Meso compounds

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

chirality centers but is achiral.

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

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

And our goal is to draw
all possible stereoisomers

for this dot structure.

So we know from earlier
videos that this carbon

is a chirality center, and so is this one.

And we would expect two
to the n stereoisomers,

where n is the number
of chirality centers.

And since we have two chiral centers here,

we would expect two to the second power,

or four stereoisomers.

So this is really just a maximum number,

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

So do we get four stereoisomers?

Let's draw out the four possibilities.

Our first stereoisomer
could have both bromines

coming out at us in space.

So let me go ahead and draw that in.

So we could have both
bromines coming out at us.

For the second possibility,
we might have both bromines

going away from us in space.

So I'll draw that in there.

For the third possibility,
we could have one bromine up

and one bromine down.

So I'll put those in.

And for the fourth, just reverse them.

Have the top bromine down
and the bottom bromine up.

like that.

Well, all right, let's
examine the relationships

between our stereoisomers.

And let's start with
stereoisomer possibility

three and four.

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

the relationship.

On the left is stereoisomer three.

We can see there's a bromine coming at us

and a bromine going away from us.

On the right is stereoisomer four.

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

and a bromine coming out at us here.

So let's compare our two stereoisomers.

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

mirror images of each other.

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

here we get one pair
of bromines to line up,

but the other pair doesn't match.

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

now the first pair doesn't match.

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

These are enantiomers.

So three and four are enantiomers.

They are non-superimposable mirror images.

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

because at this carbon we
have bromine coming out

at us in space, and
then now we have bromine

going away from us in space
for the other stereoisomer.

So that's an opposite
configuration at that carbon.

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

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

So since we have opposite
configurations at all

chirality centers, we
would expect these two

to be enantiomers of each other.

What about the relationship
between one and two?

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

because here we have a
wedge, and then over here

we have a dash and here we have a wedge,

and here we have a dash.

So that should be the
opposite configuration

at both chirality centers.

So those might be enantiomers.

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

On the left is a model of drawing one,

with the two bromines
coming out at us in space.

On the right is a model of drawing two,

with the two bromines going
away from us in space.

And if I rotate the model on the right,

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

But they are superimposable mirror images.

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

you'll see that they are superimposable.

So these actually are two
models of the same molecule.

This is a meso compound.

It's a compound that
has chirality centers,

but it is achiral, the mirror
image is superimposable.

So one and two really
represent the same molecule.

This is a meso compound,
a compound that has

chirality centers but is achiral.

The mirror image is
superimposable on itself.

So we thought we would
have four stereoisomers,

but really we only have three.

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

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

is what we did in the video.

We had the mirror image and
we were able to superimpose

the mirror image on itself.

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

a plane of symmetry.

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

and look for symmetry on either side.

So you can see, it's symmetrical.

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

but it's hard to visualize it.

So up here is a better picture.

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

And on the left side,
we have our bonds here

and then we have our bromine going up

and our hydrogen going down.

The right side is symmetrical
with the left side.

So look for symmetry on
both sides of the plane.

Let's do another example.

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

We know that we have two chiral centers.

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

So we would expect two to
the second stereoisomer.

So that's, of course, four.

So I'll put a question mark here again

because we're not sure
if we actually will get

four stereoisomers.

That's a maximum number.

Down here I have the four possibilities.

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

So we have one, two, three and four.

And let's examine the
relationship between one

and two first.

On the left is stereoisomer one,

and I've left the hydrogens
off the methyl groups

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

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

coming out at us in space.

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

and we have both OHs going away from us.

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

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

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

of each other.

But if I try to superimpose
one on the other,

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

you can see they don't match up.

So the atoms don't line up here.

So they're non-superimposable.

Doesn't matter how you do it.

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

superimpose our atoms.

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

These are enantiomers.

And we could check for
a plane of symmetry,

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

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

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

So this is not a meso compound.

So one and two are enantiomers.

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

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

our chiral centers here,
so this one has an OH

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

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

and this one has it going away from us,

so we have opposite configurations at both

chirality centers.

Let's look at three and four next.

So what is the relationship
between these two?

Well, first, we might think
these could be enantiomers

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

and then here we have OH on a dash.

And then here we have OH on a dash,

and here we have it on a wedge.

So that might be your first guess.

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

the model sets to help us out.

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

and the hydrogens off
the oxygens in the video

just to help us see the
molecule more clearly.

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

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

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

On the right is a model of drawing four.

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

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

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

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

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

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

And now we can see that
these two are mirror images

of each other.

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

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

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

the one on the right, the mirror image,

on the molecule on the left.

And notice that we can.

All of the atoms line up.

So all of the hydrogens,
carbons and oxygens

are in the same place.

So this is a compound that
has chirality centers,

but it is achiral, the mirror
image is superimposable

on itself.

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

So I'll just pick one of these models,

doesn't matter which one
because they represent

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

so we can see a plane of symmetry.

So right there is our plane of symmetry.

This is a meso compound.

So three and four actually
represent the same compound.

So this is one meso compound.

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

It's not really obvious looking at these

bond line structures that these
represent the same molecule.

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

So we thought there might
be four stereoisomers,

but actually there are only three.

We have a pair of enantiomers,

and we have one meso compound.