Sharp melting points and disappearance of the carbonyl absorbtion band in the IR spectrum along with the appearance of the C=N band clearly indicate the formation of the desired product, however strong signal in HNMR spectrum at 10.4 indicate the presence of aldehyde in the sample!
Is it possible that the imine suffers hydrolysis in the deuterated DMSO?
What is the substituent on the nitrogen?
DMSO does usually contain water so it is possible it is hydrolyzing.
Why are you using DMSO? Why not chloroform or acetone? Those solvents don't usually contain water as they are not hygroscopic like DMSO.
Nevermind I think I answered my own question about the nitrogen substituent. Try a different NMR solvent.
You are welcome! Yes thats why I usually use DMSO too. Perhaps acetone?
What aldehyde(s) are you using? You wouldn't expect to get the same resonance for all of the aldehydes. If you see a different peak for your different aldehydes than hydrolysis is probably your culprit.
Thanks Joseph, I am using substituted salicylaldehyde, the H NMR clearly indicates the presence of the starting material!
A couple things came cross my mind. First option: the product still contains the sp2 proton of the starting material, so are you sure that the singlet al 10.4 ppm is the aldehyde proton and not the imine one? Second: any chance that because of the DMSO and the hydrogen bonding network with the imine nitrogen and the pyridine, you have a shift of the phenol OH above 10 ppm?
Dear,
I synthesized many imines and always performed 1 H and 13 C NMR analysis on deuterated DMSO and the imine signal never disappeared. I believe that your Schiff base is hydrolyzing because of the water contained in the DMSO or water of the reaction
Imine bonds are easily hydrolyzed by water to form the corresponding aldehyde. You have to use dry solvents.
If it is not soluble in any other solvent than DMSO, solid state NMR can be used.
Imine hydrolysis is quite facile. If structure verification is necessary, then perhaps GC-MS or LC-MS might be a better route since you can more readily choose a solvent that will dissolve your compound. Alternatively, you could try to grow a crystal of it and determine the structure by X-ray diffraction.
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