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Author: Subject: Acetic anhydride preparation
njl
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[*] posted on 1-5-2021 at 14:15


Yes I believe in earlier pages of this thread. Sometimes a pain to read through, but I remember it being discussed. Also, 1000th post :cool:



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[*] posted on 1-5-2021 at 22:33


Quote: Originally posted by njl  
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.
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[*] posted on 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.




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[*] posted on 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.



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[*] posted on 2-5-2021 at 22:30


Quote: Originally posted by njl  
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]
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[*] posted on 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.



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[*] posted on 3-5-2021 at 08:54


Quote: Originally posted by njl  
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]
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[*] posted on 3-5-2021 at 18:22


Quote: Originally posted by Keras  
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...




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[*] posted on 3-5-2021 at 22:20


Quote: Originally posted by njl  
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.


Wikipedia reports the yield of this reaction to be very low, typically under 40%.
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[*] posted on 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.



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[*] posted on 4-5-2021 at 08:02


Quote: Originally posted by njl  
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?).
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[*] posted on 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]




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[*] posted on 11-5-2021 at 11:30


Quote: Originally posted by njl  
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 :)


IMG_0814.JPG - 1.5MB

[Edited on 11-5-2021 by Keras]

IMG_0813.JPG - 1.6MB

[Edited on 11-5-2021 by Keras]
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[*] posted on 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.
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[*] posted on 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.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 13-5-2021 at 23:24


Quote: Originally posted by clearly_not_atara  
What kind of HCl generator makes sodium bisulfate?


NaCl + H₂SO₄ → NaHSO₄ + HCl

Quote: Originally posted by clearly_not_atara  
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.
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[*] posted on 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]
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[*] posted on 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.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 18-5-2021 at 14:36


Quote: Originally posted by Keras  
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]

Quote: Originally posted by clearly_not_atara  
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]




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[*] posted on 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.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 18-5-2021 at 21:28


Quote: Originally posted by S.C. Wack  
Quote: Originally posted by Keras  
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.
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[*] posted on 18-5-2021 at 21:38


Quote: Originally posted by clearly_not_atara  
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.

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[*] posted on 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...




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 19-5-2021 at 08:47


Quote: Originally posted by clearly_not_atara  
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...


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
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[*] posted on 19-5-2021 at 14:47


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

Quote: Originally posted by clearly_not_atara  
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]




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