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Chiral molecules tend to absorb one circular polarization of light more than the other. Is this ever used to isolate a particular enantiomer?

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Yes. For some chiral compounds, each enantiomer in a racemic mixture absorbs certain frequencies of light differently depending on the circular polarization type of the light.

That means that if you can pump in sufficiently intense and energetic (e.g. ultraviolet) circular polarized light pulses into the racemic mix of such compounds, you may in some cases increase the rate of breakdown of the enantiomer that absorbs that particular circular polarization (left or right), resulting in an increase in the relative concentration of the other enantiomer.

You can view the first page of a paper on asymmetric photolysis using circularly polarized ultraviolet light here.

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  • $\begingroup$ I stand corrected...interesting paper. $\endgroup$ – Janice DelMar May 19 '12 at 5:03
  • $\begingroup$ @JaniceDelMar, not corrected, please! I liked your answer, but in digging around I found to my own surprise that there's been some interesting work on this topic. The connection of some of this research to people pondering natural amino acid chirality is particularly fascinating, but to me doesn't answer much. After all, intensely chiral sources of circular light are to the best of my knowledge extremely rare in nature, so demolishing amino acid enantiomers via such light only defers the mystery a bit. $\endgroup$ – Terry Bollinger May 20 '12 at 3:33
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    $\begingroup$ As I said, interesting paper...but I must admit, having worked in industry, I was thinking more along practical lines and this doesn't seem a particularly practical separation technique. I wonder what scale they've done this on? (Which I could probably find out with further reading.) This amino acid chirality connection seems a "reach" to me. $\endgroup$ – Janice DelMar May 20 '12 at 6:15
  • $\begingroup$ I think your comments nailed it: The paper I quoted is the most successful example I could find of people trying to develop some sort of general model for using chiral light (cheap!) to isolate chiral enantiomers (costly!). I don't think anyone has succeeded in finding a method that provides that level of generality, given that this "best I could find" example depends on too many happy coincidences and so does not generalize well. $\endgroup$ – Terry Bollinger May 21 '12 at 20:36
  • $\begingroup$ The paper seemed to indicate that part of the difficulty in performing this experiment was getting a good source of circularly polarized ultraviolet light of sufficient intensity. Laser physics has come a long way since 1977; this might be easier now than it was. $\endgroup$ – Dan May 30 '12 at 19:10
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The optical rotation is certainly used to follow the separation of optical isomers (enantiomers if they are mirror images, or diastereomers if you have more than one stero-center and the two isomers are not mirror images). You can see how the optical rotation changes as you use the normal separation techniques (chromatography, distillation, crystallization, etc.). If you know the true value for the substance, you can determine the purity. But I don't know how polarized light would be used to do the actual separation.

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