Sciencemadness Discussion Board - Acetic anhydride preparation

Also, 1000th post :cool:

Wow. I swear I didn't even notice

Besides, is it possible to get acetyl chloride by bubbling HCl into acetic anhydride? Just curious. Something I might attempt early next week.

Metacelsus - 2-5-2021 at 02:38

According to Wikipedia yes: https://en.wikipedia.org/wiki/Acetyl_chloride#Synthesis

You could separate the acetyl chloride by distillation.

njl - 2-5-2021 at 14:41

Keras, if you are going to try that would you perhaps also try the MeCN/HCl reaction for generating acetyl chloride? Assuming you have MeCN that is. The procedure is quite similar to Ac2O/HCl.

Keras - 2-5-2021 at 22:30

Keras, if you are going to try that would you perhaps also try the MeCN/HCl reaction for generating acetyl chloride? Assuming you have MeCN that is. The procedure is quite similar to Ac2O/HCl.

Yeah, I have acetonitrile. What I lack most is time, alas :/ Especially since I live one hour away from my lab.
The MeCN/HCl reaction you mention is described Chemplayer's video, right?

[Edited on 3-5-2021 by Keras]

njl - 3-5-2021 at 04:52

Yes it is, but experiments done by users on another forum showed that Chemplayer didn't use nearly enough HCl gas, or allow the reaction to run for long enough. See my reply in this thread from 4/29/21 for more info.

Keras - 3-5-2021 at 08:54

Yes it is, but experiments done by users on another forum showed that Chemplayer didn't use nearly enough HCl gas, or allow the reaction to run for long enough. See my reply in this thread from 4/29/21 for more info.

OK. I’ve dug out the original publication in the Comptes rendus de l'Académie des Sciences (being French helps for this once) – the mixing must be done at 0°C and the solution saturated with HCl must be left standing for 24 h in a closed vial. The author claims to have obtained a large number of acetamide chlorhydrate crystals and the corresponding amount of AcCl.

If you want to see it for yourself, it’s here.

[Edited on 3-5-2021 by Keras]

S.C. Wack - 3-5-2021 at 18:22

Besides, is it possible to get acetyl chloride by bubbling HCl into acetic anhydride? Just curious. Something I might attempt early next week.

It's all been posted before.

https://www.sciencemadness.org/whisper/viewthread.php?tid=32...

https://www.sciencemadness.org/whisper/viewthread.php?tid=77...

Keras - 3-5-2021 at 22:20

Wikipedia reports the yield of this reaction to be very low, typically under 40%.

njl - 4-5-2021 at 05:39

Well half of the acetonitrile is consumed as acetamide HCl. The advantage to this reaction isn't yield it's that the procedure is so simple, and yields only the volatile product and a non volatile side product. If you really wanted to you could even recycle the acetamide back to acetonitrile.

Keras - 4-5-2021 at 08:02

Well half of the acetonitrile is consumed as acetamide HCl. The advantage to this reaction isn't yield it's that the procedure is so simple, and yields only the volatile product and a non volatile side product. If you really wanted to you could even recycle the acetamide back to acetonitrile.

To be honest, I have a hard time figuring out a reasonable pathway for this reaction. Admitting the acid is transformed into the acyl chloride, as seem to be the case, there is probably an H⁺ attack on the double-bound O, in which case OH⁻ would be the leaving group, (but that’s very unlikely?).

njl - 4-5-2021 at 08:23

I think it works via addition of HCl to the nitrile to form an imidoyl chloride, this reacts with AcOH to give the mixed imidoyl/acetic anhydride, addition of chloride to the acetyl group and elimination of the bridging oxygen to give acetyl chloride and an "imidic acid" which then tautomerizes to the amide.

Edit: see the Pinner reaction for more details. Although it is conducted with a nitrile, acid, and alcohol, the mechanism is probably similar where the alcohol is replaced with AcOH.

[Edited on 5-5-2021 by njl]

Keras - 11-5-2021 at 11:30

Edit: see the Pinner reaction for more details. Although it is conducted with a nitrile, acid, and alcohol, the mechanism is probably similar where the alcohol is replaced with AcOH.
[Edited on 5-5-2021 by njl]

Will have a look.

So, I decided to try this reaction today. The setup is in the first attached picture. Dropping funnel with ~ 100 mL of concentrated sulphuric acid, 250 mL two-neck RBF filled with 60 g NaCl, second neck leading to a CaCl₂ tube. The outlet is connected to a bent glass tube (that I made myself using straight tubing, a glass cutter tool and my blowtorch, I’m pretty proud of that) running through a rubber cork plugged into the first outlet of a Claisen adapter. The tube plunges into a small, 50 mL RBF containing 0.3 mole of each acetonitrile and glacial acetic acid (circa 15 mL each), the RBF being immersed in an ice/salt bath. The second outlet of the Claisen adapter is connect to a second CaCl₂ tube.

I calculated the HCl generator for a 2 mol output, way more than the stoichiometric amount (0.6 mol).

A gentle current of argon was bubbled into the solution while the HCl generator was starting, to clear residual moisture (the weather was rainy).

At no point during the HCl bubbling did the temperature rise over -15 °C (see second picture).

But the HCl gas generator turned out to be pretty unreliable. I had a very slow stream of HCl out. Even when the addition of all the sulphuric acid was over, I got the feeling that very little had run into the small RBF. I dismantled the apparatus, transferred the contents of the RBF into a vial that I stored in the freezer. So far, (about 6 hours later), I still have nothing apparent. There was a lot of unreacted acid in the HCl generator, which took a while to neutralise properly with both lye and sodium bicarbonate. I didn't heat the HCl generating flask, and I’m sure that was a mistake, because the reaction didn't generate a lot of heat, was slow, and I had to open the generator every so often to break up the mass of ?sodium bisulfate? that formed and swirl the flask to get it run more vigorously.

I won’t be able to return to my lab before next Tuesday, but in the meanwhile the flask with the reactants is secured in the freezer. I probably can pick up where I left, assuming I can assemble a more efficient HCl generator. If anyone had a recipe for a good, reliable HCl generator, I’m all ears

[Edited on 11-5-2021 by Keras]

[Edited on 11-5-2021 by Keras]

Keras - 13-5-2021 at 00:46

EDIT: I was told that the flask is now full of crystals. I suppose it takes a while for the reaction to progress at -18 °C. I wonder, btw, if the reaction starts with the bubbling at all. Probably the bubbling gets HCl dissolved in the mixture, and it’s only after the bubbling is complete that the reaction picks up. If this is true, then it’s important to keep the flask cooled in order to avoid losing HCl.

By the way, I got no HCl escaping from the second CaCl₂ moisture trap, so I guess everything I generated got dissolved.

I’ll check what I get next Tuesday.

clearly_not_atara - 13-5-2021 at 09:59

What kind of HCl generator makes sodium bisulfate?

There are two HCl generation methodologies I know about; both involve adding azeotropic hydrochloric acid to something. "Something" is either concentrated sulfuric acid or anhydrous calcium chloride. The latter technique is significantly cheaper, but can give an unreliable gas generation rate, so H2SO4 is preferred when possible.

I also wonder if you could use a slurry of CaCl2 and 85% H3PO4, or if this would give undesirable side-reactions, or condense into an intractable solid mass.

Keras - 13-5-2021 at 23:24

What kind of HCl generator makes sodium bisulfate?

NaCl + H₂SO₄ → NaHSO₄ + HCl

I also wonder if you could use a slurry of CaCl2 and 85% H3PO4, or if this would give undesirable side-reactions, or condense into an intractable solid mass.

Never tried that. Don’t have enough CaCl₂ at present to give it a go.

Keras - 18-5-2021 at 11:35

OKKKKKKK. So, I went back to my lab, and checked on the acetic acid/acetonitrile/HCl process. The bottle I'd left the mixture in was indeed full of crystals (as a reminder, it stayed a full week in the freezer at -18 °C). I prepared it for vacuum filtration but before I could do anything my vacuum tube, which is unnecessarily unwieldy, made the Erlenmeyer flask trip and the funnel broke :/ I will have to buy another.

That was not a huge problem, apart from the outlay of 40 euros just for being clumsy, since I was able to directly decant the liquid from the crystals (and thus spared my other grade-4 Büchner funnel in the process). I washed the bottle and the crystals with a bit of cold ether, collected everything into a small Erlenmeyer flask, and set up for distillation. The collected liquid fumed strongly, but I was not able to tell whether it was because of diluted HCl that was being released as the mixture warmed, or because of acetyl chloride in the mix.

Distillation was very slow (one drop every three or four seconds). The first drops were collected around 30 °C, which was expected (ether). That 'head' also fumed, so I suppose ether carried away a bit of HCl. After like 5 ml of ether had passed, and I was already ¾ hour into it, the temp began to rise steadily, but it was really a pain. I switched the hotplate I used for a heating mantle and carried on. Unfortunately, the temp here jumped all the way from 30-35 °C to 80-82 °C, the boiling point of acetonitrile. There was absolutely no pause at 51, the b.p. of acetyl chloride. Just to be certain, I continued collecting the fraction at 82, and when it died out I was left with a flask visibly full of acetic acid, with a slight brownish tinge.

So, the yield was zilch. Traces, maybe, of acetyl chloride, but nothing I could separate from the starting materials. I dripped a few drops of the 30 °C fraction into water, and there was a slight haze of HCl emitted. As a clincher, I added a few drops of n-butanol, waited a bit, and tried to smell the result. There was definitely the characteristic aroma of butyl acetate, but it was very faint.

Yet, there were crystals of acetamide-hydrochloride. A few grams, maybe (I didn't even bother to weigh them).

The weather is quite bad over here in Western Europe these days, but I doubt that even that high level of atmospheric moisture could’ve turned all the acetyl chloride back into acetic acid and hydrogen chloride. I had, however, difficulties with my HCl generator, so I suspect the quantity of HCl that was actually bubbled into the initial mixture was way beyond what I hoped.

I don’t think ether reacts with acetyl chloride.

I’m going to redo the experiment, but this time I plan to warm the HCl generator so as to push the reaction between sulphuric acid and salt as far as I can.

[Edited on 18-5-2021 by Keras]

clearly_not_atara - 18-5-2021 at 11:47

I don't think that dripping concentrated sulfuric acid onto table salt is a very good HCl gas generator. It would probably be better to use one of the other two methods with a drying tube. I can't really speak from experience as I've never done it that way. Warmer will help, of course.

It appears there is AcCl entrained in your ether. Probably not a lot, but if you want to isolate it, you could try stirring the ether fraction with some anhydrous potassium or sodium acetate (the former is easier to dry) and re-distilling; the much larger bp difference with Ac2O will surely separate the two.

S.C. Wack - 18-5-2021 at 14:36

So, the yield was zilch.

Can you think of a reason for why this might be; while technically may not be true?

[hint: it's obvious]

I don't think that dripping concentrated sulfuric acid onto table salt is a very good HCl gas generator.

The quality of clan lab chemistry has really fallen (along with Rome) in the past 20 years...I was mistaken to believe that someone might read the books I scanned...C.W. Scheele would be even more disappointed with this...BTW the UK Alkali Act of 1863 was the first legal control of chemical emissions (target: hydrogen chloride from salt and sulfuric acid)

[Edited on 18-5-2021 by S.C. Wack]

clearly_not_atara - 18-5-2021 at 15:49

When you are dehydrating muriatic acid, the residue in the rxn flask can be heated to regenerate... more H2SO4 (unpleasant, to be fair) or more CaCl2 (easy). But if you are reacting H2SO4 with salt, you lose your reagent. See the difference? And conc. H2SO4 is a thing to come by these days -- granted dehydrating it is no fun, but at least you aren't starting from 15%.

I am of course aware of the reaction, I just wasn't aware that someone would choose it.

Keras - 18-5-2021 at 21:28

Quote: Originally posted by S.C. Wack

So, the yield was zilch.

Can you think of a reason for why this might be; while technically may not be true?

[hint: it's obvious]

So there are several possibilities: the first one is kinetics: since the mix was 'created' and left at -18 °C circa, it is possible that at this temperature the reaction is crawling.

Next, insufficient HCl being bubbled into the mix, or water vapour that made its way even though I set up a CaCl₂ trap, which did not appear to catch any water whatsoever (the pellets were still very dry when I dismantled it).

Finally, the distillation might’ve gone awry because of an azeotrope Et₂O/AcCl. Separation between the two theoretical b.p. was fair (35 °C for Et₂O, 52 °C for AcCl), I should've got both, even without using a Vigreux column.

Beyond that, apart from bad reagent quality or something of the same ilk, I’m at a loss.

Keras - 18-5-2021 at 21:38

When you are dehydrating muriatic acid, the residue in the rxn flask can be heated to regenerate... more H2SO4 (unpleasant, to be fair) or more CaCl2 (easy). But if you are reacting H2SO4 with salt, you lose your reagent. See the difference? And conc. H2SO4 is a thing to come by these days -- granted dehydrating it is no fun, but at least you aren't starting from 15%.

I agree that sulphuric acid is going to be a dainty in a few months. However, I still have 9 L left of it, and I recently bought from my local hardware store a couple of leftover bottles of drain cleaner (~70% concentration) from which I should be able to generate more than one L of concentrated stuff (or use directly in a generator), as well as 3 bottles of 37% grade, that I could use to, say, make azeotropic hydrobromic acid.

I’m not sure using sulphuric acid to dehydrate 36% HCl is very efficient. You’re right in that it doesn’t waste the sulphuric acid, however. Yet, reacting sulphuric acid with salt generates sodium bisulphate, from which you can regenerate SO₃ by heating, which you can feed into c. sulphuric acid to form more.

2 NaHSO₄ → Na₂SO₄ + SO₃ + H₂O

So you lose acid, but half of what you expected, provided you collect the residue. Besides, sodium bisulphate is cheap and easy to acquire, so as long as you have some sulphuric acid left, you can easily generate oleum that you dilute to get more sulphuric acid.

clearly_not_atara - 19-5-2021 at 08:24

Right, but you're comparing all of these distillations to heating a jar on a hot plate for a few hours.

Quote:

I’m not sure using sulphuric acid to dehydrate 36% HCl is very efficient.

It is the standard HCl gas generation methodology. It works at low temperature and gives a consistent flow rate. What more could you ask for? See e.g.:
https://www.sciencemadness.org/whisper/viewthread.php?tid=90...

Keras - 19-5-2021 at 08:47

Right, but you're comparing all of these distillations to heating a jar on a hot plate for a few hours.

Quote:

I’m not sure using sulphuric acid to dehydrate 36% HCl is very efficient.

In my case, I have only 23 % azeotropic HCl, so I'd have to concentrate it, meaning I'd have to generate HCl to then generate HCl :p

S.C. Wack - 19-5-2021 at 14:47

Can acetyl chloride be distilled from a mixture with acetic acid?

It is the standard HCl gas generation methodology.

There are a few lab manuals that mention salt with conc. HCl (Fieser, the Organikum) and of course it is standard for cookbooks (Uncle Fester, Strike), but the majority of directions either do not use HCl or it is mentioned as one possibility; when it appears it's often in a more complicated setup not using salt. (e.g. Vogel, who also has the oft-mentioned ammonium chloride)

[Edited on 19-5-2021 by S.C. Wack]

clearly_not_atara - 19-5-2021 at 15:11

^Considering I didn't say anything about using "salt" with muriatic, I'm not sure what you think you're critiquing.

In any case, this is tedious. If you have excess liters of sulfuric acid burning a hole in your pocket, obviously your problems are much different from mine. Hydrochloric acid is easily concentrated by distilling it over any number of dehydrating agents -- H3PO4, CaCl2, H2SO4, take your pick. I wish you luck in your efforts.

[Edited on 19-5-2021 by clearly_not_atara]

Keras - 20-5-2021 at 00:45

Then it might be best to boil it under reflux (to get rid of water vapour), and led the HCl gas through a calcium chloride trap. This is slightly more complex than the sulphuric acid/salt generator, but has the definite advantage of not using any sulphuric acid.

teodor - 28-7-2021 at 23:41

"Upon treating sodium acetate with chlorosulfonic acid in a little acetic
anhydride there is formed acetic anhydride". K.E. Jackson "Smoke-forming chemicals". The reference is PYTASZ AND RABEK:
Przemysl Chem. 14, 529-35 (1930).
(sorry if it was already mentioned - it is really hard to re-read the whole thread again)

solo - 6-3-2022 at 10:11

......it's a long thread , read 20 pages and didn't find any suggestion of doing a transterification of acetic acid with 4- toluene sulfonic acid in toluene using a dean stark trap to trap water. to acquire acetic anhydride ....I hava 4 toluene sulfonyl chloride and want to hydrolize it to attempt the procedure....no, no references only a theory...but would appreciate a method to hydrolyse th 4-toluene sulfonyl chloride to 4-toluene sulfonic acid.......solo

Waffles SS - 7-3-2022 at 13:16

Quote: Originally posted by solo

......it's a long thread , read 20 pages and didn't find any suggestion of doing a transterification of acetic acid with 4- toluene sulfonic acid in toluene using a dean stark trap to trap water. to acquire acetic anhydride ....I hava 4 toluene sulfonyl chloride and want to hydrolize it to attempt the procedure....no, no references only a theory...but would appreciate a method to hydrolyse th 4-toluene sulfonyl chloride to 4-toluene sulfonic acid.......solo

There is no need for hydrolyzing Tosyl chloride(4-Toluenesulfonyl chloride),Just mix with dry Sodium acetate(1:2) and distill it !

The reaction is exothermic when started( 80-100c)

(Redistillation of product maybe needed if you want odorless and colorless Ac2O)

[Edited on 7-3-2022 by Waffles SS]

solo - 7-3-2022 at 18:09

Quote: Originally posted by Waffles SS

Quote: Originally posted by solo

....thank you, what solvent would you use since tosyl chloride is insoluble in most solvents save benzene , ether and alcohol ( which might work for both tosyl chloride and sodium acetate.).....solo

SWIM - 8-3-2022 at 16:38

Sounds like waffles is describing a dry distillation to me.

solo - 9-3-2022 at 07:38

" Sounds like waffles is describing a dry distillation to me. "....you are right , that seems to be the case...thanks for pointing it out....solo

Waffles SS - 11-3-2022 at 01:12

Quote: Originally posted by solo

....thank you, what solvent would you use since tosyl chloride is insoluble in most solvents save benzene , ether and alcohol ( which might work for both tosyl chloride and sodium acetate.).....solo

As swim said,I mean Dry distillation.(i did this method many times with Tosyl chloride and even Benzene sulfonyl chloride)

Try 2:1 and 3:1 ratio of sodium acetate : Aryl Sulfonyl chloride

[Edited on 11-3-2022 by Waffles SS]

Fantasma4500 - 15-4-2022 at 01:15

so much bickering about acetaldehyde but nothing in this 1000+post thread on the oxidation of acetaldehy into acetic anhydride?

as for the construction its rather easy of what ive been told
i know one person who produced liters of acetaldehyde which he then stored as some type of ammonia adduct in freezer
a regular glass jar with metal screw lid has a hole drilled in one side of the lid, air pumped through here
the middle a piece of copper, copper wire, spiral wound is left hanging in the "air" in the jar
ethanol is put in the jar
and an outlet composed of copper tube- my guess is that soldering wouldnt be required, silicone should do plenty well
it does get quite hot however
prior assembly the copper is heated up, the reaction is autocatalytic, it heats itself as the ethanol reacts with the CuO or Cu2O, then reduced back to copper, then oxygen in air delivered repeats the cycle
i recall something about silver or platinum being able to do the same with less heating- more desirable for making formaldehyde as that reaction tends to blow up more easily

the acetaldehyde is condensed in a container with icewater

since this whole thread mentioning acetaldehyde has only mentioned the construction of acetaldehyde generator, we never got to the actually interesting part, how do we go on from that to acetic anhydride? sodium persulfate? perchloric acid?

clearly_not_atara - 15-4-2022 at 06:41

Well, for one thing, why would we choose a precursor (for Ac2O) which is a gas?

Acetaldehyde is a precursor to lots of interesting stuff, like 2-picoline, acetonitrile, glyoxal and chloral, but under most achievable conditions it will oxidize at the 2-position rather than the 1-position, which means that obtaining acetyl anything is going to be hard. Instead, acetaldehyde is interesting because it can be used to make other things.

Radical halogenation of acetaldehyde acetals should give acetals of acetyl halides. If the diol used can undergo a pinacol rearrangement, treatment with HBr@HOAc might give acetyl bromide and pinacolone; hopefully there are no further condensations. This seems unnecessarily laborious, though.

solo - 15-4-2022 at 09:00

.....cyanuric chloride will produce the acetic acid chloride , this with acetic acid will give you acetic anhydride.....Note:the article of cyanuric acid has been posted in the articles of interest in the Reference section.........solo

Fantasma4500 - 16-4-2022 at 06:02

i think i figured out why acetaldehyde has even been mentioned in the first place
it has been confused from acetone-glacial acetic acid reaction where acetone is reacted at high temperature to form ketene- which is a gas, a highly deadly gas, this gas however is bubbled into glacial acetic acid in a totally closed apparatus where it then causes it to turn into acetic anhydride

as i see it, it would be best to have 3 glacial acetic acid traps for the ketene gas as well as ventilation.. ketene may be reacted easily with ammonia gas just as a 100% scrubbing mechanism maybe?

video from dougs lab he explains hydrogen cyanide is dangerous in 50ppm where ketene is 5ppm or even less

so it was never about acetaldehyde, it does use a "ketene lamp" kinda construction, which acetaldehyde also does use, but very different setup!

the ketene in glacial acetic acid is decently attractive, but if one can just wetten anhydrous sodium acetate with glacial acetic acid, and distill that, that would be a lot safer- i didnt quite see anything on temperature of this distillation, i would expect it to be decently high? sodium acetate starts to decompose just over 150*C- so drying out NaAc has to be at most 150*C

Triflic Acid - 27-4-2022 at 13:40

Acetaldehyde with tert-butyl hypochlorite should create acetyl chloride:
"tert-Butyl hypochlorite is a versatile and inexpensive oxidizing agent, that allows the conversion of alcohols to ketones, aldehydes to acid chlorides, sulfides to sulfoxides, and hydroxylamines to nitroso compounds"
-https://www.organic-chemistry.org/chemicals/oxidations/tert-butyl-hypochlorite.shtm#:~:text=tert-Butyl%20hypochlorite%20is%20a,and%20hydroxylamines%2 0to%20nitroso%20compounds.

clearly_not_atara - 28-4-2022 at 06:49

Lots of interesting transformations are possible with tBuOCl. It turns amines to N-chloroimines, aldehydes with sodium azide to acyl azides, allylic carbons to allyl halides, incautious chemists to smoldering piles of ash, etc.

Triflic Acid - 29-4-2022 at 08:03

Well, there is this really annoying chemist who works 3 hoods away from me at work

clearly_not_atara - 17-5-2022 at 19:29

In the "acetonitrile method", is it really necessary to gas HCl? Couldn't you get a similar effect by adding an AcOH solution of H2SO4 or TsOH to a solution of NaCl in AcOH/MeCN? Sodium chloride appears to be somewhat soluble in AcOH, and hopefully MeCN will not crash it out:
https://pubs.acs.org/doi/pdf/10.1021/ja01276a009

Waffles SS - 19-5-2022 at 12:22

In the "acetonitrile method", is it really necessary to gas HCl? Couldn't you get a similar effect by adding an AcOH solution of H2SO4 or TsOH to a solution of NaCl in AcOH/MeCN? Sodium chloride appears to be somewhat soluble in AcOH, and hopefully MeCN will not crash it out:
https://pubs.acs.org/doi/pdf/10.1021/ja01276a009

Maybe this be unclear for this reaction but i had success on synthesis Methyl Phenyl Butenone(Not for P2P) by AcOH solution of H2SO4 in aldol condensation of BzH and MEK instead of gassing HCl.

I think your idea is worth trying just as much as my idea.

[Edited on 19-5-2022 by Waffles SS]

clearly_not_atara - 4-6-2022 at 11:20

https://patents.google.com/patent/US3513180

"A mixture of 60 g. of benzoic acid and 0.5 g. of trimellitic anhydride in a 250 ml. flask was heated to reflux for 6 hours. Water vapor (about 2 ml.) produced by the dehydration reaction was condensed in a Dean-Stark trap. The product consisted of a mixture of benzoic acid and benzoic anhydride."

If Bz2O reacts with NaOAc without a proton donor, I think Ac2O is the most volatile possible product. Benzoic acid is about three times as strong as acetic acid.

Waffles SS - 4-6-2022 at 23:58

https://patents.google.com/patent/US3513180

"A mixture of 60 g. of benzoic acid and 0.5 g. of trimellitic anhydride in a 250 ml. flask was heated to reflux for 6 hours. Water vapor (about 2 ml.) produced by the dehydration reaction was condensed in a Dean-Stark trap. The product consisted of a mixture of benzoic acid and benzoic anhydride."

If Bz2O reacts with NaOAc without a proton donor, I think Ac2O is the most volatile possible product. Benzoic acid is about three times as strong as acetic acid.

As patent mentioned there is no need for NaOAc and Ac2O produced directly from Catalyst and Acetic acid.(I hope this route works because i was unsuccessful in Phthalic anhydride + acetic acid)

Quote:

It has now been found that this objective may be accomplished by carrying out the dehydration reaction in the presence of a small amount of a 1,2-dicarboxylic acid or the corresponding anhydride which acts to catalyze the dehydration of the monocarboxylic acids.

Monocarboxylic acids that may be dehydrated by the process of the invention include saturated and unsaturated aliphatic, alicyclic, aromatic and heterocyclic acids having from about 2 to 18 carbons, as well as derivatives of these acids. Examples are acetic, propionic, caprylic, lauric, stearic, crotonic, oleic, linoleic, cyclopropanecarboxylic, cyclohexanecarboxylic, cyclohexenecarboxylic, cyclodecanecarboxylic, benzoic, toluic, phenylacetic, cinnamic, 1-naphthylacetic, nicotinic and benzofuroic acids.

[Edited on 5-6-2022 by Waffles SS]

clearly_not_atara - 5-6-2022 at 05:06

You are correct, the patent method can work directly with AcOH. But that doesn't lower the reaction temperature of 230-260 C, meaning that you need a pressurized setup. I thought it would be more practical to use benzoic acid at atmospheric pressure for most members.

Waffles SS - 5-6-2022 at 11:28

You are correct, the patent method can work directly with AcOH. But that doesn't lower the reaction temperature of 230-260 C, meaning that you need a pressurized setup. I thought it would be more practical to use benzoic acid at atmospheric pressure for most members.

Thats true.
Monocarboxylic acids with a boiling point of less than 230 degrees do not lead to the formation of anhydride at atmospheric pressure,because below this temperature, mentioned dicarboxylic acid(Phthalic acid) will not lose water and make anhydride itself.

The mechanism of this reaction is interesting for me.
I think each mole of Dicarboxylic acid react with two mole of Monocarboxylic acid and make 1 mole Asymmetric Dicarboxylic anhydride and above 230c 5 members Dicarboxylic anhydride regenerate and lead to formation of carboxylic anhydride.

[Edited on 5-6-2022 by Waffles SS]

clearly_not_atara - 15-6-2022 at 11:23

The preferred catalyst in the patent is trimellitic acid, which is not exactly something everyone has. But I'm pretty sure that it could easily be produced by the action of permanganate on naproxen, which is 2-(6-methoxy-naphth-2-yl)-propanoic acid. Naproxen is also known as Aleve:

That permanganate reacts with naproxen is documented in this M.S. thesis:
https://smartech.gatech.edu/bitstream/handle/1853/33870/gibs...
but the products are not identified. However, it is known that phthalic acid is produced by the action of permanganate on naphthalene:
https://researchgate.net/profile/Bijudas-K/publication/31211...
so it is a natural extension to infer that trimellitic acid is produced by oxidation of the naproxen ring and alkyl chain. Permanganate is also known to oxidize phenylacetic acid:
https://research.monash.edu/en/publications/heterogeneous-pe...

So this catalyst might be very easy to get after all. And the method should scale nicely.

BAV Chem - 9-4-2023 at 15:44

Quote: Originally posted by dicyanin

GB299342 looks interesting as sodium metaphosphates are commercially available OTC. Sodium hexametaphosphate is a common food additive (E452i) and water softener, and sodium trimetaphosphate is used in construction. Calgon according to some sources is sodium polymetaphosphate Na₂[Na₄PO₃]₆ but polycarboxylate to others. Still I've seen 1lb bags of pure sodium hexametaphosphate offered for under 10 euros as a water softener.

The patent from 1927 claims "the production of acetic anhydride from a mixture of sodium metaphosphate, anhydrous sodium acetate, and infusorial earth moistened with glacial acetic acid, the reaction being effected at 150-180°C". Sounds too good to be true, but it could work, and yield wouldn't really be an issue considering the requirements. I've heard mixed results from a similar low-yielding process, heating sodium pyrosulfate with sodium acetate.

That patent (see attachment) also claims that acetic anhydride can be produced by heating a mixture of glacial acetic acid and sodium hexametaphosphate which doesn't work as far as I can tell.

Here's what I did:
127g of anhydrous sodium acetate and 170g of concentrated (~90%) phosphoric acid were thoroughly mixed and refluxed for some hours. I then distilled off the concentrated acetic acid that was formed in the reaction and heated the remaining sodium dihydrogen phosphate to ~700°C in order to convert it into some nice and glassy sodium hexametaphospate. This was broken up and ground to a more or less fine powder. Next all of the hexametaphosphate and acetic acid was refluxed in a round bottom flask for another several hours. During this time the solid really caked to the bottom of the flask (stirring would've helped). Finally I distilled everything to dryness but the vapour temperature never rose above 110°C. Everything I got was some (nearly) glacial acetic acid with a freezing point of 10°C.

Glacial acetic acid should freeze at 16°C
and it's meant to boil at ~120°C.
Acetic anhydride boils at ~140°C.

That doesn't quite mean it can't work at higher temperatures but simply boiling sodium hexametaphosphate in acetic acid doesn't do it. I'm kinda skeptical about the rest of that patent now.

Attachment: GB299342A_Original_document_20230331211754.pdf (1.5MB)
This file has been downloaded 266 times

[Edited on 9-4-2023 by BAV Chem]

ManyInterests - 24-5-2023 at 02:53

I feel like this might be old news for some people here, but I just read this paper on the production of acetic anhydride from phosphorous pentoxide, calcium chloride, and even simply sodium acetate. Of all the organic reagents I've seen, making acetic anhydride seemed difficult due to the difficulty (for me) in obtaining some key reagents, but with this it seems like a no brainer.

https://www.ijraset.com/fileserve.php?FID=24897

Is this true? Is there a downside to this?

[Edited on 24-5-2023 by ManyInterests]

clearly_not_atara - 24-5-2023 at 07:30

P2O5 alone was tried and failed extensively. Using calcium chloride as a cofactor is new, though.

ManyInterests - 24-5-2023 at 07:55

P2O5 alone was tried and failed extensively. Using calcium chloride as a cofactor is new, though.

The word cofactor is new to me. I looked up what it means and I think I got the concept (organic chemistry isn't my focus). But to make it clear, do you think the simple use of anhydrous calcium chloride and glacial acetic acid is sufficient to produce acetic anhydride? As well as possible anhydrous sodium acetate?

Because if that is the case, I can probably make it quite easily without the need for difficult to obtain and/or very toxic chemicals.

solo - 24-5-2023 at 08:13

......this might help...solo

Reference Information

https://www.chegg.com/learn/topic/synthesis-of-acetic-anhydr...

clearly_not_atara - 24-5-2023 at 08:27

CaCl2 alone will not do it. See:

https://pubs.acs.org/doi/pdf/10.1021/ja01852a002

ManyInterests - 24-5-2023 at 08:47

CaCl2 alone will not do it. See:

https://pubs.acs.org/doi/pdf/10.1021/ja01852a002

Yes I forgot, sulfuric acid is involved. But that is something I can easily obtain and have extensive experience with. It is no biggie. I will read that pub and edit this response later.

dettoo456 - 24-5-2023 at 10:59

Quote: Originally posted by BAV Chem

Quote: Originally posted by dicyanin

BAV, I’m not sure if the hexametaphosphate salts would do the trick as described in that patent but metaphosphoric acid would definitely dehydrate GAA to Ac2O, along with most, if not all carboxylic acids to their anhydrides - I can’t find the patent at the moment but o know of at least two old ones from like the early 1900s that mentioned MPA and GAA with some other acyl anhydrides.

I’m not sure if someone else in the thread mentioned it before, but MPA (metaphosphoric acic) is a fantastic dehydrating agent. Magpie demonstrated its ability to generate oleum from 98-% H2SO4 even. You will need a quartz glass setup to run the rxns in though.

The SM thread ‘(HPO3)n Dehydrating agent (polyphosphoric or metaphosphoric acid)’ has some more info and Magpie’s write up is very detailed.

Lionel Spanner - 24-5-2023 at 16:31

I posted this method for acetic anhydride a little while back, while apparently not seeing this stickied thread. It's probably better placed here than in its own thread.
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

tyro - 15-8-2023 at 09:29

Quote: Originally posted by dettoo456

Quote: Originally posted by BAV Chem

Quote: Originally posted by dicyanin

When I read the patent, it seemed to indicate working under pressure to achieve higher temperatures when using the metaphosphate salts and only GAA (middle of page 9). And. In examples 1 and 2, where sodium metaphosphate, sodium acetate, and GAA is used, GAA is "added to the mixture at such speed that [it] appears just moist". Presumably gradual addition of the GAA, as stated in the latter case, would allow for higher reaction temperatures to be achieved at ambient pressure.

It might be worth giving this another shot. I may make an attempt when I have some free time again.

clearly_not_atara - 15-8-2023 at 12:52

Quote:

Previous tests reported in this thread by multiple users show that metaphosphoric acid does not produce any isolable AA under any conditions.

Trimetaphosphoric acid H3P3O9 is a superacid at the first deprotonation (pKa < -10) which catalyzes uncontrolled aldol condensation of carboxylic anhydrides. Whether it could be used for benzoic or pivalic anhydrides might be an interesting question; whether it works for AA has been answered with a resounding "no" and also a "why don't people read the thread before posting this crap?".

tyro - 15-8-2023 at 16:54

clearly_not_atara - Apologies for resurrecting a previously disproven route. I've read through this thread casually a few times, but it's a considerable amount of information to retain.

I feel like it would be useful to have a table or some such which lists discussed routes along with whatever conclusions have been drawn for massive threads like this. Problem is, it would take a fair bit of effort, and I'm not sure how much it would help in the end. Adding a section in the wiki, a Google spreadsheet... etc. Aside from the maintenance of such a document, the key would be to have a good way of raising visibility in all parts of the thread in order to prevent cyclical comments.

I'd be interested in any thoughts you might have on this.

WalterB - 9-12-2023 at 21:15

Hi.
Just wondering. This online synthesis of Ac2O doesn´t work?
Anhydrous sodium acetate, dropping acetyl chloride, kept protected from moisture with calcium chloride tube.. than distilled 2x. of course there is more to do, this but this is the base.

clearly_not_atara - 9-2-2024 at 08:18

I just don't think it's considered interesting. Mostly we're interested in preparing acetic anhydride without the use of the strong dehydrating reagents that require hazmat shipping and attract regulatory scrutiny.

But like the bromine thread, this one has mostly been "solved" by the acetonitrile + HCl method and/or the benzoic acid thermal dehydration (260 C) with catalytic phthalic acid (to benzoic anhydride, therefrom to Ac2O). The remaining interest is further simplifying those methods, like maybe you can use TsOH + NaCl in MeCN + AcOH instead of gassing. Maybe diglycolic acid or adipic acid is easier to use than phthalic. Etc.

EDIT: not adipic, unfortunately: https://pubs.rsc.org/en/content/articlelanding/1927/jr/jr927...

[Edited on 9-2-2024 by clearly_not_atara]

Keras - 9-2-2024 at 10:17

I’m not sure why the SCl route isn't more used. Sure it needs synthesising SCl, but that’s not such a hassle.

clearly_not_atara - 9-2-2024 at 14:22

S2Cl2 is not so bad itself, but it's infamous for ruining any container you keep it in. So you probably want to use it all quickly after you make it. But that's difficult for the on-again, off-again work most of us do.

Loptr - 10-2-2024 at 09:06

I just don't think it's considered interesting. Mostly we're interested in preparing acetic anhydride without the use of the strong dehydrating reagents that require hazmat shipping and attract regulatory scrutiny.

But like the bromine thread, this one has mostly been "solved" by the acetonitrile + HCl method and/or the benzoic acid thermal dehydration (260 C) with catalytic phthalic acid (to benzoic anhydride, therefrom to Ac2O). The remaining interest is further simplifying those methods, like maybe you can use TsOH + NaCl in MeCN + AcOH instead of gassing. Maybe diglycolic acid or adipic acid is easier to use than phthalic. Etc.

EDIT: not adipic, unfortunately: https://pubs.rsc.org/en/content/articlelanding/1927/jr/jr927...

[Edited on 9-2-2024 by clearly_not_atara]

clearlynotatara,

Has the benzoic acid dehydration with phthalic anhydride been discussed on this forum before? I tried to find it using Google to no avail.

That would be interesting because you can recycle the phthalic acid.

[Edited on 10-2-2024 by Loptr]

clearly_not_atara - 10-2-2024 at 17:30

Loptr: It was discussed on the previous page. In order to better keep track of the enormous amount of information in this thread, I have updated the Wiki articles:

http://www.sciencemadness.org/smwiki/index.php/Organic_acid_...

http://www.sciencemadness.org/smwiki/index.php/Acyl_chloride

Perhaps the article "organic acid anhydride" should be moved to "carboxylic anhydride" in order to distinguish it from other anhydrides such as the sulfonic anhydrides, which are prepared differently.

Attached is the reference describing the preparation of acetic anhydride in excellent yields by pyrolysis of silver acetate at 380-400 C in an argon atmosphere. This reaction is widely discussed but poorly understood.

Attachment: kirshenbaum1953.pdf (667kB)
This file has been downloaded 215 times

clearly_not_atara - 11-2-2024 at 22:21

I can't edit that post anymore, but I wanted to add that in the absence of phthalic acid, a high-boiling dicarboxylic acid can probably be obtained by the Diels-Alder cyclocondensation of itaconic anhydride, formed by pyrolysis of citric acid, with alpha-phellandrene, found in dill essential oil. I think this is a simpler than trying to oxidize naphthalene or naproxen. Dill oil is surprisingly popular among the hippies and is about 1/3 alpha-phellandrene; the other components probably won't interfere with the cyclization.

solo - 15-2-2024 at 11:02

Reference Information

Base-Mediated Synthesis of Anhydrides from Activated Amides
Iliyasu Aliyu Bashir and Sunwoo Lee
J. Org. Chem.
2023, 88, 9, 6159–6167
doi:10.1021/acs.joc.2c03098

ABSTRACT:
Symmetrical anhydrides were synthesized from activated amides such as N-benzoylsaccharins and N-Boc-protected benzamides. The activated amides reacted with H2O in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) at 25 °C to produce the corresponding symmetrical anhydrides in high yields through C−N bond cleavage. In addition, N-benzoylsaccharins reacted with benzoic acid derivatives to generate unsymmetrical anhydrides in high yields.

...posted by fcttc from hyperlab

Attachment: Base-Mediated Synthesis of Anhydrides from Activated Amides.pdf (1.7MB)
This file has been downloaded 204 times

greenlight - 22-5-2024 at 10:11

Has anyone on here had any success with the reaction of phosphorus pentoxide and glacial acetic acid?
I know its rare in some countries but I have some phosphorus pentoxide and I gave it a go using that one suspicious paper that everyone refers to where they also make it from calcium chloride "apparently".

Handling the phosphorous pentoxide is a pain, even just transferring it straight from a sealed jar as quickly as possible with a spatula in small portions to the reaction mix was a whole ordeal. Water is so in l9ve with it that it jumps onto it faster then you spell tar.. And indeed tar was the result even with an ice bath under the round-bottom with stirring and the fastest spatula transfer skills which I am sure made a world record.

Regardless, the mixture was refluxed and attempted distillation which just yield unreacted acetic acid and this time tar with some added foaming bubbles.

Does this whole setup have to be run in a dry box and then refluxed with a drying tube on top the of the condenser?

I have an exotic synthesis I've been wanting to attempt for a long time now and have acquired 11 out of 12 reagents with the only thing standing in my way being acetic anhydride :mad:

Any help greatly appreciated

[Edited on 22-5-2024 by greenlight]

clearly_not_atara - 23-5-2024 at 05:48

Has anyone on here had any success with the reaction of phosphorus pentoxide and glacial acetic acid?

No, but around three million people (including you) have failed. My interpretation is that phosphoric anhydride polymerizes Ac2O faster than it dehydrates AcOH.

There is a paper out there about the rxn of P2O5 with CaCl2 giving POCl3 under some conditions: you could just do that and then use POCl3 as normal.

dettoo456 - 23-5-2024 at 06:27

If someone needs the Ac2O for a simple friedel crafts (or some other carboxylic anhydride for that purpose), then PPA + GAA (or PPA + dry propionic, butyric, etc) should work fine on its own. Or, if the Ac2O is to be used for nitrations, just dehydrate WFNA with PPA to N2O5.

I have not seen any instance of amine protection with GAA + PPA though.

Other than that, you will need at least POCl3 like atara said.

Keras - 23-5-2024 at 11:42

No, but around three million people (including you) have failed. My interpretation is that phosphoric anhydride polymerizes Ac2O faster than it dehydrates AcOH.

I’ll try that using DMSO as a solvent. Maybe it can work?

clearly_not_atara - 23-5-2024 at 14:01

No, but around three million people (including you) have failed. My interpretation is that phosphoric anhydride polymerizes Ac2O faster than it dehydrates AcOH.

I’ll try that using DMSO as a solvent. Maybe it can work?

Did you mean to quote the other part of the post? I have attached the paper. It appears that the reaction with NaCl actually begins at a lower temperature (250 C) than with CaCl2 (400 C). This is the opposite of what I would have expected. I think I only read the abstract when I found it the first time. But CaCl2 will have a better solubility in organic solvents. LiCl may be worth considering.

I think P2O5 might form an adduct with DMSO similar to the SO3 adduct. Possibly a less nucleophilic solvent would be good. The bp of propylene carbonate (240 C) is close to the rxn temperature for P2O5 + NaCl found by Tarbutton et al, though I have no idea if the solvent would be stable under reflux with these reagents present.

Attachment: tarbutton1941.pdf (799kB)
This file has been downloaded 160 times

greenlight - 26-5-2024 at 09:58

Thankyou for the replies,
Would it be possible to run the reaction by slowly dripping glacial acetic acid into phosphorus pentoxide which is suspended in dry dichloromethane with stirring?
Phosphorus pentoxide is not soluble in DCM and the glacial acetic acid is miscible as well as acetic anhydride and if it ran successfully the AA could be distilled off afterwards.
The only problem is the maximum temperature that can be achieved is low due to DCM's boiling point ~38°C.

Maybe hexane could work otherwise?

As long as the solvent is not an alcohol and doesn't react with the P2O5 or acetic acid would this be viable as it also keeps the P2O5 from reacting with moisture in the air in the flask and during addition like in the original method.

I am probably overlooking something crucial here as it is late at night but the idea just came to me.

[Edited on 26-5-2024 by greenlight]

paulll - 26-5-2024 at 13:51

Thankyou for the replies,
Would it be possible to run the reaction by slowly dripping glacial acetic acid into phosphorus pentoxide which is suspended in dry dichloromethane with stirring?
Phosphorus pentoxide is not soluble in DCM and the glacial acetic acid is miscible as well as acetic anhydride and if it ran successfully the AA could be distilled off afterwards.

I was contemplating trying something like that until I realised that the theoretical maximum yield from all 100g of the P2O5 I had on-hand, was approximately frig-all squared.

Keras - 27-5-2024 at 00:10

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.

greenlight - 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.

Keras - 27-5-2024 at 11:05

@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!

Keras - 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… :/

Fery - 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.

Keras - 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.

clearly_not_atara - 11-6-2024 at 06:19

Did you get a headache during the procedure?

Keras - 11-6-2024 at 10:41

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?

clearly_not_atara - 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.

Keras - 11-6-2024 at 11:36

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.

greenlight - 30-6-2024 at 09:30

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… :/

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]

Fery - 30-6-2024 at 21:26