In this video, we're going to look at pairs of molecules and
see if they relate to each other in any obvious way or
maybe less than obvious way.
So these first two right here, they actually look like a
completely different molecules.
So your gut impulse might be to say that these are
completely different molecules.
And it wouldn't be completely off, but we look a little bit
closer, you see that this guy on the left has one, two,
three, four carbons, and so does this guy on the right.
It has one, two, three, four carbons.
This guy on the left has two, four, six,
seven, eight hydrogens.
This guy on the right has two, four, six, eight hydrogens.
And they both have one oxygen.
So both of the molecular formulas for both of these
things are four carbons, eight hydrogens, and one oxygen.
They're both C4H8O.
So they have the same molecular formula.
They're made up of the same thing, so these are going to
be isomers.
They're going to be isomers, and they're a
special type of isomers.
In this situation, we don't have the same bonds.
We're made up of the same things, but the bonds, what is
connected to what is different.
So we call this a constitutional isomer.
So we are essentially made up of the same things, but we are
actually two different molecule, actually, two very
different molecules here.
Now let's look at this next guy over here.
So if we look at this molecule, it does look like
this carbon is chiral.
It is an asymmetric carbon.
It is bonded to four different groups: fluorine, bromine,
hydrogen, and then a methyl group.
And so's this one.
And they're both made up of the same things.
You have the carbon-- and not only are they made up of the
same things, but the bonding is the same.
So carbon to a fluorine, carbon to a fluorine, carbon
to a bromine, carbon to a bromine, carbon to hydrogen in
both of then carbon to the methyl group in both.
But they don't look quite the same.
Are they mirror images?
Well, no.
This guy's mirror image would have the fluorine popping out
here, the hydrogen going back here, and then would have the
bromine pointing out here.
Let's see if I can somehow get from this guy to that guy.
Let me flip this guy first. So let me-- a good thing to do
would be to just flip to see the fastest way I could
potentially get there.
Let me just flip it like this.
So I'm going to flip out of the page, you can imagine.
I'm going to flip it like this.
So I'm going to take this methyl group and then put it
on the right-hand side.
And you can imagine, I'm going to turn it so it would come
out of the page and then go back down.
So if I did that, what would it look like?
I would have the carbon, this carbon here.
I would have the methyl group on that side now.
And then since I flipped it over, the bromine was in the
plane of the page.
It'll still be in the plane of the page, but since I flipped
it over, the hydrogen, which was in the back, will now be
in the front.
The hydrogen will now be in the front and the fluorine
will now be in back because I flipped it over.
So the fluorine is now in the back.
Now, how does this compare to that?
Let's see if I can somehow get there.
Well, if I take this fluorine and I rotate it to where the
hydrogen is, and I take the hydrogen and rotate it to
where-- that's all going to happen at once-- to where the
bromine is, and I take the bromine and rotate it to where
the fluorine is, I get that.
So I can flip it and then I can rotate it around this bond
axis right there, and I would get to that molecule there.
So even though they look pretty different, with the
flip and a rotation, you actually see that these are
the same a molecule.
Next one.
So let's see, what do we have here?
Let me switch colors.
So over here, this part of both of these
molecules look the same.
You have the carbons on both of them.
This carbon looks like a chiral center.
It's bonded to one, two, three different groups.
You might say, oh, it's two carbons, but this is a methyl
group, and then this side has all this business over it, so
this is definitely a chiral carbon.
And over, here same thing.
It's a chiral carbon.
And this has the same thing.
It's bonded to four different things.
So each of these molecules has two chiral carbons, and it
looks like they're made up of the same things.
And not only are they made up of the same things, but the
bonds are made in the same way.
So this carbon is bonded to a hydrogen and a fluorine, and
the two other carbons, same thing, a
hydrogen and a fluorine.
Carbon, it looks like it's a hydrogen.
It's bonded to a hydrogen and a chlorine, so it's made up of
the same constituents and they're
bonded in the same way.
So these look like-- but the bonding is
a little bit different.
Over here on this one on the left, the hydrogen goes in the
back, and over here, the hydrogen's in the front.
And over here, the chlorine's in back, and over here, the
chlorine's in front.
So these look like sterioisomers.
You saw earlier in this video, you saw structural isomers,
made up of the same things but the
connections are all different.
Stereoisomers, they're made up of the same thing, the
connections are the same, but the three-dimensional
configuration is a little bit different.
For example, here on this carbon, it's connected to the
same things as this carbon, but over here, the fluorine's
out front, and over here-- out here, the
fluorine's out front.
Over here, the fluorine's backwards.
And same thing for the chlorine here.
It's back here and it's front here.
Now, let's see if they're related in a more nuanced way.
You could imagine putting a mirror behind.
I guess the best way to visualize it, imagine putting
a mirror behind this molecule.
If you put a mirror behind this molecule, what would its
reflection look like?
So if you put a mirror behind it, in the image of the
mirror, this hydrogen would now, since the mirror's behind
this whole molecule, this hydrogen is actually closer to
the mirror.
So then the mirror image, you would have a hydrogen that's
pointed out, and then you would have the carbon, and
then you would have the fluorine being further away.
And same thing in the mirror image here.
You would have the chlorine coming closer since this
chlorine is further back, closer to the mirror, and then
you would have the hydrogen pointing outwards like that.
And then, obviously, the rest of the molecule would look
exactly the same.
And so this mirror image that I just thought about in white
is exactly what this molecule is: hydrogen pointing out in
front, hydrogen pointing out in front.
You might say, wait, this hydrogen is on the right, this
one's on the left.
It doesn't matter.
This is actually saying that the hydrogen's pointing out
front, the fluorine is pointing out back, hydrogen up
front, fluorine back, chlorine out front, hydrogen back,
chlorine out front, hydrogen back.
So these are actually mirror images, but they're not the
easy mirror images that we've done in the past where the
mirror was just like that in between the two.
This one is a mirror image where you place the mirror
either on top of or behind one of the molecules.
So this is a class of stereoisomers, and we've
brought up this word before.
We call this enantiomers.
So if each of these are an enantiomers, I'll say they are
enantiomers of each other.
They're steroisomers.
They're made up of the same molecules, so that they have
the same constituents.
They also have the same connections, and not only do
they have the same connections, that so far gets
us a steroisomer, but they are a special kind of stereoisomer
called an enantiomer, where they are actual mirror images
of each other.
Now, what is this one over here in blue?
Just like the last one, it looks like it's made up of the
same things.
You have these carbons, these carbons, these carbons and
hydrogens up there.
Same thing over there.
You have a hydrogen, bromine, hydrogen and a bromine,
hydrogen, chlorine, hydrogen, chlorine, hydrogen, chlorine,
hydrogen, chlorine.
So it's made up of the same things.
They're connected in the same way, so they're definitely
stereoisomers.
Well, we have to make sure they're not-- well, let's make
sure they're not the same molecule first. Here,
hydrogen's in the front.
There, hydrogen's in the back.
Here, hydrogen is in the back.
Here, hydrogen is in the front.
So they're not the same molecule.
They have a different three-dimensional
configuration, although their bond connections are the same,
so these are stereoisomers.
Let's see if they're enantiomers.
So if we look at it like this, you put a mirror here, you
wouldn't get this guy over here.
Then you would have a chlorine out front and a hydrogen.
So you won't get it if you get a mirror over there.
But if we do the same exercise that we did in the last pair,
if you put a mirror behind this guy, and I'm just going
to focus on the stuff that's just forward and back, because
that's what's relevant if the mirror is
sitting behind the molecule.
So if the mirror's sitting behind the molecule, this
bromine is actually closer to the mirror than that hydrogen.
So the bromine will now be out front and then the hydrogen
will be in back.
This hydrogen will be in the back.
I'm trying to do kind of a mirror image if it's hard to
conceptualize.
And then that would all look the same.
And then this chlorine will now be out front, and this
hydrogen will now be in the back in our mirror image, if
you can visualize it.
And then we have another one.
And this chlorine is closer to the mirror that it's kind of
been sitting on top of.
So in the mirror image, it would be pointing out, and
then this hydrogen would be pointing back.
Now let's see, is our mirror image the same as this?
So the mirror image, our bromine is pointing in the
front, hydrogen in the back there.
Then we have hydrogen in-- then in our mirror image, we
have the hydrogen in back, chlorine in front.
Same there.
So far, it's looking like a mirror image.
And then in this last carbon over here, chlorine in front,
hydrogen in back.
But here, we have chlorine in the back, hydrogen in front.
So this part, you could think of it this way.
This is the mirror image of this, this is the mirror image
of this part, but this is not the mirror image of that part.
So when you have a stereoisomer that is not a
mirror, when you have two stereoisomers that aren't
mirror images of each other, we call them diastereomers.
I always have trouble saying that.
Let me write it.
These are diastereomers, which is essentially saying it's a
stereoisomer that is not an enantiomer.
That's all it means: a stereoisomer, not an
enantiomer.
A stereoisomer's either going to be an enantiomer or a
diastereomer.
Now, let's do this last one.
Let's see we have two-- we have this cyclohexane ring,
and they have a bromo on the number one and the number two
group, depending how you think about it.
It looks like they are mirror images of each other.
We could put a mirror right there, and they definitely
look like mirror images.
And this is a chiral carbon here.
It's bonded to one carbon group that is different than
this carbon group.
This carbon group has a bromine.
This carbon group doesn't.
It just has a bunch of hydrogens on it, if you kind
of go in that direction.
And it's hydrogen and then a bromine, so that is chiral.
And then, same argument, that is also chiral.
And obviously, this one is chiral and that is chiral.
But if you think about it, they are mirror images of each
other, and they each have two chiral centers
or two chiral carbons.
But if you think about it, all you have to do is flip this
guy over and you will get this molecule.
These are the same molecules.
So it is the same molecule.
So this is interesting, and we saw this when we first learned
about chirality.
Even though we have two chiral centers, this is
not a chiral molecule.
It is the same thing as its mirror image.
It is superimposable on its mirror image.
So even though it has chiral carbons in it, it is not a
chiral molecule.
And we call these meso compounds.
And we can point to one of them because they really are
the same compound.
This is a meso compound.
It has chiral centers.
It has chiral carbons, I guess you could say it.
But it is not a chiral compound.
And the way to spot these fairly straightforward is that
you have chiral centers, but there is a
line of symmetry here.
There's a line of symmetry right here.
These two sides of the compound are mirror images of
each other.
Now, these would not be the same molecule if I change that
to a fluorine and I change that to a fluorine.
Then all of a sudden, you do not have this symmetry.
These are mirror images, but they would not be
superimposable.
So if that was a fluorine, these would actually be
enantiomers.
And this would not be only one meso compound, it would be two
different enantiomers, and one of them would have an R
direction and one of them would have an S direction if
we go with the naming conventions that we learned.