Sciencemadness Discussion Board - Tar filtration in work-up

Tar filtration in work-up

Keras - 4-5-2024 at 23:06

Folks,

I wonder if there are a sort of FAQ or set of recipes to get rid of the tarry substances which sometimes ‘pollute’ the result of reactions, for example, nitrosations or nitrations of aromatic compounds. I know steam distillation can be used, but if the desired product is steam volatile, which is not always the case. Gravity filtration with filter paper can be used if the desired product is dissolved in the mother liquor.

Is there any other procedure that can by tried in other cases?

Specifically, I’m still trying to purify p-nitrosophenol made by nitrosation of phenol. The reaction produces a small amount of tar which darkens the colour of the crystals and make them stick to one another. Unfortunately both product and tar are solid, and both dissolve in strongly basic aqueous solutions. p-nitrosophenol is not steam volatile (only a negligible amount is carried over in steam, while this might only be the small amount of o-nitrosophenol formed).

I’m thinking about :

• Trying to filter out the nitrosophenol while the reaction is proceeding, rather than waiting the end of it to filter it all at once. Apparently, tarry substances arise from the prolonged contact of nitrosophenol with the phenol in the solution.

• Somehow try to add both reactants (phenol/sodium nitrate on the one side – diluted sulphuric acid on the other side) simultaneously rather than drip B into A.

• Reduce the nitrosophenol to the amine (since this is what I want to do anyway with sodium dithionite) and acidify with hydrochloric acid to get soluble 4-aminophenol hydrochloride while the tar should remain solid.

Any opinion?
Thanks!

Metacelsus - 5-5-2024 at 03:16

If the tar doesn't affect the reduction, you should just reduce everything and purify the 4-aminophenol.

Texium - 5-5-2024 at 06:04

Yeah I would just reduce the crude intermediate. Don’t count on the tar remaining solid when you acidify the solution though. Extracting the product into HCl is a good start, but it’s not going to be totally clean and some of the tar will likely still pollute the aqueous. After you neutralize and extract the product back into an organic solvent, then you can clean it up further by treating it with activated charcoal and filtering through celite (diatomaceous earth). Usually in similar situations I treat my solution of product with a mixture of activated charcoal and anhydrous MgSO4, then filter through celite so that tar and water are removed simultaneously before evaporating the solvent.

[Edited on 5-5-2024 by Texium]

Keras - 5-5-2024 at 06:12

Quote: Originally posted by Texium

Yeah I would just reduce the crude intermediate. Don’t count on the tar remaining solid when you acidify the solution though. Extracting the product into HCl is a good start, but it’s not going to be totally clean and some of the tar will likely still pollute the aqueous. After you neutralize and extract the product back into an organic solvent, then you can clean it up further by treating it with activated charcoal and filtering through celite (diatomaceous earth). Usually in similar situations I treat my solution of product with a mixture of activated charcoal and anhydrous MgSO4, then filter through celite so that tar and water are removed simultaneously before evaporating the solvent.

Thanks to both. Excellent ideas.

Keras - 5-5-2024 at 10:44

Quote: Originally posted by Texium

Yeah I would just reduce the crude intermediate.

Finally, after much digging I found a recipe in a Vietnamese paper where they invert the order of the synthesis, so to speak: instead of dripping acid into the solution of phenol + sodium nitrite, you drip phenol + sodium nitrite into acid. And it works much better: 0 tar. Probably because all the reactants are consumed immediately, leaving nothing to react with the product (see picture, the orange mixture).

In any case, I was able to reduce the p-nitrosophenol immediately after into very pure 4-aminophenol (white crystals). The next step to paracetamol is trivial, with a few mL of acetic anhydride.