Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Rutherford on May 28, 2013, 10:00:32 AM
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There are 5 pairs of different enantiomers in a mixture. If in chromatography an achiral sorbent is used, 5 different fractions will be observed, but if a chiral sorbent is used, 10 different fractions will be spotted.
Why can't an achiral sorbent separate enantiomers but a chiral can? How does a chiral sorbent do it?
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This is effectively the formation of diastereoisomers which have different retention properties and can be separated.
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But why can a chiral sorbent separate the enantiomers?
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Because you form a series of diastereoisomers as the enantiomer moves down the column. Diastereoisomers are separable. In this case one of them is the column material, so what comes out the bottom is the pure enantiomer.
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I understand that diastereoisomers are separable, but how the 2*5 enantiomers that make the 5 enantiomer pairs get separated to form 10 (not 5) fractions when a chiral sorbent is used?
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I understand that diastereoisomers are separable, but how the 2*5 enantiomers that make the 5 enantiomer pairs get separated to form 10 (not 5) fractions when a chiral sorbent is used?
Consider that your pairs are S1, R1; S2, R2; S3, R3; S4, R4; S5, R5. With an achiral sorbent both Sn and Rn interact the same given the achiral mobile phase too, but with a chiral sorbent, say Sx, you get S1:Sx, R1:Sx; S2:Sx, R2:Sx,... R5:Sx, which act as diastereoisomers, and hence differently wrt to the achiral mobile phase
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How can one chiral sorbent form a diastereoisomer pair with each if these are different compounds?
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How can one chiral sorbent form a diastereoisomer pair with each if these are different compounds?
Consider two products PR and PS interacting with an enantiomerically pure chiral stationary phase AR.
You get two complexes: PR:AR and PS:AR these are diastereomeric complexes.
The energy associated with the interaction PR:AR is not the same as PS:AR, hence PR and PS have different retention times on stationary phase AR
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I begun to understand this. Could you give me an example of the complex that can be formed in a specific case?
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I begun to understand this. Could you give me an example of the complex that can be formed in a specific case?
Consider for instance Organic Syntheses, Coll. Vol. 5, p.932 (1973); Vol. 49, p.93 (1969); with the modification of polymer-bound tartaric acid or similar.
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Okay, this will require some time to analyze.
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It's just like a left hand fits more easily into a left glove than a right hand fits into a left glove.
Imagine the stationary phase is made of left gloves, and you pass through a mixture or left and right hands.
The interaction of the left hands with the left gloves is stronger than the interaction of the right hands with the left gloves.
So, the right hands move through the left gloves more quickly because the interaction is weaker. The left and right hands are separated.
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Okay, thanks.