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Exchangeable Protons Absent in CDCl3 and DMSO-d6

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CinnabarCherry:
Hello,

I've synthesized a few compounds with a hydroxamic acid moeity (C(=O)NHOH). The NHOH peaks should appear around 10.6 and 8.9 ppm.

The thing is, more than often I do not see these peaks in solvents considered non-exchangeable. To my knowledge I never see the peaks in CDCl3, I typically do in DMSO-d6 except on one occasion.

Does anyone have a good reason why I don't see these peaks in CDCl3 or DMSO-d6?

Babcock_Hall:
My recollection concerning hydroxyl hydrogens is that I usually do not see them.  There is a small amount of water in CDCl3, which might be exchanging fast enough to produce a single peak.  The exchange rate might be a function of acidic catalysts.

CinnabarCherry:
Thank you for your reply Babcock_Hall.

I'm just trying to wrap my head around why exchanging protons for protons weakens detection. I understand if the exchange is with deuterium but not protons.

Babcock_Hall:
Suppose that a hydrogen nucleus has two environments, such as being on an alcohol and being in water.  Now suppose that it the exchange reaction that puts this nucleus into one or the other environment is very fast.  Then the hydrogen experiences an average environment, and it will produce a single peak with a chemical shift at the weighted average of the chemical shifts of the two environments.  It has been a while since I have thought about chemical exchange, and you might want to look at a good intermediate-level textbook for a more detailed explanation.

rolnor:
Is it not a a little DCl in CDCl3? That would give a D-H exchange. Its normaly stabilized with Ag-metal but it can still be a little acidic. CHCl3 is normaly stabilized with ethanol.

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