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Author Topic: optical rotation  (Read 1502 times)

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kriggy

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optical rotation
« on: February 11, 2017, 06:50:24 AM »

Im trying to separate some enantiomers of axialy chiral acid and for the ee measurments I used chiral HPLC. Since the HPLC takes a while (like few days till I get the results due to moving the sample to different building etc) I decided to try to measure optical rotation. The idea is that if I see other number than 0 then there is some ee and I can move it for chiral HPLC (I dont know the optical rotation of my enantiomers becauese I dont have them in pure form).
I did the measurment and got values of specific rotation about +1° suggesting that I have the reacemate, however, as I obtained my acid as a salt with chiral amine, I would expect that the amine would interfere with the measurment? Therefore the +1° value seems quite odd.
I did two crystalizations, one with R amine and the other with S amine. If my compound is racemic, then I should be able to see the rotation of just the amine right? If it is not racemic, I would expect that different atropoisomers crystalize (more like precipitate actualy) therefore one sample having positive sign an one being negative but they are both at about +1.
Any thoughts? Should I liberate my free acid before measurement or can I calculate the rotation of my compound if I know the specific rotation of the amine? Or more sample?  I had 10 mg/ml which should be enough but maybe Im wrong.

Thank you

sjb

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Re: optical rotation
« Reply #1 on: February 11, 2017, 07:18:29 AM »

From what I understand, you have created diastereomeric salts - e.g. R,R and R,S from pure R amine and some mixture of R and S acid -  correct?

I don't think you can guarantee that R,R and R,S salts in this instance will have opposing signs for α.
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kriggy

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Re: optical rotation
« Reply #2 on: February 11, 2017, 08:37:41 PM »

I have a sample that is acid+R amine and acid+S amine. I dont know if the sample is just racemic acid + amine or there is one of the enantiomers of acid in excess. The acid should be as s salt with the amine (i did not acidify after isolation).
Im sorry if OP was not clear
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