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teodor
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I'm sorry if this question was already asked, this is quite a long thread. Where can I get the information regarding boiling points of AcOH/Ac2O
mixture? If it can be calculated somehow based on physical parameters of each ocompound please enlight me.
Update: already found https://doi.org/10.1021/je60012a004
Thanks who bothered to read my message.
[Edited on 20-3-2025 by teodor]
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Waffles SS
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Quote: Originally posted by Waffles SS  | | Quote: Originally posted by clearly_not_atara | | 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.
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another interesting patent about synthesis of anhydrides from dicarboxylic anhydride.
Patent suggested Glutaric anhydride but another anhydride tested too.
Attachment: US_2021347722_A1.pdf (1MB)
This file has been downloaded 38 times
Chemistry = Chem + is + Try
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clearly_not_atara
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That sure is a fancy process. Let me see if I understand the approach here.
Normally, if you try to equilibrate acetic acid with glutaric anhydride and distill conventionally, you run into a bad case of Le Chatelier's
principle. Acetic acid is the most volatile component, so it boils first, and the equilibrium shift causes more acetic anhydride to decompose. Even if
you can make Ac2O, you can't get it out of the reaction mixture efficiently.
Instead, you use flash distillation, as a flow process, which distills the mixture faster than the equilibrium can establish itself. This allows you
to separate a fraction containing primarily Ac2O and AcOH and a second fraction containing primarily glutaric acid and its anhydride. These can then
be separated by conventional methods. Everything is reused except acetic acid, so the economics are basically optimal.
This is pretty cool, but I don't have any idea how to set up a flash distillation on the tabletop. In principle, it shouldn't be harder than a ketene
lamp, and you could definitely make a whole lot of Ac2O if you actually built this thing.
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MrDoctor
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based on what i read, some sort of complex is forming that decomposes to produce a low percentage of anhydride, or perhaps not but, ultimately
yielding a low output of anhydride per individual step or batch, and what seems to be important is the use of a vacuum sufficiently hard enough to
prevent the acetic anhydride from hydrolyzing, so this follows a similar mechanic that was mentioned at some earlier point in this thread where
phosphoric acid of some dehydrated state was suggested, if only one could overcome the issue of the anhydride forming alongside water under conditions
that would rapidly cause it to hydrolyze back together before any sort of separation could occur.
read [0046] as well as the example process given right under the table.
the mixture of acetic acid and the glutaric anhydride are heated together for 15 minutes before being distilled under a strong vacuum, i would
probably describe this as a flash boil. What isnt clear to me is how long the two need to remain in contact.
I do also have an idea for how one might run this continuously, vaguely. what immediately comes to mind is the idea of putting a chunk of glass frit
inside a tube and then melting the tube and clamping it around the frit. using a strong vacuum line if one were to draw up the "transanhydrization"
mixture, at say, 100C, it would flow through the frit very slowly, and boil aggressively as it reaches all the way through. furthermore it would be
trivial to use a heater of some sort to maintain 100C in the tube itself. the image i thought of was the capillary valve in an air conditioning
system, where theres high pressure on one side and low on the other, using a capillary to allow a differential.
If silicone tubing is compatible with acetic acid and the anhydride, you could move the fluid around from reactor to reactor using peristaltic pumps
to produce the series of reactions it asks for to get up to 20% or so conversion.
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