Stereoisomers, enantiomers, diastereomers, constitutional isomers and meso compounds | Khan Academy
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Stereoisomers, enantiomers, diastereomers, constitutional isomers and meso compounds | Khan Academy


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. 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.

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