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Author: Subject: Acetic anhydride preparation
stygian
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[*] posted on 6-12-2005 at 17:41


Yes, as the propionic procedure (only substituting acetate for propionate) does say to 'add, with stirring, keeping the temp under 50C'
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[*] posted on 6-12-2005 at 20:16


Garage chemist wrote:


Quote:

We need to focus on different procedures. Back to the drawing board.


"Left are:

1. the ketene process
2. the N2O4 + acetate process
3. the S2Cl2 process"

Did you purposely leave out the "acetyl chloride + acetate" process? Is acetyl chloride disallowed?




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[*] posted on 6-12-2005 at 20:52


Maybe not disallowed, Magpie, but for me, I don't see the point of making acetic anhydride if you have acetyl chloride, and vice-versa. They are for the most part interchangeable in syntheses...

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[*] posted on 6-12-2005 at 21:19


Ok, so, because gc decided to totally ignore the procedure partway through, we should focus on different methods? o-kay.
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[*] posted on 6-12-2005 at 22:54


OK sparkgap. I thought it was something like that but I just wanted to make sure. Are we then just trying to make a powerful reagent without using the holy grail of organic chemistry? ;)



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[*] posted on 7-12-2005 at 03:33


Sorry, it seems like I haven't expressed myself correctly.
Of course everyone can try the bromine/sulfur process if he wants, maybe someone has more luck. I just stopped further research on this because I don't like the method and the product is apparently impure with bromine compounds.
The procedure for acetic anhydride didn't specify cooling, nor did the report of the successful preparation. It also didn't say to add the bromine-sulfur miy gradually. Maybe that's important.
Also there's the problem of the Ac2O addition on the beginning, which we aren't able to do.

I left out the acetyl chloride process because acetyl chloride is just as hard to prepare as acetic anhydride and, like sparkgap said, it is interchangeable with the anhydride in syntheses.
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[*] posted on 10-12-2005 at 01:22


Quote:
The synthesis yields something which looks like Ac2O and distills at the same teperature as Ac2O but is not Ac2O.


Boiling point is quite heavy evidence of right product.. I still think that you've got Ac2O.
Will Ac2O really evolve heat when mixed with water, specially in less than 5g quantity? It suppose to hydrolise quite slowly.

Easier would be to take a known quantity, reflux for some time with water and titrate the formed acetic acid.
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[*] posted on 10-12-2005 at 02:18


This is what Practical Methods of Organic Chemistry (Gattermann) says about detecting acetic anhydride:

Quote:

5 c.c. of water are treated with 0,5 c.c. of acetic anhydride. The latter sinks to the bottom and does not dissolve even on long shaking. If the mixture be warmed, solution takes place.


Quote:

2 c.c. of alcohol are added to 1 c.c. of acetic anhydride in a test-tube, and heated gently for several minutes. It is then treated with water and carefully made slightly alkaline. The acetic ester can be recognised by its characteristic pleasant odour.


Quote:

Add 1 c.c. of aniline to 1 c.c. of acetic anhydride, heat to incipient ebullition, and then, after cooling, add twice the volume of water. The crystals of acetanilide separate out easily if the walls of the vessel be rubbed with a glass rod.
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[*] posted on 10-12-2005 at 05:02


Ah, thanks a lot!

Good that I saved the product from the last try.
I honestly did hear the exact opposite of the first test, namely that acetic anhydride reacts violently with water.
Seems like there's still a chance that I have gotten Ac2O.
I'll boil some of this longer with ethanol and look if I get some ethyl acetate.
I also have aniline, which I'm going to use for the acetanilide test.
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[*] posted on 17-1-2006 at 07:48


I told madscientists opinion, the method oxidizing acetic acid beside other methods shoult walk, but in oxygen free atmosphere i think it`s not necessary.
A metal or metal oxide catalyst should walk, copper, chromium ... and the the acetic acid/acetic acid anhydride should passed often thrue the catalyst.

A suggestion is:

Place the acetic acid in a beaker, the beaker is connected with a claisen to a long glas-tube with the catalyst inside (glas wool, metal oxide) and a another beker is connected with a claisen to the end of the glas-tube. The acetic acid is destilled over the bp. and the tube with the catalyt is heated > 140 gedree celcius until the liquid is passed over. The procedure is repeated often until the acic acid is oxidized. (purifying the anhydride by destilation over the bp. when the rest of the aa content is removed by destillation < 138 gegree celsius.)

I think the method should walk.



[Edited on 19-1-2006 by Madandcrazy]
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[*] posted on 17-1-2006 at 09:42


Madandcrazy, that would just give Ketene and not Ac2O.
Also 140°C are not enough, by far.

As to my product: I repeated the ester formation with an excess of Ethanol, and the smell of ethyl acetate is only faint, even after 10 minutes of boiling.
Pouring into water gives no ester at all.

Also, someone who uses to work with Ac2O smelled my product, and said that it was highly impure since the Ac2O he handles smells much different.


For Ac2O I am now going the same route as with carbon disulfide: I am trying to order it (250 ml).
It is a "watched" chemical where I live, but its possession is not forbidden.
I will have to fill out a lot of paperwork and send a copy of my personal identity card, give information about my type of education and so on.
This is more of an "experiment" to find out if you will actually get Ac2O as a private person if you fill out all of the papers or if this is just "bluff" to deter most people from ordering and you actually will never get it.
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[*] posted on 17-1-2006 at 20:57


Has anyone actually performed the bromine path to anhydride or is that just theory? it seemed kind of vague - it mentioned keeping the flask in cold water so im guessing you need to keep the reaction slow and cooled - the rest seems pretty easy - make some bromine - dry sodium acetate in an oven - melt some sulfur and crumble into the mix - fraction distill - is the anhydride absolutely needed to start with?
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[*] posted on 19-1-2006 at 07:22


garage chemist, i meant the oxidation of acetic acid is surely possible with this method between the boiling point, the decompositions point of the acetic acid and the acetic acid anhydride. The temperature is not precise know by me.
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[*] posted on 19-1-2006 at 15:14


Why would you want to oxidize acetic acid? The goal is acetic anhydride, meaning you've removed a water molecule, not like, acetic...oxide...or something. Besides, oxidizing the most active(?) part would just give CH4 or CH3OH, and CO2.

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[*] posted on 22-1-2006 at 07:55


Read in the forums for removing water of delute acetic acid to obtain glacial acetic acid. Oxidizing of acetic acid is only a easy way ;) when no chemicals available, i think rather than
a industrial method.


[Edited on 22-1-2006 by Madandcrazy]
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[*] posted on 10-2-2006 at 04:31


I finally received my order with acetic anhydride from a chemical supplier today (250ml).

And bad news: it smells very different than my product from Br2+S+NaOAc. The AA from the supplier smells very pungent and strongly stings in the nose.

I'm going to make some nice syntheses with it, the first one being cinnamic acid (with benzaldehyde, which I also got in the order today).
The next thing could be acetylcellulose.
And when I am successful with my synthesis of salicylaldehyde, I'm going to make cumarin (is said to smell very nice).
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[*] posted on 12-3-2006 at 19:17


Anyone have any experience with this method of Acetic anhydride synthesis from Dick's encyclopedia.
Mix in a glass retort, well fused acetate of potassa with half its weight of chloride of Benzoyle, apply gentle heat, collect the liquid that distills over.
Interested in opinions.
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[*] posted on 13-3-2006 at 07:56


Quote:
Originally posted by learner
Anyone have any experience with this method of Acetic anhydride synthesis from Dick's encyclopedia.
Mix in a glass retort, well fused acetate of potassa with half its weight of chloride of Benzoyle, apply gentle heat, collect the liquid that distills over.
Interested in opinions.


This will produce acetyl chloride, since there is an equilibrium between sodium acetate + benzoyl chloride and acetyl chloride + sodium benzoate.
Because the acetyl chloride has a low boiling point it can be constantly distilled off, shifting the reaction to the right.

Reacting this acetyl chloride with anhydrous sodium acetate will make the desired acetic anhydride.

However, maybe the proportions in the above preparation are adjusted to 2 mol NaOAc and 1 mol C6H5-COCl, this should directly give acetic anhydride since the acetyl chloride would react further with the excess of sodium acetate.
Though the so- produced acetic anhydride would contain acetyl chloride. It should be fractionated in order to get rid of this.
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[*] posted on 19-3-2006 at 22:12


In JCS 18, 21 (1865), the author puts 20 grams of dried Pb(II) acetate in each of several glass tubes and adds enough CS2 to produce "on agitation with the acetate, a mixture of a creamy consistency. The tubes being then rather more than one-third full, were sealed and heated in the oil bath, to a temperature of 165C. The mixture quickly assumed a black colour, and after some hours heating, one of the tubes, by exploding with great violence, gave warning that a large amount of gas had been liberated. After this it was found advisable to open the tubes once a day, in order to let the carbonic acid escape; and with this precaution the digestion was continued, till it was found that that on opening the tubes, only a slight rush of gas took place."

The liquid was decanted, and the wet PbS was distilled to dryness, the distillate combined with the mother liquor, and the whole distilled. CS2 comes over, then "some acetic acid, with a trace of acetone", and the remainder distilled at 137C.

The author, in his only article published by the RSC, does not give an exact figure nor the most vague hint of yield. This cannot be due to a spareness with words. He also says "Experiments with acetate of silver gave a similar result, with even greater readiness."

http://dx.doi.org/10.1039/JS8651800021
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[*] posted on 15-4-2006 at 13:26


"I'm not sure, but I think that it's possible that I prepared acetic anhydride by accident today. I mixed 164g CH3COONa (contaminated with a small amount of NaHCO3, around 1-2g) with 75g 94% H2SO4. I then poured that into a flask; began heating it, and condensing the vapors (typical distillation). I noticed a very strange, sickly-sweet odor; very difficult to describe. I got a whiff of a very small amount of it, causing me to choke for a few moments. The condensed liquid (which I got 22mL of, if I remember correctly) was still liquid, showing no signs of imminent freezing, at -10C. Now, if that had been acetic acid, it would have frozen at a far higher temperature than that... acetic anhydride, on the other hand, would not freeze until the temperature was FAR lower. I'm postulating that the following reactions were occuring, forming at least a fair-sized quantity of acetic anhydride (there is probably a significant amount of acetic acid remaining). Keep in mind that there was a slight stoichemical excess of sulfuric acid."


I just got done distilling from a mixture of nearly anhydrous sodium acetate 47 grams worth, and about 50 ml's conc. sulfuric acid (30mls added at first then 20mls more to help dissolve the rest of the sodium acetate), I collected about 45mls of distillate, and then I added about 35mls of conc sulfuric acid to that and distilled it again. I got about 25-30mls of distillate that time, the distillate I was thinking was GAA, but I put it in the freezer and it got down to 0 Celsius and it is still very runny and looks like water at room temp, it has a horrible pungent smell at that temp, also from the last distillations I had about 50mls left of the sulfuric acid mix, this was an even darker color then when I started out with, and my H2SO4 is pretty pure, its a brownish tan color, now it is a black color.

To get a better understanding of what this strange compound may be, I took a small pipette and dripped a small amount into a test tube full of water, thinking if it was acetic anhydride it would form a little bit and sink to the bottom, but when the drop went into the water, it formed a solution after a while. as it was dissolving it looked really strange, it was mixing with the water but you could see where it was and see the current in the test tube.
I also added a drop to a tiny bit of sodium bicarbonate and of course, it bubbled

I don't think it is acetic anhydride, but I know it is not acetic acid and if it is it has something else mixed with it, my question is what on earth could this other compound possibly be?

Also when I was doing the distillation I did not use a thermometer and I don't know what temp it got to, but i was carefully and made sure I didn't distill over to much, meaning it couldn't have gotten to hot for the H2SO4 to let off its water.

From both are observations, the acetic acid is definitely turned into some other compound, isn't it?

[edit] It is now -5*C and some crystals are forming so this may be a mixture of GAA and acetic anhydride or another compound

[Edited on 15-4-2006 by kclo4]




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[*] posted on 15-4-2006 at 14:43


Keep it cold, let the crystals grow, then pour off the liquid and do some more tests on both. Can you fractionally distill it maybe?

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[*] posted on 15-4-2006 at 15:36


This is interesting, I have to try this out too and distill the resulting strange liquid over a column.
Or better redistill over some more anhydrous sodium acetate. This is listed as a method to remove acetic acid impurities from acetic anhydride. Of course it won't work when there is a large amount of GAA in there.

Note that AA smells very different than acetic acid! It has an own, peculiar smell, it also stings in the nose and upper throat about three seconds after smelling it.
GAA just horribly reeks of vinegar, and irritates the hell out of the nose instantly.

Does AA form an azeotrope with GAA?

EDIT: To separate GAA and AA, one could add it to some anhydrous sodium carbonate. No protons here, only in the GAA, which will react with the Na2CO3, but the AA will not react.
Keep adding Na2CO3 until the fizzing ceases, then redistill.



[Edited on 15-4-2006 by garage chemist]




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[*] posted on 15-4-2006 at 17:14


Na2CO3 + 2CH3COOH =CH3COONa + H2O + CO2
The reaction makes water, and then that would react with acetic anhydride, to form more acetic acid, correct?

would chloroform extract only the acetic anhydride and leave the acetic acid

Perhaps H2SO4 and acetic acid could be added, and then add chloroform, the chloroform would take the acetic anhydride out of the reaction so then no equilibriums can form with acetic acid, water, sulfuric acid, and acetic anhydride present


now my liquid has been in the freezer for half the day and is -13 C and has the constancy of about pudding, there are crystals and liquid
I opened the reagent bottle and smelled the it for a little while, after around 3seconds of not smelling hardly anything other then a little sting here and there but nothing really, it seemed to all of a sudden have a burning smell, like sniffing in something like maybe say hydrogen chloride gas, it just add a very sharp sting to it. I am pretty sure that my distillate has a low conc of acetic anhydride in it, for most of it is frozen, how ever like I had said earlier I stopped distillation so the hydrated sulfuric acid didn't decompose and let off water. The boiling point of acetic anhydride is much higher then that off acetic acid only a little bit anhydride should have came over, and so it did!

Now I do not know at what temp H2SO4 lets off water, but the boiling point of acetic anhydride is something like 285deg F, and i think sulfuric acid should have let off quite a bit of water by then, so the reason that H2SO4 "doesn't" work to make acetic anhydride from acetic acid is that the reaction: 2CH3COOH = (C2H3O)2O + H2O is shifted to the left when it is heated, and only a little bit of acetic anhydride can make it over. so possibly as vulture was saying maybe add anhydrous MgSO4 to it, to join dehydrating powers to distill more acetic anhydride

Is this hard to understand? I hope I am explaining it good enough!

so the way I am seeing it is that you need to take the acetic anhydride out of the H2SO4 solution, with out using heat.
How might this be done? Chloroform, this should be a good way to extract the anhydride, for it could be done at low temperatures, possible lower then the freezing point of sulfuric acid, the acetic anhydride would change the melting point i would think. maybe a possible way to tell if anhydride is present is to take GAA and it to H2SO4 and see if it will freeze, in the freezer, if it does not, then there would be acetic anhydride in it

tomorrow I am planning on taking the ~30mls of distillate and add it to about 40mls of conc H2SO4 and distill it again, and also add a small amount to chloroform and then take the chloroform out, and let it evaporate off and see what is left!

But what I am saying may just be baloney, for that if acetic anhydride is present in a mixture of sulfuric acid and acetic acid, why would they just make acetate plastic this way, instead of using acetic anhydride? But then again it doesn't mean it will not work this way, now does it?



I also have a question, would ethyl acetate work to extract acetic anhydride in the situation that H2SO4 and GAA are mixed, saying that acetic anhydride is present, or would it disolve?

[Edited on 16-4-2006 by kclo4]




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[*] posted on 16-4-2006 at 02:00


You are right, reaction with Na2CO3 produces water. Sorry about that, it was too late yesterday evening, and I couldn't think clearly.

Hmm, I doubt that adding it to H2SO4 and redistilling will do any good. Pure acetic anhydride reacts with H2SO4 in a nasty way, forming some crazy useless compound of H2SO4 with AA. Someone here tried it out in hope to make Oleum, and it didn't work, it just got hot and red and it was a waste of AA.

Your best bet would be a simple redistillation, without anything added. The boiling points of GAA and AA are far enough apart to allow efficient separation with a column or even without a column if you distill slowly enough and constantly watch the thermometer.

Alternatively, try adding a few ml of the mix to an excess of icecold water, stirring gently, and allow to settle. AA, if present, will sink to the bottom as an oil and can be extracted with chloroform. Drying the chloroform with CaCl2 and redistillation should afford some purer AA.
Don't use too little water, or the acetic acid will act as a cosolvent and make the AA dissolve.

Don't use H2SO4 here, it will react with your AA!




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[*] posted on 16-4-2006 at 02:08


ok, why not just add chloroform to the GAA/acetic anhydride mix?
it should leave the acetic acid on top and take out the anhydride

What do you think about adding chloroform, GAA and H2SO4 to each other, shaking them and what not, and then taking out the chloroform and distilling it off to leave behind the anhydride?

LOL i waiting till 4 in the moring for a reply tonight haha so if i sound like a moron thats why!




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