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Resolution of enantiomers | Chemical processes | MCAT | Khan Academy

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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.
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Title:
Resolution of enantiomers | Chemical processes | MCAT | Khan Academy
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Video Language:
English
Team:
Khan Academy
Duration:
05:02

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