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- [Instructor] A meso compound
is a compound that has

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chirality centers but is achiral.

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We're gonna come back to this
definition in a few minutes.

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Right now, let's focus in on this drawing.

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And our goal is to draw
all possible stereoisomers

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for this dot structure.

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So we know from earlier
videos that this carbon

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is a chirality center, and so is this one.

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And we would expect two
to the n stereoisomers,

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where n is the number
of chirality centers.

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And since we have two chiral centers here,

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we would expect two to the second power,

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or four stereoisomers.

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So this is really just a maximum number,

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so I'm gonna put a
question mark right here.

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So do we get four stereoisomers?

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Let's draw out the four possibilities.

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Our first stereoisomer
could have both bromines

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coming out at us in space.

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So let me go ahead and draw that in.

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So we could have both
bromines coming out at us.

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For the second possibility,
we might have both bromines

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going away from us in space.

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So I'll draw that in there.

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For the third possibility,
we could have one bromine up

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and one bromine down.

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So I'll put those in.

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And for the fourth, just reverse them.

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Have the top bromine down
and the bottom bromine up.

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like that.

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Well, all right, let's
examine the relationships

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between our stereoisomers.

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And let's start with
stereoisomer possibility

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three and four.

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So let's compare these
and let's figure out

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the relationship.

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On the left is stereoisomer three.

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We can see there's a bromine coming at us

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and a bromine going away from us.

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On the right is stereoisomer four.

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Now we have a bromine going
away from us at the top carbon

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and a bromine coming out at us here.

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So let's compare our two stereoisomers.

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If I rotate the one on the
right, we can see these are

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mirror images of each other.

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And if I try to superimpose
one on top of the other,

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here we get one pair
of bromines to line up,

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but the other pair doesn't match.

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If we try to get the other
pair of bromines to line up,

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now the first pair doesn't match.

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So these are non-superimposable
mirror images of each other.

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These are enantiomers.

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So three and four are enantiomers.

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They are non-superimposable mirror images.

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And we could have guessed that
by looking at the drawing,

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because at this carbon we
have bromine coming out

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at us in space, and
then now we have bromine

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going away from us in space
for the other stereoisomer.

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So that's an opposite
configuration at that carbon.

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And at this carbon, we go
from a dash to a wedge.

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So that's an opposite
configuration at this one too.

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So since we have opposite
configurations at all

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chirality centers, we
would expect these two

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to be enantiomers of each other.

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What about the relationship
between one and two?

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Well, at first we might say
oh, those are enantiomers

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because here we have a
wedge, and then over here

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we have a dash and here we have a wedge,

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and here we have a dash.

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So that should be the
opposite configuration

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at both chirality centers.

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So those might be enantiomers.

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But let's go to the video
to see if that's true.

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On the left is a model of drawing one,

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with the two bromines
coming out at us in space.

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On the right is a model of drawing two,

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with the two bromines going
away from us in space.

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And if I rotate the model on the right,

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we can see that these are
mirror images of each other.

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But they are superimposable mirror images.

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So if I put that one on top of the other,

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you'll see that they are superimposable.

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So these actually are two
models of the same molecule.

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This is a meso compound.

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It's a compound that
has chirality centers,

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but it is achiral, the mirror
image is superimposable.

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So one and two really
represent the same molecule.

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This is a meso compound,
a compound that has

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chirality centers but is achiral.

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The mirror image is
superimposable on itself.

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So we thought we would
have four stereoisomers,

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but really we only have three.

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We have a pair of enantiomers
and we have one meso compound.

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So to look for a meso compound,
one thing you could do

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is what we did in the video.

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We had the mirror image and
we were able to superimpose

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the mirror image on itself.

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Another way to look for a
meso compound is to look for

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a plane of symmetry.

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So if I draw a line here, think
about this as being a plane,

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and look for symmetry on either side.

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So you can see, it's symmetrical.

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I drew in the plane of symmetry
with a dashed line here,

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but it's hard to visualize it.

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So up here is a better picture.

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Here you can see the plane
dividing the molecule in half.

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And on the left side,
we have our bonds here

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and then we have our bromine going up

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and our hydrogen going down.

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The right side is symmetrical
with the left side.

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So look for symmetry on
both sides of the plane.

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Let's do another example.

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This one's a little bit
harder than the last one.

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We know that we have two chiral centers.

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So that's a chiral center
and so is this one.

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So we would expect two to
the second stereoisomer.

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So that's, of course, four.

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So I'll put a question mark here again

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because we're not sure
if we actually will get

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four stereoisomers.

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That's a maximum number.

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Down here I have the four possibilities.

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So I've drawn them out
just to save some time.

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So we have one, two, three and four.

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And let's examine the
relationship between one

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and two first.

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On the left is stereoisomer one,

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and I've left the hydrogens
off the methyl groups

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and the OH just so we can
see the models better.

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So here's our carbon
chain, and we have both OHs

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coming out at us in space.

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And then for stereoisomer
two, here's the carbon chain,

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and we have both OHs going away from us.

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So I'm gonna hold them in the
way they are in the drawing

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and I'm gonna rotate the one on the right.

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And when I do that, we can see
that these are mirror images

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of each other.

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But if I try to superimpose
one on the other,

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so I'll just rotate this one
back here and flip it over,

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you can see they don't match up.

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So the atoms don't line up here.

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So they're non-superimposable.

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Doesn't matter how you do it.

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I'll rotate it again,
and we can see we can't

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superimpose our atoms.

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So these are non-superimposable
mirror images of each other.

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These are enantiomers.

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And we could check for
a plane of symmetry,

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so I could take one of
these and I can rotate it

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so we have our hydrogens
going away from us in space.

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And I look for a plane of
symmetry, but I don't see one.

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So this is not a meso compound.

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So one and two are enantiomers.

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They're non-superimposable
mirror images of each other.

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And we could've guessed
that because if we look at

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our chiral centers here,
so this one has an OH

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coming out at us, and
then that one has it going

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away from us, this one
has an OH coming out at us

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and this one has it going away from us,

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so we have opposite configurations at both

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chirality centers.

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Let's look at three and four next.

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So what is the relationship
between these two?

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Well, first, we might think
these could be enantiomers

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because at this carbon,
we have OH on a wedge,

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and then here we have OH on a dash.

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And then here we have OH on a dash,

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and here we have it on a wedge.

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So that might be your first guess.

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But let's look at the
video and let's look at

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the model sets to help us out.

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Remember, I'm leaving the
hydrogens off the methyl groups

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and the hydrogens off
the oxygens in the video

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just to help us see the
molecule more clearly.

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On the left, we have a
model of drawing three.

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So here's our carbon chain
with an OH going away from us

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in space, and an OH
coming out at us in space.

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On the right is a model of drawing four.

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Here's our carbon chain
with an OH coming out at us

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in space and an OH going
away from us in space.

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So I'll hold the two models
and we'll compare them.

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First let's see if they are
mirror images of each other.

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So I'll take the one on the
right and I'll rotate it

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and I'll hold it up next
to the one on the left.

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And now we can see that
these two are mirror images

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of each other.

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So next, let's see if one is
superimposable on the other.

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So I'll go back to the starting
point and I'll rotate it

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around like that, and let's
see if we can superimpose

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the one on the right, the mirror image,

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on the molecule on the left.

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And notice that we can.

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All of the atoms line up.

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So all of the hydrogens,
carbons and oxygens

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are in the same place.

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So this is a compound that
has chirality centers,

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but it is achiral, the mirror
image is superimposable

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on itself.

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So we should be able to
find a plane of symmetry.

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So I'll just pick one of these models,

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doesn't matter which one
because they represent

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the same compound, and I'll
rotate, I'll rotate it around

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so we can see a plane of symmetry.

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So right there is our plane of symmetry.

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This is a meso compound.

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So three and four actually
represent the same compound.

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So this is one meso compound.

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So these two are the same,
and we have one meso compound.

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It's not really obvious looking at these

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bond line structures that these
represent the same molecule.

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So definitely get a model set
and try this out for yourself.

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So we thought there might
be four stereoisomers,

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but actually there are only three.

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We have a pair of enantiomers,

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and we have one meso compound.