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

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from each other.

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Enantiomers are like your[br]left and right hands.

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They are mirror[br]images of each other,

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but they look almost identical.

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Remember that much like we[br]use right and left to describe

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which hand is which, scientists[br]use the letters S and R

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to designate which[br]enantiomer is which,

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when you only have[br]one chiral center.

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However, when you have[br]multiple chiral centers,

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there are other ways of[br]designating enantiomers.

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But we won't be getting[br]into that today,

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because that's much[br]more complicated.

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Here we have a set[br]of enantiomers.

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This is the S confirmation[br]of thalidomide,

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and here on the right[br]is the R confirmation.

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Why does it matter that we have[br]two different confirmations?

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Well, you can see the[br]difference quite clearly

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at the chiral center,[br]where one of the groups

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points into the screen and the[br]other points out of the screen.

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And just because of the[br]simple change in confirmation,

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that S version was found to[br]lead to terrible birth defects

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when consumed by mothers.

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And because of[br]this, drug companies

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

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is only one[br]particular enantiomer.

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So how would we go about[br]separating these two?

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One technique that you[br]could use is chiral column

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chromatography.

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You would need a stationary[br]phase that is chiral,

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meaning something that[br]will only bind either

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to the R confirmation or the[br]S confirmation of your desired

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enantiomer.

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So how does the chiral[br]stationary phase only

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bind to one of the enantiomers?

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Picture the two enantiomers[br]as your right and left hand.

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If your right hand tries[br]to shake another person's

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

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But if your right hand tries[br]to shake your own left hand,

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

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That's the exact same[br]thing that happens

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with the chiral stationary[br]phase and the wrong enantiomer.

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

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So on top here, you[br]might see that you

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have some kind of[br]band of your mixture.

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This is racemic,[br]meaning that it has

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a 50/50 mixture of enantiomers.

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

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If we take a closer[br]look, you'll see

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that this has some[br]of the S confirmation

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and some of the R[br]confirmation too thrown in.

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And as this moves through[br]the stationary phase,

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

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is that if the R[br]enantiomer was the one that

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binds tightly to the[br]stationary phase,

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it won't move very quickly.

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But with the S enantiomer,[br]it might be racing through

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since it's not[br]really interacting

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that much with the[br]stationary phase,

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and prefers to interact[br]with the mobile phase.

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

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all you'll have[br]left in the column

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is the R enantiomer, which[br]is pretty tightly bound

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to the chiral stationary phase.

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

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you'd want to pour[br]in lots of solvent

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

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Because as this pushes[br]down through the column,

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it will take the R[br]enantiomer with it,

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giving you just the R[br]enantiomer in your flask.

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And there you've done a[br]successful chiral resolution.

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The same principle can also be[br]applied to gas chromatography.

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Let's quickly review how[br]gas chromatography works.

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You insert your sample in[br]here, a gas flows through,

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and then it goes into[br]this long to that

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contains the stationary[br]phase and mobile phase,

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and goes to the detector.

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And if we were to[br]zoom in on this--

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and draw this just[br]kind of a long tube--

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

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was attracted to the[br]S enantiomer instead,

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

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sticking to the[br]stationary phase,

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while the R enantiomer races[br]through with the mobile phase.

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So there are actually[br]a number of other ways

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you can separate[br]enantiomers, but those

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tend to be much[br]more complicated.

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

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And in both of[br]them, whether you're

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doing column chromatography with[br]a solid stationary phase or gas

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chromatography was a[br]liquid stationary phase,

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the important thing to remember[br]is that your stationary phase

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