Enantiomers and diastereomers

- [Narrator] We've already
spent a lot of time

talking about enantiomers.

These are stereoisomers
that are nonsuperimposable

mirror images of each other,

and they have opposite configurations

at all chirality centers.

Diastereomers are also stereoisomers,

but these are stereoisomers
that are nonsumperimposable,

non-mirror images of each other.

So these are stereoisomers
that are not enantiomers.

And diastereomers have
opposite configurations

at some chirality centers.

If we look at this compound up here,

we have a cyclopropane ring
with a bromine coming off,

and a chlorine coming off.

We know from earlier videos that there

are two chirality centers.

So this carbon is a chiral
center, and so is this one.

The total number of
stereoisomers is two to the n,

where n is equal to the
number of chiral centers.

And since n is equal to
two for this drawing,

we would expect to be able to
draw two to the second power,

or four stereoisomers.

So there should be a total
of four stereoisomers.

Actually, two to the n is just a maximum,

and we'll talk about that in later videos.

So let's draw all four stereoisomers,

and let's look at the
relationship between them.

Let's think about how to
draw our four stereoisomers.

For the first one, we
could have both halogens

coming out at us in space,

so I put the bromine on a wedge,

and I put the chlorine on a wedge.

Next, we could have
both halogens going away

from us in space.

So I put the bromine on a dash,
and same with the chlorine.

Next, for our third
stereoisomer, we could have

one halogen coming out at us,

so I'll make that the bromine,

and one going away from us.

And for the last one, we
could just reverse it.

We could have the bromine
going away from us,

and the chlorine coming out at us.

Next, let's look at the relationships

between our stereoisomers.

And let's start with the relationship

between stereoisomer one
and stereoisomer two.

Model sets really help
in stereo chemistry,

so we're gonna look at
videos for a lot of these.

Let's go look at the video
comparing stereoisomers one

and stereoisomer two.

On the left we have stereoisomer one.

Both halogens are coming
out at us in space.

On the right is stereoisomer two,

where both halogens are
going away from us in space.

If I hold these two
stereoisomers next to each other,

and I rotate the one on the right,

we can see that they're
actually mirror images

of each other, and they're
nonsuperimposable mirror images.

If I line up the chlorines,
then the bromines

are not in the right position.

And if I try to line up the bromines,

now the chlorines are not
in the right position.

So these are nonsuperimposable
mirror images of each other.

These are enantiomers.

So we saw in the video that
one and two are enantiomers

of each other.

They are nonsuperimposable mirror images,

and they have opposite configurations

at all chirality centers.

And that's easy to see if you
look at the drawings, here.

So at this carbon, we
have bromine on a wedge,

and if we change it to a
dash, right, we see we have

this one on the right.

We look at this chiral center,
we have chlorine on a wedge,

and here it's changed to a dash.

So that's an opposite configuration
at both chiral centers,

and so that's how we know,
that's one way of knowing

that this one on the
right is the mirror image

of the one on the left.

They are enantiomers.

Let's look at the relationship
between stereoisomers

three and four.

On the left is stereoisomer
three, with bromine up

and chlorine down.

On the right is four, with
bromine down and chlorine up.

If we hold them together and
I rotate the one on the right,

it's easy to see that these are
mirror images of each other,

and they are nonsuperimposable.

If I put the chlorines
on top of each other,

now the bromines don't line up,

and if I try to line up the bromines,

then the chlorines don't.

So these are nonsuperimposable
mirror images of each other.

These are enantiomers.

So three and four are
enantiomers of each other.

They are nonsuperimposable mirror images,

and they have opposite configurations

at all chirality centers.

So at this chiral center,
we have bromine on a wedge,

and over here, we have bromine on a dash.

At this chiral center, we
have chlorine on a dash,

and over here, we have it on wedge.

So we have opposite configurations

at both chirality centers.

Next, let's compare
stereoisomers two and three.

So what's the relationship
between two and three?

On the left is stereoisomer
two, with the bromine

and the chlorine going
away from us in space.

On the right is three, with bromine up

and chlorine down.

If I hold the two stereoisomers
next to each other,

and I rotate the one on the right,

we can see that these are not
mirror images of each other.

The bromines look right,
but the chlorines don't.

One chlorine is up, and
one chlorine is down.

If I try to superimpose these,

I can get the bromines to match,

but not the chlorines.

And if I try to make
the chlorines line up,

then the bromines won't.

So these are nonsuperimposable,
non-mirror images.

These are diastereomers.

We saw in the video that two and three

are nonsuperimposable,

and they're also non-mirror images.

Therefore they are diastereomers.

So let me write that down, here.

So two and three represent
a pair of diastereomers,

and diastereomers have
opposite configurations

at some chirality centers.

So if we look at this carbon,
we have bromine on a dash,

and over here, we have bromine on a wedge.

So that's opposite.

But if we look at this one,
we have chlorine on a dash,

and over here, we have chlorine on a dash.

So that's the same.

So we only have an opposite configuration

at one chiral center.

And so these are diastereomers.

What about comparing
two, stereoisomer two,

with stereoisomer four?

So let's look at the video for that.

On the left is stereoisomer two,

with the bromine and
chlorine going away from us.

On the right is four, with
bromine down and chlorine up.

So we hold the two stereoisomers
next to each other,

and we rotate the one on the right.

And we can see that these
are not mirror images.

The chlorines look right,
but the bromines don't.

One bromine's down, and one bromine is up.

If we try to superimpose
one on top of the other,

the chlorines line up,
but not the bromines,

and if I try matching the bromines,

now the chlorines don't.

So these are nonsuperimposable,
non-mirror images.

These are diastereomers.

So two and four are diastereomers.

They are nonsuperimposable,
non-mirror images of each other.

And they only have opposite configurations

at some chirality centers,
in this case, one.

So if we look at this chiral center,

the bromine's on a dash,

and we look at this one, the
bromine's also on a dash.

So that's the same.

If we look at this one,
the chlorine is on a dash,

and over here, the chlorine is on a wedge.

So that's an opposite configuration

at only one chiral center.

So two and four are diastereomers.

What about comparing one and three?

So, thinking about one and three.

So we don't need a
video for this, anymore.

I think we've got the hang of it.

And if we look at this carbon,
we have bromine on a wedge,

and at this carbon, we
have bromine on a wedge.

So that's the same.

At this carbon, we have
chlorine on a wedge,

and at this one, we
have chlorine on a dash.

So that's different.

So we only have an opposite configuration

at one chirality center.

So one and three are
diastereomers of each other.

And the same thing if you're thinking

about one and four.

Those would also be
diastereomers of each other.