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Introduction to Chirality

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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
stereocenter.
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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Introduction to Chirality

Introduction to Chirality