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Author: Subject: Purifying anthranilic acid
TYOP213
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[*] posted on 6-2-2017 at 13:42
Purifying anthranilic acid


Im having trouble purifying my crude anthranilic acid. I have tried recrystallizing it in hot water multiple times but it always form dark brown colored crystals. Some loss of anthranilic acid with each re crystallization. I don't know any solvent that would work or any other way to remove the impurities. Does anyone have any experience with this?

Prepared anthranilic acid from phthalimide and old bleach
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Chlorine
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[*] posted on 6-2-2017 at 13:55


Your impurity is also water soluable, try recyrstalizing from ethanol.
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TYOP213
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[*] posted on 6-2-2017 at 15:49


Ill give it a shot in a few days
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TYOP213
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[*] posted on 6-2-2017 at 16:17


What is the likely cause of the brown-yellow impurity? Could it have been the decomposition products of sodium hypochlorite more specifically chlorate ions reacting with the phthalimide? The reaction mixture turned dark red when i added the bleach.
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[*] posted on 6-2-2017 at 16:53


Basic solutions of anthranilate tend to turn black because of oxidation.



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Ex42k9j
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[*] posted on 8-2-2017 at 00:12


Assuming your recrystallization and filtration techniques are sound, I second the use of ethanol. Chloroform-ether also works well here but depends on its availability to you. From my experience on this, a water recrystallization did very little more than make the powder form some pretty, still brown colored, needle like crystals. It is a pain to get this any cleaner than a tan color, let alone white.

Glad the synthesis worked out for you in the end though. Every time I end up with a red solution (upon hypochlorite addition as well), it ends up going black and the yields suffer greatly.

Me thinks the issue lies with the hypochlorite not being chilled enough or the addition being to rapid. Sometimes I see the red color form immediately upon addition. To me it seems that if everything is kept at 5C or less the addition goes smoothly. The temperature doesn't raise above 11C and the complete addition results in an off yellow colored solution.
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chemplayer...
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[*] posted on 8-2-2017 at 06:13


It is actually quite hard to purify anthranilic acid to the point that it's a nice pale pure powder. At the temperature at which it's highly soluble in water (i.e. during recrystallisation) it tends to form red-brown impurities, possibly due to self-condensation or oxidation.

Another option is to react to form a salt (using acid or alkali) and then very carefully adjust the pH back to create the neutral species, but this is also really hard and when we tried resulted in poor yields (but a nice looking yellow tan coloured product), and the possibility of salt impurities as well.

We never had a problem with follow-up reactions using the brown recrystallised product though so we suspect that the brown colour is due to an intensely coloured impurity which is only present in small quantities.




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[*] posted on 8-2-2017 at 07:26


My anthranilic acid is a tan-colored powder and, as Chem Player said, the impurity is likely intensely colored and not present in a substantial quantity.

As for evidence, this is a GC of my nice brown anthranilic acid after over 3 months in storage:

Anthranilic acid.PNG - 11kB

Anything past the first peak is simply noise as is common on this machine; no identifiable compounds to speak of.
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Boffis
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[*] posted on 8-2-2017 at 21:52


Have you simply tried using decolourizing carbon/charcoal? This is my first port of call in such purifications. Try adding a little at a time, stir and keep warm for a few minutes and then filter hot through a preheated buchner funnel. If the crystals are still too stained repeat the process. I haven't tried this with the free acid but a while back I tried this with a tea-coloured sodium salt and a single treatment gave a light straw coloured sodium anthranilate which was OK for me.
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[*] posted on 9-2-2017 at 09:09


Just to point out, methyl/ethyl anthranilate esters are available online-OTC as bird repellent. This could likely be distilled to purify and an acid hydrolysis done perhaps, followed by careful neutralization of the acid.
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veganalchemist
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[*] posted on 9-2-2017 at 09:40


My purification book says to recrystallize from water using charcoal.

Hoy\t filtration is best done using a flutted filter paper with a stemless funnel.

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TYOP213
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[*] posted on 10-2-2017 at 08:31


So what i ended up doing was dissolve in ethanol and crash the crystals out with water to my surprise the crystals collected was yellow compared to the brown stuff i began with. Anthranilic acid dissolved fairly easy in ethanol and when I filtered it about a fourth of the weight was lost.
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[*] posted on 10-2-2017 at 08:34


I think activated carbon would also work, would give it a try sometime
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[*] posted on 15-2-2017 at 07:17


Let us know how you get on!
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[*] posted on 5-2-2020 at 11:27


As a purification step anthranilic aicd is often converted to it’s copper salt. To liberate the acid from it’s salt form hydrogen sulfide is bubbled in a warm solutiom of water and copper anthranilate. Because I experienced the nice smell and being aware of the toxicity of hydrogen sulfide gas I decided to never ever preform this procedure again.
Yesterday I bubbled co2 instead of hydrogen sulfide in a cold solution of water anthranilate. Then I warmed the solution untill nearly boiling point and vacuum filtered the hot solution. Copper carbonate stayed in The filter and while cooling down white crystals starting to fall out of the solution. The process is not as effeciant as the hydrogen sulfide method so I had to repeat the gassing 3 times to get everything converted. 2/3 of the filtrate was evaporated and at cooling I had a solution of nice pure anthranilic acid.
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