Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kettchy on January 07, 2021, 03:50:56 PM
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Hello!
This last week I was making a benzaldehyde oxime from benzaldehyde and hydroxylamine (+K2CO3 and MeOH). After the reaction I performed an H-NMR, however the hydrogens from the aldehyde and the oxime were not there.
Are there any other products that could have formed instead of the benzaldehyde oxime? I've seen benzoic acid being mentioned, but it doesn't correspond with my H-NMR.
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How did you prepare your sample for NMR analysis? Did you see any evidence of -OCH3 in your spectrum? I don't think that this is likely, but I also can't think of many explanations.
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For the NMR, I only added some chloroform. Before that, I used a rotavap to remove MeOH and DCM (I used DCM for LLE). The H-NMR only showed DCM, MeOH and the hydrogen atoms on the benzene. There were no other peaks.
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Is it possible that the -OH is exchanging with residual water and so not being observed in the expected position? Have you taken a 13C NMR?
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From memory these are more different on TLC than you might suspect. Are you following a lit procedure (I think I used sodium acetate a lot of the time when I was doing this conversion)?
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This method of synthesizing benzaldoxime, if properly carried out, gives a practically quantitative conversion of the aldehyde to the oxime (reflux, TLC control).
So you must have made some fatal mistakes in the synthesis, isolation, purification, or interpretation of the NMR spectrum. Based on the information provided (or rather the lack of it), I can guess all possible errors.
If you want real help, you must provide all the details of your procedure and the NMR spectrum.
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@OP, Do you have a small sample of the known product that could be used in TLC or other tests?