Dear Kadja Chagas , I found only a few references in the literature describing Schiff bases obtained from 5-aminoindole and aromatic aldehydes (see attached). Unfortunately, they are all published in journals where I don't have access to the full texts. I suggest that you try to recrystallize your compounds e.g. from n-pentane or toluene. In case that they are highly soluble in these non-polar hydrocarbons, acetonitrile is often a very good alternative as solvent for recrystallization.

1st you have to know if the schiff base or the aldehyde are soluble in any organic or aqueous solvent, so you can try by simple washing. Or try to push the reaction to completion by removing water as side product using molecular sieves.

I have a couple of ideas you can try:

1. You mention that chromatography doesn't work. Try using a different stationary phase (for example, basic alumina).

2. As mentioned by Frank T. Edelmann recrystallisation is always a good approach to purification. It might require a lot of try and error, but if you succeed you will never want to do another column ever again.

3. If your Schiff base is not soluble in water, you can try a rather obscure and old organic chemistry technique. Sodium metabisulfite (Na2S2O5) it's known to form aldehyde-bisulfite addition compounds, rendering them water-soluble. Just dissolve the crude product in an organic solvent and wash it with a solution of Na2S2O5, collect the organic phase, dry it, and remove the solvent to afford the product clean of aldehyde (if you have other contaminants you'll need to remove them separately). Also, if you adjust the pH of the aqueous solution to 10 you can recover the aldehyde, by extraction with an organic solvent.

Dear Kadja Chagas, the suggestion by Alvaro Etcheverry-Berrios to use sodium metabisulfite (sodium disulfite) to separate the aldehyde from your product is an excellent one! You should really give this a try!

cf. also on RG:

https://www.researchgate.net/post/How_do_I_break_benzaldehyde-sodium_bisulphite_complex

Dear Kadja Chagas

I fully agree with the answer provided by Frank T. Edelmann , but I recommend to use ethyl acetate as a recrystallized solvent.

All the best

https://books.google.iq/books?id=OlPRBQAAQBAJ&pg=PA21&lpg=PA21&dq=remove+aldehyde+from

Frank T. Edelmann I also found it difficult to find a good methodology for this synthesis. I appreciate your research. In general, recrystallization is not totally effective for removing aldehyde, but it usually removes a good part.

Alvaro Etcheverry-Berrios I even tried to use sodium bisulfite, but for my aldehydes there was no removal ... changing the stationary phase is interesting, but I have no option but silica.

Yasser Fakri Mustafa thanks for the reply. I recently tried recrystallization from ethyl acetate and ether, but traces of the aldehyde used still remain.

Kadja Chagas If recrystallisation is working but you still find traces of the aldehyde, you can keep recrystallising. After two-three recrystallisations, you should have a pure product. Also, how are you measuring purity?

Alvaro Etcheverry-Berrios The traces of aldehyde that I still observe are seen through TLC, still significantly. New recrystallizations make me lose a lot of product.

Dear Kadja Chagas, did you try purification by column chromatography? When you say that you observe "traces of aldehyde", I wonder how important it is to remove these traces? Perhaps you could add another small amount of amine to make the remaining aldehyde react?

I follow

Kadja Chagas, TLC is not good for measuring purity. Sometimes small traces can appear as large spots in the TLC, but when you run an NMR you barely see the signals of the impurity (something similar happened to me with a synthesis involving aldehydes). It's also important to think why do you need a certain purity of your product; for example, if you want to use your product as a precursor in another organic synthesis you could just use it as it is, and in the next step the purification can become simpler. If you need it for biological assays, perhaps you will need a compromise between yield and purity. If you want to test for catalysis, for sure you'll need 99.99% of purity.

From your first post, you mention some difficulties with chromatography. What difficulties you are referring to? Does your product stay in the column without eluting?

Dear, Frank T. Edelmann I tried to remove by column chromatography, but the aldehyde always persists ... I tried to add more equivalents, but only what is observed is the appearance of by-products.

Dear, Alvaro Etcheverry-Berrios I am beginning to assess how important it is that the disappearance of the aldehyde is complete ... because the NMR spectra have not observed its presence. I will take these compounds for bio-analytical tests. The aldehyde ends up being eluted with the product of interest.