WEBVTT

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If I were to draw a hand, and
let me just draw a hand really

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fast, so I'll draw
a left hand.

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It looks something like that.

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That is a left hand.

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Now, if I were to take its
mirror image, let's say that

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this is a mirror right there,
and I want to take its mirror

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image, and I'll draw the
mirror image in green.

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So its mirror image would look
something like this.

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Not exact, but you
get the idea.

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The mirror image of a left
hand looks a lot

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like a right hand.

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Now, no matter how I try to
shift or rotate this hand like

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this, I might try to maybe
rotate it 180 degrees, so that

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the thumb is on the other side
like this image right here.

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But no matter what I do, I will
never be able to make

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this thing look like
that thing.

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I can shift it and rotate it,
it'll just never happen.

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I will never be able to
superimpose the blue hand on

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top of this green hand.

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When I say superimpose,
literally put it exactly on

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top of the green hand.

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So whenever something is not
superimposable on its mirror

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image-- let me write this down--
we call it chiral.

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So this hand drawing
right here is an

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example of a chiral object.

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Or I guess the hand is an
example of a chiral object.

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This is not superimposable
on its mirror image.

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And it makes sense that it's
called chiral because the word

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chiral comes from the
Greek word for hand.

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And this definition of
not being able to be

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superimposable on its mirror
image, this applies whether

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you're dealing with chemistry,
or mathematics, or I guess,

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just hands in general.

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So if we extend this definition
to chemistry,

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because that's what we're
talking about, there's two

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concepts here.

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There are chiral molecules,
and then there are chiral

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centers or chiral-- well, I call
them chiral atoms. They

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tend to be carbon atoms, so
sometimes they call them

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chiral carbons.

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So you have these
chiral atoms.

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Now, chiral molecules are
literally molecules that are

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not superimposable on
their mirror image.

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I'm not going to write
the whole thing.

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You know, not superimposable--
I'll just

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write the whole thing.

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Not superimposable
on mirror image.

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Now, for chiral atoms, this is
essentially true, but when you

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look for chiral atoms within a
molecule, the best way to spot

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them is to recognize that these
generally, or maybe I

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should say usually, are carbons,
especially when we're

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dealing in organic chemistry,
but they could be phosphoruses

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or sulfurs, but usually are
carbons bonded to four

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different groups.

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And I want to emphasize
groups, not just four

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different atoms. And to kind of
highlight a molecule that

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contains a chiral atom or chiral
carbon, we can just

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think of one.

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So let's say that I have a
carbon right here, and I'm

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going to set this up so this
is actually a chiral atom,

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that the carbon specific is a
chiral atom, but it's partly a

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chiral molecule.

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And then we'll see examples
that one or both

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of these are true.

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Let's say it's bonded
to a methyl group.

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From that bond, it kind of
pops out of the page.

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Let's say there's a
bromine over here.

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Let's say behind it, there is a
hydrogen, and then above it,

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we have a fluorine.

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Now if I were to take the mirror
image of this thing

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right here, we have your carbon
in the center-- I want

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to do it in that same blue.

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You have the carbon in the
center and then you have the

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fluorine above the carbon.

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You have your bromine now
going in this direction.

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You have this methyl group.

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It's still popping out of the
page, but it's now going to

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the right instead of to
the left, So CH3.

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And then you have the hydrogen
still in the back.

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These are mirror images, if you
view this as kind of the

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mirror and you can see on both
sides of the mirror.

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Now, why is this chiral?

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Well, it's a little bit of a
visualization challenge, but

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no matter how you try to rotate
this thing right here,

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you will never make it exactly
like this thing.

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You might try to rotate it
around like that and try to

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get the methyl group over here,
to get it over there.

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So let's try to do that.

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If we try to get the methyl
group over there, what's going

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to happen to the other groups?

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Well, then the hydrogen group is
going-- or the hydrogen, I

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should say.

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The hydrogen atom is going to
move there and the bromine is

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going to move there.

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So this would be superimposable
if this was a

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hydrogen and this was a
bromine, but it's not.

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You can imagine, the hydrogen
and bromine are switched.

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And you could flip it and do
whatever else you want or try

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to rotate it in any direction,
but you're not going to be

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able to superimpose them.

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So this molecule right here is
a chiral molecule, and this

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carbon is a chiral center, so
this carbon is a chiral

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carbon, sometimes called
an asymmetric

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carbon or a chiral center.

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Sometimes you'll hear
something called a

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stereocenter.

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A stereocenter is a more general
term for any point in

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a molecule that is asymmetric
relative to the different

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groups that it is joined to.

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But all of these, especially
when you're in kind of in

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introductory organic chemistry
class, tends to be a carbon

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bonded to four different
groups.

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And I want to to stress that
it's not four different atoms.

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You could have had a methyl
group here and a propyl group

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here, and the carbon would still
be bonded directly to a

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carbon in either case, but that
would still be a chiral

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carbon, and this would still
actually be a chiral molecule.

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In the next video, we'll
do a bunch of examples.

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We'll look at molecules, try
to identify the chiral

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carbons, and then try to
figure out whether the

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molecule itself is--

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