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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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greenlight
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Is it viable to go from benzyl alcohol to benzaldehyde, then to benzoyl chloride, then reaction with sodium acetate and subsequent distillation to
acetyl chloride. Then finally to acetic anhydride.
I know it's a very long sight seeing and smelling route to get there but I am at my whitts end trying to obtain this reagent.
[Edited on 26-11-2024 by greenlight]
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Fery
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Seems to be interesting method.
Benzoyl chloride 99,9% 1 L costs 40 EUR. This EU sellers sells to individuals:
https://www.laboratoriumdiscounter.nl/en/benzoyl-chloride-99...
Though I cannot recommend them, my first order from them was also my last one.
[Edited on 27-11-2024 by Fery]
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Keras
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What problem did you have with them? I routinely order chemicals and stuff from them. It’s not entirely smooth, but so far I had no big issue. Also,
really the route with sulfur chloride and sodium acetate seems way less intricate.
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greenlight
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Stepwise, yes the sulfur chloride route is less intricate.
I'm not sure how the yields or time would compare between the two routes if you are after 150ml or more of AA.
I guess the benefits would be no sulfur contamination in the final anhydride and you have the option of stopping at acetyl chloride if that is
required as it is a hard to get reagent as well in most countries.
Might have to try it and report whether it's worthwhile, got some benzyl alcohol here gathering dust.
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Keras
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Also, you can easily get triphenylphosphine. Bubbling chlorine in triphenylphosphine will get you triphenylphosphine dichloride, which can be used to
get acyl chlorides from carboxylic acids.
[Edited on 28-11-2024 by Keras]
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greenlight
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Step 1: Benzyl alcohol route to acetic amhydride
Okay, so far I have done the following:
Prepared a dilute aqueous solution of 78 ml of 70% nitric acid and 520 ml distilled water in a 1000ml round bottom flask. Then prepared a sodium
nitrite (5.00g) in distilled water (8 ml) which was poured into the round bottom flask inside a fume cupboard which resulted in minor nitrogen dioxide
evolution.
Benzyl alcohol (156 ml, 99.99% food grade) was then added and a liebig condenser attached with ice water flowing and the flask placed in a hot water
bath on a hotplate stirrer. Reflux was performed at approximately 90°C (water bath 95°C allowing for difference in thermal transfer between bath
and flask wall) for 4 hours before cooling to room temperature.
After cooling, the crude benzaldehyde was collected using a seperatory funnel. The benzaldehyde was washed twice with 200ml saturated sodium
bicarbonate solution to neutralise residual acid which also resulted in small spillage when the stopper was not on properly during venting. There is
a fair quantity of gas evolved so very frequent venting is required for the first wash.
The crude benzaldehyde was then washed with 200 ml saturated sodium chloride solution before being placed in a reagent bottle with anhydrous calcium
chloride in the bottom to thoroughly dry the product before a vacuum distillation can be performed. The crude product is very red in colour as can be
seen in attached image.Step 1:
This should then be able to be reacted into benzoyl chloride, then acteyl chloride and finally acetic anhydride if all goes well.
[Edited on 11-12-2024 by greenlight]
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