February 28, 2020, 05:33:29 PM
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Topic: How to use NMR to calculate Enanatiomeric Excess?  (Read 306 times)

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Offline Amalthea-x

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How to use NMR to calculate Enanatiomeric Excess?
« on: November 12, 2019, 02:28:17 PM »
I have NMR spectra for an enantiomerically pure mixture, a racemic mixture and my derivatised compound. I'm supposed to calculate the enantiomeric excess by comparing the integrals from the racemic mixture and my compound.

From literature research, I understand that I need to get a ratio for the enantiomers from these integrals. This is the bit I am stuck on. I've tried to google it every which way and in literature they just tell you what the ratio is, not HOW they got it. This is something we have no lecture notes on and I have had no luck with any of my textbooks.

If someone could explain HOW I use the integrals to find the ratio (or point me in the direction of a useful resource) that would be a massive help. I understand what to do after that point (I think).

I have managed to get an answer for the enantiomeric excess, but I want to be sure I am using the correct method as this is my last year and I want to get the best grade I can.

If you have any questions, or need more info, please let me know!

Offline clarkstill

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Re: How to use NMR to calculate Enanatiomeric Excess?
« Reply #1 on: November 13, 2019, 04:58:13 AM »
What do you mean by "derivatised compound"?

It is not possible to determine ee by NMR (since enantiomers are magnetically equivalent) unless (i) you use a chiral solvent (unlikely); (ii) you add a chiral shift reagent; (iii) you do some reaction that turns your mixture of enantiomers into a mixture of diastereomers (e.g. by forming Mosher's esters).

Certainly the NMR spectra of your "enantiomerically pure mixture" and "racemic mixture" will be of no use at all (unless you are using a chiral shift reagent) - if the compounds are pure then these spectra should look exactly the same.

Can you explain the exact procedure you have conducted?

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