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Author: Subject: Acetic anhydride preparation
Keras
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[*] posted on 27-5-2024 at 00:10


Quote: Originally posted by greenlight  

The only problem is the maximum temperature that can be achieved is low due to DCM's boiling point ~38°C.

Hexane won’t mix with acetic acid, which is polar.
That’s why I’ll try DMSO. It’s polar, aprotic and has a boiling point well above that of Ac₂O.
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[*] posted on 27-5-2024 at 08:51


@paulll, yes, stoichiometrically the yields from phosphorus pentoxide reagent are terrible haha. I only need just over a 100 ml but even that's probably pushing it.

@keras, are you sure glacial acetic acid isn't soluble in hexane? The SM wiki says it is and I'm sure I looked through a paper that said glacial acetic acid is soluble all the way up to around octane the other day.




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[*] posted on 27-5-2024 at 11:05


Quote: Originally posted by greenlight  

@keras, are you sure glacial acetic acid isn't soluble in hexane? The SM wiki says it is and I'm sure I looked through a paper that said glacial acetic acid is soluble all the way up to around octane the other day.

Well, best way is try it. :) But I would be surprised!
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[*] posted on 9-6-2024 at 11:52


So today I've tried AcOH in DMSO + phosphorus pentoxide.

First I had to wait for DMSO and AcOH to melt, because when I drew them from the shelves, they were frozen solid.

Got some liquid AcOH (10 mL), diluted in DMSO (30 mL), added P₂O₅ (~ 15 g) 'spoonwise'. First, working with phosphorus pentoxide is a pain. The weather was clear, but the powder gets instantly sticky with what small amount of atmospheric moisture exists, it then remains in the spatula, etc. In other words, nasty as hell.

The first spoonfuls dissolved with a hiss, and I really wonder if my DMSO (which I didn't distill beforehand) was really anhydrous. In any case, I almost got the solvent to boil because of the exothermicity of the dissolution. I must say, I never had any charring like what you experience when you add pentoxide directly to AcOH. After a while, the pentoxide refused to dissolve, and I obtained a sort of sticky gooey white ball of P₂O₅ that the stir bar couldn't move. I tried to mix it manually using a small glass rod, got nothing really stellar so I just pulled that gum out, dissolved it into running water and basta.

During all this time, the addition was accompanied by the expected odour of dimethyl sulphide (garlic like, though not a pungent as garlic) into which DMSO apparently easily disproportionates above 80 °C.

I then set up for distillation. Very surprisingly, something did distill, but it was somewhat around 100 °C, so not Ac₂O. I thought at first it was water, but that is not possible. I’m not sure what it was. Very acidic, but no smell like vinegar, so definitely not acetic. No idea. When that first distillate was collected, the temperature dropped. I cranked up the heat somewhat to 200 °C, the solution turned from light to dark yellow then charred up until I had a dark brown froth going up my still head. I stopped there.

So, verdict: no acetic anhydride. No immediate charring, but no product either. It was worth trying, but that’s one more failure to add to the list… :/
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[*] posted on 10-6-2024 at 20:48


If you obtained some acetic anhydride, at least part of it (or maybe all???) could undergo cyclisation by P4O10. I did the cyclisation experiment:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
If I'm not mistaken also glacial acetic acid undergoes the cyclisation, just with lower yield than acetic anhydride.
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[*] posted on 11-6-2024 at 04:31


Well I suppose I should redo the experiment with less P₂O₅ so that it is all consumed by the Ac₂O production.
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[*] posted on 11-6-2024 at 06:19


Did you get a headache during the procedure?



Quote: Originally posted by bnull  
you can always buy new equipment but can't buy new fingers.
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[*] posted on 11-6-2024 at 10:41


Quote: Originally posted by clearly_not_atara  
Did you get a headache during the procedure?


Except while figuring out how to properly add the deliquescent pentoxide to the DMSO solution, no. Why?
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[*] posted on 11-6-2024 at 11:29


The only acid boiling at 100 C is formic acid. Under the wrong conditions this decomposes to carbon monoxide.



Quote: Originally posted by bnull  
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[*] posted on 11-6-2024 at 11:36


Quote: Originally posted by clearly_not_atara  
The only acid boiling at 100 C is formic acid. Under the wrong conditions this decomposes to carbon monoxide.


Oh. Thanks. I appreciate your concern, that’s very very kind. I always operate outside, so even if that happened, that’s not an issue for me. And yes, I remember treating formic acid with concentrated sulphuric acid at dusk and lighting the resulting carbon monoxide.
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