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- In the last video we learned a little bit about what a chiral
- molecule or what a chiral carbon or a chiral atom is.
- What I want to do in this video is go through a bunch of
- examples and see if we can identify if there are any
- chiral atoms and to also see if we're dealing
- with a chiral molecule.
- So let's look at our examples here.
- So here I have, what is this?
- This is chlorocyclopentane.
- So the first question is do we have any chiral atoms?
- And when we look at our definition that we thought of
- chiral atoms, it all comes from this notion of handedness
- and not being able to be superimposable on your mirror
- image, but we said that they're usually carbons bonded
- to four different groups.
- Let's see, do we have any carbons here bonded to four
- different groups?
- Well, all the CH2's, they're bonded to another CH2
- and then two H'2.
- I could draw it like this: H and H.
- So they're bonded to two of the same group, so none of
- these CH2's are good candidates for being a chiral
- center or chiral carbon.
- They're both bonded to-- or all of them are bonded to two
- hydrogens and two other very similar-looking CH2 groups,
- although you have to look at the entire group
- that it's bonded to.
- But they're all definitely bonded to two hydrogens, so
- it's not four different groups.
- If we look at this CH right here, we could
- separate it out like this.
- We could separate the H out like this, and so since it's
- bonded to a hydrogen.
- This carbon is bonded to a chlorine, and then it's bonded
- to-- well, it's not clear when you look at it right from the
- get-go whether this group is different than this group.
- But if you go around, if you were to split it half-way like
- this, or maybe another better way to think about is if you
- were to go around this molecule in that direction,
- the counterclockwise direction, you would encounter
- a CH2 group, and then you encounter a CH2 group, and
- then you would encounter a third, and then you would
- encounter a fourth CH2 group, then you would come back to
- where you were before.
- So you would encounter four CH2's and then you'd come back
- to where you were before.
- If you go in this direction, what happens?
- You encounter one, two, three, four CH2's and you come back
- to where you were before.
- So all of this, this bottom group, depending on how far
- you want to extend it, and this top group, are really the
- same group.
- So this is not a chiral center or not a chiral carbon.
- It's not bonded to four different groups.
- And this is also not a chiral molecule, because it does not
- have a chiral center.
- And to see that it's not a chiral molecule-- let me see
- if I can backtrack this back to the way I
- wrote it right before.
- So you see that it's not a chiral molecule.
- There's a couple of ways you could think about it.
- The easiest way, or the way my brain likes to think about it,
- is just to think about its mirror image.
- Its mirror image will look like this.
- So if that's the mirror, you would have a chlorine.
- Then you have a CH, CH2, CH2, and you have a CH2, CH2, and
- then you complete your cyclopentane.
- Now, in this situation, is there any way to rotate this
- to get this over there?
- Well, if you just took this molecule right here and you
- just rotated it 180 degrees, what would it look like?
- Well, maybe a little over-- yeah, well, not quite 180
- degrees, but if you were to rotate it so that the chlorine
- goes about that far, you would get this exact molecule.
- You would get something.
- It would look a little bit different.
- It would look like this.
- Let me see if I can do it justice.
- It would look like this.
- You would have a CH2.
- So let me let me do it up here where I have a
- little bit more space.
- If I were to rotate this about that far, I would get a CH.
- You get the chlorine and then you have your CH2, and then
- you have another CH2, CH2, and then you would
- have your CH2 up there.
- If you were to rotate this all the way around, or actually
- this is almost exactly 180 degrees, it
- would look like this.
- And the only difference between this and this is just
- how we drew this bond here.
- I could have easily, instead of drawing that bond like
- that, I could draw it facing up like that, and these are
- the exact same molecule.
- So this molecule is also not chiral.
- So let's go to this one over here.
- So what is this?
- This is a bromochlorofluoromethane, just
- to practice our naming a little bit.
- But it's very clear that we are bonded to
- four different groups.
- All of the different groups, or the atoms in this case that
- are bonded to this carbon, are different, so this carbon is a
- chiral center.
- And it should also be pretty clear that it is
- also a chiral molecule.
- If you were to take its mirror image, and this is very
- similar to the example we did in the first video on
- chirality, but its mirror image will look like this.
- You have the bromine on the right now.
- The hydrogen is still in back, and you have the
- fluorine above it.
- No matter how you try to rotate this thing, if you try
- to get the bromine all the way over there, all the way to
- that position, then the hydrogen would be in this
- position and the chlorine would be in that position.
- And no matter how you try to flip this around or rotate it
- or shift it, you will never be able superimpose this molecule
- on that molecule right there.
- So that is a chiral center and this is a chiral molecule.
- And there's a word for these two versions.
- We're going to go into the naming of them later on.
- It's a little bit more involved.
- We'll have a whole separate video on it.
- But these two versions of bromochlorofluoromethane, they
- sometimes have different chemical properties.
- And these are called enantiomers.
- And enantiomers are just the mirror images.
- Each enantiomer is a mirror image of each other, but they
- are stereoisomers.
- This is all just terminology.
- Stereoisomers.
- You're familiar with the word isomer, and isomer just means
- that you have the same atoms in your molecule.
- But then you have different types of isomers.
- You have constitutional isomers that say, OK,
- different things are connected to different things.
- Stereoisomers, the same things are all
- connected to the same things.
- You have a carbon connected to only a fluorine, a chlorine
- connected to the carbon, a hydrogen connected to the
- carbon, a bromine connected to the carbon.
- So all of the same things are connected to the same things,
- but they're a three-dimensional
- configuration.
- That's where we're dealing with the stereo part.
- Stereochemistry is the study of three-dimensional
- chemistry, as essentially understanding the actual
- three-dimensional structure of things.
- So stereoisomers mean that we have the same constituents,
- the same atoms. They have the same
- connections to each other.
- Bromine is still connected to carbon, which is still
- connected to hydrogen.
- That's all true over here.
- But their three-dimensional
- orientation is still different.
- And in this case where they are mirror images of each
- other, we call them enantiomers.
- And I should probably make one clarification.
- In the last few videos, I've been a little bit, you know,
- sometimes I'll say configuration and sometimes I
- use the word conformation.
- So sometimes I'll use the word configuration and sometimes I
- use the word conformation, and I actually should be a little
- bit more, or I should have been a little bit more exact
- about these.
- When you're talking about a configuration, you're actually
- talking about a different structure.
- To go from one configuration to another configuration, you
- would actually have to break bonds and kind
- of reassemble them.
- So these are different configurations.
- Because in order for them to be able to be the same thing,
- you would have to swap maybe the bromine and the hydrogen
- in there where they are relative to the carbon, so
- these are different configurations.
- Confirmations are really just different shapes or different
- orientations of the same molecule.
- So when we talked about cyclohexane being in a boat,
- so this is cyclohexane being in a boat conformation, or
- this is cyclohexane being in a chair conformation, it's the
- exact same molecule with the exact same connections.
- We didn't detach any bonds or reattach any bonds.
- They just flipped around a little bit.
- So these are two different conformations.
- These are two different configurations.
- To go from one configuration to another, you have to
- rearrange bonds.
- Now let's look at this molecule over here.
- Can we identify any stereocenters or chiral
- carbons or chiral atoms?
- And you have this carbon right here.
- Let's see, this carbon right here is bonded to a chlorine,
- a hydrogen, a bromine, and then another carbon.
- So this is bonded to four different things, so this is a
- chiral carbon.
- Sometimes they put a little asterisk there.
- If we look at this carbon right here you can-- well,
- it's bonded to a fluorine and another carbon, but it's
- bonded to two hydrogens, so it's not chiral.
- It has two of the same things that it's bonded to.
- You can even see a little axis of symmetry through it.
- If you look at that, you can kind of flip it over, and it's
- going to be the same thing.
- But this one right here, that is a chiral center.
- That is a chiral center, or chiral carbon, or chiral atom,
- or a symmetric carbon.
- You'll see it used in different ways.
- And because this molecule has got that chiral center, you'll
- see that if you were to take its mirror image, it would be
- an enantiomer.
- This is not superimposable on its mirror image.
- We could even try to draw it.
- And just so you know, you don't always have to do the
- mirror image on the right side.
- We can draw the mirror image on the left side.
- So if we want to draw its mirror image, it
- would look like this.
- You would have a fluorine, carbon, carbon, chlorine.
- You have your two hydrogens, and then you would have a
- hydrogen here, and then you would have your bromine here.
- No matter what you try to do, if you try to flip this around
- or whatever, you will never be able superimpose this on top
- of this, so these are enantiomers.
- These are both stereoisomers relative to each other.
- And either of these, regardless of which one you
- pick, are chiral molecules.
- I'm over the time that I normally want to go in the
- video, so in the next video I'll do even more.