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Keras
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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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greenlight
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@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.
Be good, otherwise be good at it 
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Keras
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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.
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Well, best way is try it. But I would be surprised!
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Keras
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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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Fery
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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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Keras
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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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clearly_not_atara
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Did you get a headache during the procedure?
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Keras
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Except while figuring out how to properly add the deliquescent pentoxide to the DMSO solution, no. Why?
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clearly_not_atara
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The only acid boiling at 100 C is formic acid. Under the wrong conditions this decomposes to carbon monoxide.
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Keras
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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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greenlight
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| Quote: Originally posted by Keras | 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, but that’s one more failure to add to the list… :/
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Respect for giving it a shot
Your experience sounds very similar to mine when I attempted it with just glacial acetic acid and no solvent.
It did not tar up immediately, it first went through that same stage where it became so thick that it was unable to be stirred with a magnetic stir
bar then turned to tar before I could get to the distillation stage. The brown tar climbed up my flask in a similar fashion to what you describe.
Totally agree with you on the absolute pain phosphorus pentoxide is to transfer from its bottle to any reaction vessel.
This is one of those reaction mechanisms that looks great on paper but refuses to cooperate in reality.
Back to the drawing board it is once again.
[Edited on 30-6-2024 by greenlight]
Be good, otherwise be good at it
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Fery
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| Quote: Originally posted by Keras | So today I've tried AcOH in DMSO + phosphorus pentoxide.
...
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… :/ |
Man!!! Be happy that you did not get any acetic anhydride. DMSO reacts with acetic anhydride into 2,4-dithiapentane (truffle scent)
https://www.youtube.com/watch?v=zBCY6kYptJc
[Edited on 1-7-2024 by Fery]
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