WEBVTT

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Today, we'll be talking about
how to separate enantiomers

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from each other.

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Enantiomers are like your
left and right hands.

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They are mirror
images of each other,

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but they look almost identical.

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Remember that much like we
use right and left to describe

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which hand is which, scientists
use the letters S and R

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to designate which
enantiomer is which,

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when you only have
one chiral center.

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However, when you have
multiple chiral centers,

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there are other ways of
designating enantiomers.

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But we won't be getting
into that today,

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because that's much
more complicated.

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Here we have a set
of enantiomers.

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This is the S confirmation
of thalidomide,

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and here on the right
is the R confirmation.

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Why does it matter that we have
two different confirmations?

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Well, you can see the
difference quite clearly

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at the chiral center,
where one of the groups

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points into the screen and the
other points out of the screen.

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And just because of the
simple change in confirmation,

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that S version was found to
lead to terrible birth defects

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when consumed by mothers.

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And because of
this, drug companies

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now try to make sure that the
active ingredient in their drug

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is only one
particular enantiomer.

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So how would we go about
separating these two?

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One technique that you
could use is chiral column

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chromatography.

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You would need a stationary
phase that is chiral,

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meaning something that
will only bind either

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to the R confirmation or the
S confirmation of your desired

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enantiomer.

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So how does the chiral
stationary phase only

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bind to one of the enantiomers?

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Picture the two enantiomers
as your right and left hand.

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If your right hand tries
to shake another person's

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right hand it seems normal,
the two fit together properly.

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But if your right hand tries
to shake your own left hand,

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it doesn't seem like
they line up quite right.

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That's the exact same
thing that happens

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with the chiral stationary
phase and the wrong enantiomer.

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Next, what you do is you'd load
that mixture of enantiomers.

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So on top here, you
might see that you

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have some kind of
band of your mixture.

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This is racemic,
meaning that it has

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a 50/50 mixture of enantiomers.

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So that's what you're
seeing here in the yellow.

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If we take a closer
look, you'll see

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that this has some
of the S confirmation

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and some of the R
confirmation too thrown in.

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And as this moves through
the stationary phase,

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so once you open up the
stop cock, what you'll see

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is that if the R
enantiomer was the one that

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binds tightly to the
stationary phase,

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it won't move very quickly.

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But with the S enantiomer,
it might be racing through

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since it's not
really interacting

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that much with the
stationary phase,

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and prefers to interact
with the mobile phase.

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Once you've collected all of
the S enantiomers in your flask,

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all you'll have
left in the column

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is the R enantiomer, which
is pretty tightly bound

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to the chiral stationary phase.

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Next, what you'd do is
when you have this column,

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you'd want to pour
in lots of solvent

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so that you can get the
R enantiomer to come out.

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Because as this pushes
down through the column,

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it will take the R
enantiomer with it,

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giving you just the R
enantiomer in your flask.

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And there you've done a
successful chiral resolution.

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The same principle can also be
applied to gas chromatography.

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Let's quickly review how
gas chromatography works.

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You insert your sample in
here, a gas flows through,

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and then it goes into
this long to that

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contains the stationary
phase and mobile phase,

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and goes to the detector.

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And if we were to
zoom in on this--

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and draw this just
kind of a long tube--

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again what you'd see is that if
this time the stationary phase

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was attracted to the
S enantiomer instead,

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you'd see that the S enantiomer
is sticking to the sides,

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sticking to the
stationary phase,

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while the R enantiomer races
through with the mobile phase.

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So there are actually
a number of other ways

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you can separate
enantiomers, but those

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tend to be much
more complicated.

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These are just two of the
common ways you can do it.

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And in both of
them, whether you're

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doing column chromatography with
a solid stationary phase or gas

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chromatography was a
liquid stationary phase,

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the important thing to remember
is that your stationary phase

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should be chiral and bind to
the enantiomer that you want.