Introduction to Chirality

Introduction to Chirality

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--