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00:11:07,750 --> 00:11:11,510
It looks like they are mirror
images of each other.

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00:11:11,510 --> 00:11:15,050
We could put a mirror right
there, and they definitely

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00:11:15,050 --> 00:11:16,640
look like mirror images.

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00:11:16,640 --> 00:11:18,520
And this is a chiral
carbon here.

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00:11:18,520 --> 00:11:20,790
It's bonded to one carbon group
that is different than

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00:11:20,790 --> 00:11:21,550
this carbon group.

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00:11:21,550 --> 00:11:23,170
This carbon group
has a bromine.

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This carbon group doesn't.

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00:11:24,080 --> 00:11:25,750
It just has a bunch of hydrogens
on it, if you kind

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of go in that direction.

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00:11:27,210 --> 00:11:30,640
And it's hydrogen and then a
bromine, so that is chiral.

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00:11:30,640 --> 00:11:33,370
And then, same argument,
that is also chiral.

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And obviously, this one is
chiral and that is chiral.

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00:11:36,050 --> 00:11:38,880
But if you think about it, they
are mirror images of each

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00:11:38,880 --> 00:11:42,370
other, and they each have
two chiral centers

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00:11:42,370 --> 00:11:44,350
or two chiral carbons.

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00:11:44,350 --> 00:11:46,840
But if you think about it, all
you have to do is flip this

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guy over and you will
get this molecule.

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00:11:49,830 --> 00:11:51,970
These are the same molecules.

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So it is the same molecule.

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00:11:56,380 --> 00:11:59,050
So this is interesting, and we
saw this when we first learned

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about chirality.

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00:12:00,220 --> 00:12:03,350
Even though we have two chiral
centers, this is

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00:12:03,350 --> 00:12:04,650
not a chiral molecule.

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00:12:04,650 --> 00:12:06,690
It is the same thing as
its mirror image.

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00:12:06,690 --> 00:12:10,180
It is superimposable on
its mirror image.

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00:12:10,180 --> 00:12:23,410
It is superimposable on
its mirror image.

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00:12:23,410 --> 00:12:27,540
So even though it has chiral
carbons in it, it is not a

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00:12:27,540 --> 00:12:29,380
chiral molecule.

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00:12:29,380 --> 00:12:31,530
And we call these
meso compounds.

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00:12:34,700 --> 00:12:36,510
And we can point to one of them
because they really are

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00:12:36,510 --> 00:12:37,660
the same compound.

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00:12:37,660 --> 00:12:41,590
This is a meso compound.

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00:12:41,590 --> 00:12:43,770
It has chiral centers.

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00:12:43,770 --> 00:12:45,960
It has chiral carbons, I
guess you could say it.

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00:12:45,960 --> 00:12:48,360
But it is not a chiral
compound.

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00:12:48,360 --> 00:12:50,350
And the way to spot these fairly
straightforward is that

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00:12:50,350 --> 00:12:52,500
you have chiral centers,
but there is a

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00:12:52,500 --> 00:12:54,170
line of symmetry here.

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00:12:54,170 --> 00:12:56,110
There's a line of symmetry
right here.

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00:12:56,110 --> 00:12:59,690
These two sides of the compound
are mirror images of

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00:12:59,690 --> 00:13:00,430
each other.

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00:13:00,430 --> 00:13:05,190
Now, these would not be the same
molecule if I change that

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00:13:05,190 --> 00:13:09,310
to a fluorine and I change
that to a fluorine.

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00:13:09,310 --> 00:13:11,780
Then all of a sudden, you do
not have this symmetry.

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00:13:11,780 --> 00:13:14,180
These are mirror images,
but they would not be

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00:13:14,180 --> 00:13:15,060
superimposable.

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00:13:15,060 --> 00:13:17,650
So if that was a fluorine,
these would actually be

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00:13:17,650 --> 00:13:18,790
enantiomers.

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00:13:18,790 --> 00:13:22,110
And this would not be only one
meso compound, it would be two

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00:13:22,110 --> 00:13:27,860
different enantiomers, and one
of them would have an R

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00:13:27,860 --> 00:13:30,340
direction and one of them would
have an S direction if

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00:13:30,340 --> 00:13:33,040
we go with the naming
conventions that we learned.