stephill92
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Need help filtering PABA
I just tried doing a reduction of p-nitrobenzoic acid to p-aminobenzoic acid. I followed this paper http://web.mst.edu/~tbone/Subjects/TBone/chem228/FS16Synthpr... up to where it says to use ammonia since I don't have any on hand right now.
Instead I added NaOH and filtered the reaction mixture and was left with a murky solution so I added HCl and a precipitate came out of solution and
then redissolved. I thought that if I had made PABA I may have formed the Hydrochloride salt so I added more NaOH then sulfuric acid and a precipitate
formed again, but it too dissolved back into solution. I tried cooling it since PABA is soluble in hot water, but nothing happened. Would it still be
possible to salvage anything assuming I did the reaction right in the first place?
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UC235
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They used ammonia because it buffers the solution. If you add too much NaOH, you make PABA sodium salt which is soluble. If you add too much HCl, you
protonate the aniline and form soluble PABA Hydrochloride. Not to mention, the initial pH of 8 is to selectively precipitate SnO2 which is amphoteric
and will also dissolve in both acid and base. I suppose at pH 8, it is probably minimally soluble while PABA is soluble as the ammonium salt.
They then use acetic acid to bring the pH down to 3-4 which is presumably PABA's isoelectric point, where it is neither cationic or anionic but
predominantly a overall charge neutral zwitterion (and at which point it is its least soluble).
It's possible that your solution is massively diluted and loaded with salt at this point and you won't get a precipitate even if you did things right
now since PABA is still somewhat soluble even at ideal conditions. You need to actually monitor the pH and adjust carefully with weak acid and base if
there's any hope for product now.
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stephill92
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Posts: 23
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I tried it again today with pH paper and ammonia and am waiting for the final solution to cool, but it's not cloudy so I don't expect it to work. I
don't know what I did wrong though since I followed the paper but it seemed different from what actually happened. I ran the reaction for over an hour
and a half and still had 2.5g of tin still left in the bottom of the flask, and after I raised the pH with ammonia it was still cloudy after filtering
with coffee filters multiple times when it said it should be clear. I decided to use sodium bisulfate to lower the pH to 3~4 but it made the solution
clear so I don't really know what I could do differently next time. What seemed strange to me about it though is that the reaction is supposed to be
able to be sustained on its own but I needed to keep it in a hot water bath pretty much the entire time. Any advice would be appreciated.
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