Keras
International Hazard
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Registered: 20-8-2018
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Notes on preparing anthranilic acid
Some notes on the synthesis of anthranilic acid I attempted this weekend:
• Prepare phthalimide the standard way (fuse at 120 °C phthalic anhydride and urea). This reaction is very easy, works well, doesn’t let out any
ammonia (as far as I've noted) and is high-yielding.
• In an ice bath, prepare a solution of hypobromite by dissolving bromine into a freezing solution of sodium hydroxide with stirring. The hardest
part is managing to keep the bromine in the pipette and avoiding it escaping all at once. Needs some safety measures (place your bromine flask and the
sodium hydroxide flask very close together so as to avoid leaking drops of bromine while you’re handling the pipette. Wear gloves). At the end your
solution must be slightly yellow from a tiny presence of free bromine in solution.
• Add the phthalimide to this solution (make it pasty before by adding a few mL of water). It should dissolve completely. Let it stir for 10
minutes.
• Now pull the beaker out of the ice bath on to a hot plate and add extra sodium hydroxide in several portions (always with stirring). This will
raise the temperature of the solution, but is ok.
• Once all the sodium hydroxide has been dissolved, raise the temperature until you reach 80 °C. The solution will become redder and redder. The
change in colour is apparently not due to the release of bromine, which would boil out of solution. Contrarily to what happens with hypochlorite and
urea in the hydrazine synthesis, the solution does not bubble up. Carbon dioxide is sequestered by an excess of sodium hydroxide and stays in solution
as sodium carbonate. A few seconds after reaching 80 °C, the solution will change and transition from clear to opaque as excess sodium carbonate
crashes out of solution. The temperature is maintained for ten minutes circa (unlike hydrazine, anthranilic acid will not evaporate) and then allowed
to cool, and filtered (what is left in the filter is discardable sodium carbonate).
• The solution is neutralised with hydrochloric acid until pH 7 (lots of carbon dioxide released, produces odourless fumes), then acetic acid is
added (more carbon dioxide bubbling out) to lower the pH down to 4, precipitating the anthranilic acid. Apparently, despite the ionisable groups, the
acid is not very soluble in water. I suppose its solubility increases at very low and very high pHs, that’s why acetic acid is used to acidify the
solution, and not an excess of HCl.
• Filter out the acid from the cooled solution.
• Recrystallise from boiling water. I did it twice and added some pieces of activated carbon but couldn't get rid of a dark yellow/brown tinge.
Might be traces of oxidation products. It could be helpful to add a dash of sodium hydrosulphite to avoid that.
• Yield to be evaluated later, because the crystals were still wet. Seems like 50/60%. Hoffman rearrangements are never very high yielding, to the
best of my knowledge.
→ → anthranilic acid to be used later to synthesise indigo.
[EDIT: Pictures.]
[Edited on 2-3-2025 by Keras]
 
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