Introduction to Chirality Introduction to Chirality
Introduction to Chirality
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- If I were to draw a hand, and let me just draw a hand really
- fast, so I'll draw a left hand.
- It looks something like that.
- That is a left hand.
- Now, if I were to take its mirror image, let's say that
- this is a mirror right there, and I want to take its mirror
- image, and I'll draw the mirror image in green.
- So its mirror image would look something like this.
- Not exact, but you get the idea.
- The mirror image of a left hand looks a lot
- like a right hand.
- Now, no matter how I try to shift or rotate this hand like
- this, I might try to maybe rotate it 180 degrees, so that
- the thumb is on the other side like this image right here.
- But no matter what I do, I will never be able to make
- this thing look like that thing.
- I can shift it and rotate it, it'll just never happen.
- I will never be able to superimpose the blue hand on
- top of this green hand.
- When I say superimpose, literally put it exactly on
- top of the green hand.
- So whenever something is not superimposable on its mirror
- image-- let me write this down-- we call it chiral.
- So this hand drawing right here is an
- example of a chiral object.
- Or I guess the hand is an example of a chiral object.
- This is not superimposable on its mirror image.
- And it makes sense that it's called chiral because the word
- chiral comes from the Greek word for hand.
- And this definition of not being able to be
- superimposable on its mirror image, this applies whether
- you're dealing with chemistry, or mathematics, or I guess,
- just hands in general.
- So if we extend this definition to chemistry,
- because that's what we're talking about, there's two
- concepts here.
- There are chiral molecules, and then there are chiral
- centers or chiral-- well, I call them chiral atoms. They
- tend to be carbon atoms, so sometimes they call them
- chiral carbons.
- So you have these chiral atoms.
- Now, chiral molecules are literally molecules that are
- not superimposable on their mirror image.
- I'm not going to write the whole thing.
- You know, not superimposable-- I'll just
- write the whole thing.
- Not superimposable on mirror image.
- Now, for chiral atoms, this is essentially true, but when you
- look for chiral atoms within a molecule, the best way to spot
- them is to recognize that these generally, or maybe I
- should say usually, are carbons, especially when we're
- dealing in organic chemistry, but they could be phosphoruses
- or sulfurs, but usually are carbons bonded to four
- different groups.
- And I want to emphasize groups, not just four
- different atoms. And to kind of highlight a molecule that
- contains a chiral atom or chiral carbon, we can just
- think of one.
- So let's say that I have a carbon right here, and I'm
- going to set this up so this is actually a chiral atom,
- that the carbon specific is a chiral atom, but it's partly a
- chiral molecule.
- And then we'll see examples that one or both
- of these are true.
- Let's say it's bonded to a methyl group.
- From that bond, it kind of pops out of the page.
- Let's say there's a bromine over here.
- Let's say behind it, there is a hydrogen, and then above it,
- we have a fluorine.
- Now if I were to take the mirror image of this thing
- right here, we have your carbon in the center-- I want
- to do it in that same blue.
- You have the carbon in the center and then you have the
- fluorine above the carbon.
- You have your bromine now going in this direction.
- You have this methyl group.
- It's still popping out of the page, but it's now going to
- the right instead of to the left, So CH3.
- And then you have the hydrogen still in the back.
- These are mirror images, if you view this as kind of the
- mirror and you can see on both sides of the mirror.
- Now, why is this chiral?
- Well, it's a little bit of a visualization challenge, but
- no matter how you try to rotate this thing right here,
- you will never make it exactly like this thing.
- You might try to rotate it around like that and try to
- get the methyl group over here, to get it over there.
- So let's try to do that.
- If we try to get the methyl group over there, what's going
- to happen to the other groups?
- Well, then the hydrogen group is going-- or the hydrogen, I
- should say.
- The hydrogen atom is going to move there and the bromine is
- going to move there.
- So this would be superimposable if this was a
- hydrogen and this was a bromine, but it's not.
- You can imagine, the hydrogen and bromine are switched.
- And you could flip it and do whatever else you want or try
- to rotate it in any direction, but you're not going to be
- able to superimpose them.
- So this molecule right here is a chiral molecule, and this
- carbon is a chiral center, so this carbon is a chiral
- carbon, sometimes called an asymmetric
- carbon or a chiral center.
- Sometimes you'll hear something called a
- A stereocenter is a more general term for any point in
- a molecule that is asymmetric relative to the different
- groups that it is joined to.
- But all of these, especially when you're in kind of in
- introductory organic chemistry class, tends to be a carbon
- bonded to four different groups.
- And I want to to stress that it's not four different atoms.
- You could have had a methyl group here and a propyl group
- here, and the carbon would still be bonded directly to a
- carbon in either case, but that would still be a chiral
- carbon, and this would still actually be a chiral molecule.
- In the next video, we'll do a bunch of examples.
- We'll look at molecules, try to identify the chiral
- carbons, and then try to figure out whether the
- molecule itself is--
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