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Old May 24th 2017, 10:41 AM   #1
SR3
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Urgent! Unexpected NMR signal

I synthesized aspirin in the lab and ran an H-NMR test. My solvent was CDCl3.
In the spectrum a doublet signal appears at 4.77,4.82 ppm. I have tried thinking about any byproducts that might have caused this signal and searched the web with countless search terms trying to find a reasonable answer to this but nothing of use came up.
At one point I thought this might be due to residue water however the signal for water in my solvent should appear around 1.6 ppm and is a singlet.
I have found a resource claiming that at 4.5-6.5 ppm we are dealing with c=c double bonds, however, none of the byproducts I know of include any such bonds (except of course on the ph group which appears at a much higher ppm).
Thanks so much to any helpers!
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Old May 27th 2017, 06:07 AM   #2
anoymous
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can you post a picture of the nmr and some data about the experiment?
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Old May 27th 2017, 06:52 AM   #3
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Originally Posted by anoymous View Post
can you post a picture of the nmr and some data about the experiment?
The NMR spectrum is attached.
The experiment was a standard synthesis of aspirin from salicylic acid and acetic anhydride.
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File Type: jpg IMG_20170527_174543.jpg (92.1 KB, 3 views)
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Old May 27th 2017, 10:50 AM   #4
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that's a weird one indeed
you cleaned every bit of glasswork?
what did you use as purification?
what did you use as solvent (during the reaction, not for the nmr)?

Last edited by anoymous; May 27th 2017 at 10:53 AM.
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Old May 28th 2017, 07:07 AM   #5
SR3
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Originally Posted by anoymous View Post
that's a weird one indeed
you cleaned every bit of glasswork?
what did you use as purification?
what did you use as solvent (during the reaction, not for the nmr)?
We did our best cleaning the glass work. The solvent was the acetic anhydride and for purification we redissolved the product in heated ethanol and water and set the solution aside for re-cristalization.
We are thinking this strange signal is actually the proton on the carboxylic acid which moved downscale because of the specific environment.
The NMR solvent was CDCl3.
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Old May 28th 2017, 08:03 AM   #6
anoymous
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Originally Posted by SR3 View Post
We did our best cleaning the glass work. The solvent was the acetic anhydride and for purification we redissolved the product in heated ethanol and water and set the solution aside for re-cristalization.
We are thinking this strange signal is actually the proton on the carboxylic acid which moved downscale because of the specific environment.
The NMR solvent was CDCl3.
carboxylic acid cant couple
and for CHCl3 (there are always some remaints) is the signal too low i think
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Old May 29th 2017, 05:49 AM   #7
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Originally Posted by anoymous View Post
carboxylic acid cant couple
and for CHCl3 (there are always some remaints) is the signal too low i think
I realize that carboxylic acid doesn't couple however I figured that it did due to some interferance or resonance. The signal for CHCl3 is higher up in ppm.
Anyway I have reached my deadline and so turned in the report.
Thanks for all the help!
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