I am trying to synthesize a Schiff base by condensation of an arylbenzaldehyde and a sulfonamide. I tried the conventional method (in ethanol/methanol, reflux...) but no precipitate was observed. I tried slow evaporation and used rotavap, the final product looked like some gluey brown paste. Recrystallization in ethanol, methanol and DCM didn't worked (same paste).
If the Schiff Base is soluble in ethanol/methanol, which solvent should I use ? (my starting amine is soluble in ethanol, acetone, methanol, DMSO, and alkali hydroxides).
Is there any suggestions concerning the method of synthesis, espcially for sulfonamides where the amine group is on the sulfur atom (see the picture attached) ?
Thank you !
Dear Mohamed,
many thanks for sharing this very interesting technical question with the RG community. First of all, please note that the reaction products between sulfonamides and aldehydes are not classified as Schiff bases. Instead, such compounds are commonly called N-sulfonyl imines. Due to the adjacent electron-withdrawing sulfonyl group, the –NH2 functional group in sulfonamides is not reactive enough to directly react with aldehydes. Thus the normal protocols for making Schiff bases (i.e. heating of the starting materials in methanol or ethanol until a precipitate forms) will normally not work in this case. You need to add a dehydrating agent (e.g. TiCl4, molecular sieves, MgSO4) or a catalyst to make the reaction work. For more information please search the "Publications" section of RG for the term "N-sulfonyl imines". This will provide you with a number of relevant articles about this reaction:
https://www.researchgate.net/search.Search.html?type=researcher&query=N-sulfonylimines
Good luck with your work and best wishes, Frank Edelmann
Prof. Edelman, first of all thank you for your answer, it really did clear things for me. I have another question about the amount of drying agent I should use. Any suggestions please ? Same thing for the catalyst. Thank you again !
Hello Mohamed,
thank you for your kind resonse and questions. If you check the literature reference suggested by Adel Amer you will find that the authors use P4O10 on silica as dehydrating agent. The reagent is easy to prepare, although the grinding of P4O10 and silica should prefereably done in a dry-box and the resulting reagent must be stored under the exclusion of moisture. Please also note that the authors of this article use the outdated formula P2O5. The compound P2O5 does not exist! It should always be written as P4O10. When you check the stoichiometric amounts you will see that the reagent P4O10 / SiO2 is also a catalyst as it is employed in sub-stoichiometric quantities. In any case this protocol is certainly worth a try.
P.S. Dear Mohamed, as I mentioned earlier, you should check the relevant publications about the synthesis of N-sulfonylimines which have been posted by other RG members. There you will find details about other dehydrating regents / catalysts. For example, please see the following interesting RG link:
Solvent-Free Synthesis of N-Sulfonylimines Using Microwave Irradiation
Article ChemInform Abstract: Solvent-Free Synthesis of N-Sulfonylimi...
Several other closely related papers are also freely accessible as public full texts.
Dear Prof. Edelmann, thank you so much for your genuine help. I red the article Prof. Adel Amer suggested, I wasn't that comfortable to try solvent-free methods but I am going to give it a try.
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