Phthalic acid synthesis

Our forum member Prepic asked me to post the preparation of phthalic acid from naphtalene + KMnO4.
There are already 3 old threads discussing the same topic:
https://www.sciencemadness.org/whisper/viewthread.php?tid=65...
https://www.sciencemadness.org/whisper/viewthread.php?tid=87...
https://www.sciencemadness.org/whisper/viewthread.php?tid=17...
I don't wan't to open new thread for that so I post it here. I would like to kindly ask moderator(s) to eventually move to appropriate place / merge / split etc.

Here the method which I used long time ago - scanned page from the book and its translation into English:

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naphtalene 2,6 g (0,02 mol)
KMnO4 16g (0,10 mol)
conc. H2SO4 (I certainly used diluted accumulator 38% H2SO4 because as 15 years young boy I hadn't access to conc. acid... well I had 1 L bottle thank to my schoolmate, but also plenty of accumulator acid which was possible to buy in drugstore, so certainly used accumulator acid keeping concentrated acid to experiments where diluted couldn't be used)

In 250 ml RBF with condenser attached (I suggest Liebig with wide tube and not too long, certainly shorter than your longest glass rod) is heated mixture of 2,6 g naphtalene, 16 g KMnO4 and 160 ml of water. It is heated until disappeareance of violet color of KMnO4 (this is hard to observe as MnO2 prevents to see the color of water solution, it is necessary to let MnO2 to settle a little and observe the clear liquid above it) and until the liquid loses color above the settled MnO2 (1,5 - 2 hours). It is necessary to periodically stop the water circulation through condenser and push sublimed naphtalene back into the flask using glass rod. After finishing heating MnO2 is filtered out, filtrate boiled down to 1/3 of its original volume (to approximatelly 60 ml), 4 ml of conc. H2SO4 is added (I certainly added equivalent amount of 38% H2SO4 and boiled the volume to 50 ml as the volume of the diluted acid was bigger than concentrated acid). While still hot the paste of MnO2 is added until it dissolved (IIRC not too much of it dissolved and the solution soon started to contain suspension of MnO2 particles). Already during the addition the phthtalic acid starts to precipitate in a form of crystalline product. Reaction mixture is heated to boil and such amount of water is added which dissolves the precipitated product. After finished oxidation a little of active charcoal is added and hot solution is filtered. Filtrate is concentrated and let to crystallize. Phthalic acid crystallizes in a form of white crystals, which are sucked off and dried between 2 sheets of filter paper. By its sublimation is its anhydride is obtained, m.p. 128 C. Yield 2 g which is 60,6% of theory. Phthalic acid is white crystalline substance with m.p. 203 C, only very slightly soluble in water, soluble in alcohol, little in ether.

As I did not have camera that time, I don't have pictures of the experiment itself. But I remember I "cooked" the reaction on a wood stove in my gradma summer kitchen, so the boil was only very mild, certainly not vigorous. I also remember, that I waited for KMnO4 discoloration for maybe 4-5 hours and only then realized, that it already reacted and violet color was gone, maybe already hours ago, just the colorlessness was not visible due to MnO2. I also suggest not to boil it too vigorously as then naphtalene sublimes faster. Also stirring should improve performance (mg. stirrer or at least swirling the apparatus using hand - I did that manual swirling occasionally as melted naphtalene is as a top layer and it is necessary to increase surface contact to made it to react). Maybe the boil itself is sufficient to mix both phases, but vigorous boil = too fast sublimation of naphtalene which settles in condenser. I wonder whether it couldn't be performed in a flask without condenser, sitting on mg. stirrer (or at worst periodically swirl in a hand) with its neck just covered by a plastic foil, at such temp just below the boil (e.g. in boiling water bath so the content in flask couldn't reach 100 C), so the plastic foil keeps the flask closed and prevents naphtalene loses by escaping.
I still have the vial with the product, the vial is 10 ml. Pictures made just now attached. The label is still readable although it already lost some color and the paper is somewhat yellowish.



I found solubilities of phthalic acid in water here:
https://www.bc.edu/content/dam/bc1/schools/mcas/Chemistry/pd...
"Phthalic acid is very soluble in boiling water, 18 g/100 mL, and is much less soluble in chilled (14°C) water, 0.54 g/100 mL."

[Edited on 9-4-2021 by Fery]

In France.

I have CrO₃ flakes, but really each time I open the bottle they're stored inside, I recoil instinctively

Ok so I tried this morning oxidising 1 mmol (~ 100 mg) of naphthalene with 5 mmol (~ 800 mg) of potassium permanganate. Rationale is, you oxidise four carbons with two oxygens each, so each molecule of permanganate gives up two oxygens, ergo one needs at least four times as much permanganate as naphthalene.

Naphthalene dissolved in DCM (a few drops are enough) in a 25 mL RBF with dropping funnel and reflux condenser (useless, in hindsight). Heated until the DCM boils, dripped in the permanganate (~15 mL + 2 mL 50% sulphuric acid). Probably almost instantly particles of manganese dioxide stuck to the wall of the flask. Continued stirring for one hour at 70 °C. Dismantled the thing, geared up for vacuum filtration. No traces of sublimated naphthalene. No real smell, too.

Assuming 100% yield, 1 mmol of phthalic acid weighs 166 mg, soluble in 30 mL of water, especially hot. So I put a bed of Celite™ to avoid clogging my sintered disc with particles of manganese dioxide and got a clear, straw yellow filtrate. No trace of DCM, no purple colour, so I guess all the permanganate had reacted.

Boiled the ~30 mL of water down to 10 mL. Put it in the fridge → Nothing. Yellow water, is all. Faint aroma of something decisively different from naphthalene, but zero product.

I’m not sure what happened. Could all naphthalene have been oxidised to carbon dioxide? Did I use too much permanganate? Bad solvent? No idea at this point.

Going to retry later in the week using other oxidising products: Oxone, sodium percarbonate, bleach, ammonium persulphate.

PS: Cleaning the flasks and stir bar is easy using HCl which dissolves manganese dioxide.

[Edited on 25-11-2024 by Keras]

Quote: Originally posted by Keras

Boiled the ~30 mL of water down to 10 mL. Put it in the fridge → Nothing. Yellow water, is all. Faint aroma of something decisively different from naphthalene, but zero product. [Edited on 25-11-2024 by Keras]

Quote: Originally posted by Fery

While still hot the paste of MnO2 is added until it dissolved [Edited on 9-4-2021 by Fery]

Quote: Originally posted by Gualterio_Malatesta

Quote: Originally posted by Keras   Boiled the ~30 mL of water down to 10 mL. Put it in the fridge → Nothing. Yellow water, is all. Faint aroma of something decisively different from naphthalene, but zero product. [Edited on 25-11-2024 by Keras] Isn't it supposed to be in a form of dipotassium o-phthalate (as someone mentioned here on the forums) after the oxidation reaction?

Quote: Originally posted by Sir_Gawain

Working with sub-gram amounts, it’s very easy to lose your product (or volatile reactants). 87 mg is such a tiny amount it’s not surprising you didn’t get any product. It’s also possible that the acidified permanganate solution was too harsh of an oxidizer and simply oxidized to CO2. If I have some free time this week I’ll try this reaction myself.

Quote: Originally posted by Keras

In France. I have CrO₃ flakes, but really each time I open the bottle they're stored inside, I recoil instinctively

Quote: Originally posted by bnull

The yellow may be due to alpha-naphthoquinone.

Quote: Originally posted by bnull

If you can spare another 800 mg of permanganate, try the procedure again, this time adding a small quantity of dichromate (the tip of a penknife will do) and no acid. Dichromate seems to catalyse oxidations by permanganate. I'm not really sure of how it works, maybe dichromate acts like an acid.

Quote: Originally posted by teodor

You can look https://library.sciencemadness.org/library/books/Experiments... Page 233 for further inspiration with the acetic acid solvent. Also, I recommend just to buy it glacial.

Quote: Originally posted by Keras

I have no personal grudge with permanganate. Is just that it is difficult to source because it’s listed as a drug precursor over here in the EU. So you have to make it yourself from manganese oxide by the fusion process, which takes time.

Quote: Originally posted by Keras

I have no personal grudge with permanganate. Is just that it is difficult to source because it’s listed as a drug precursor over here in the EU. So you have to make it yourself from manganese oxide by the fusion process, which takes time. [Edited on 3-12-2024 by Keras]