Here's the brief:
1. I synthesized alkyl gallates (ethyl gallate, butyl gallate, and amyl gallate) using Fischer esterification.
2. I used TsOH instead of concentrated H2SO4 as the catalyst.
3. After refluxing for about 7 hours (with no gallic acid spot observed in TLC), I evaporated the excess alcohol and solvent (toluene).
4. After that, the crude product was diluted in ethyl acetate and washed with water.
5. The aqueous phase was extracted 3x with ethyl acetate.
6. The organic phases were combined and washed with a 5% NaHCO3 solution.
My question:
When I washed the "combined" organic phase with a 5% NaHCO3 solution, the aqueous phase turned dark green. Why did this happen? Is it due to the TsO- salt, or is there another explanation?
Important Note:
I already repeat this reaction/process about 3-4 times and I always end up with that same dark green color.
The gallate anion is green. Possibly, you achieve hydrolysis of your esters and/or you have unreacted gallic acid left which gets transferred into the aqueous phase. What is the pH of your NaHCO3 solution? Also note that even strong colorings can result from only traces of a compound. So, maybe there is no reason to worry.
I see. The pH of my 5% NaHCO3 solution is 9 (I just checked it).
I just learned from you, Sir, that the gallate anion is green. Are there any literature references mentioning the green color of the gallate anion?
No. It's just my experience. Dissolve some gallic acid in your NaHCO3 solution and see how it looks like. (Certainly faster than searching for a literature reference. ;) )
I found a very old literature reference in a german handbook, now online, that describes formation of dark green or even brown products from condensation products of alkali gallates in aquous solutions and if there is oxygen and light. Probably it is not the gallate, that is green, but a reaction product. Also traces of metals can give coloured compounds with gallates.
Nevertheless, if your procedure includes the washing step, it is for removing excess gallic acid or unwanted gallic acid reaction products. So, the more important question is, are you satisfied with the products in the organic phase, are they pure ?, is there sufficient quantity ? and so on. Do not worry about the pretty colours in the washing step.
Ricarda Koopmann I'm quite satisfied with the purity and the quantity of my product in the organic phase. I believe it's fairly pure based on the IR spectra. Thank you very much for your response and advice
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