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Author: Subject: Preparation of Acetic Anhydride
Magpie
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[*] posted on 9-12-2010 at 08:52
Preparation of Acetic Anhydride


Preparation of Acetic Anhydride


by Magpie


12/9/10



Introduction
This procedure is for the preparation of ~ 10 mL of acetic anhydride (Ac2O) using sulfur, bromine, and anhydrous sodium acetate (NaOAc). It is based on the 4/29/09 post of benzylchloride1 of the ScienceMadness forum.

Preliminarily, disulfur dibromide (S2 Br2) is formed:

2S + Br2 --> S-Br-Br-S

Then, Ac2O is formed as follows, according to Thorpe (ref 1):

2 Ac2O eqns.gif - 6kB

Orshansky and Bograchov (ref 2) indicate instead the following reaction:

8CH3OONa + S +3Br2 = 4(CH3O)2O + 6NaBr + Na2SO4

The reactions of Thorpe, however, are supported by the observations of this author of free sulfur and a liberated gas.

CAUTION
S2 Br2, a red, fuming liquid, is produced during this preparation. It has a terrible sulfur stench and is toxic. SO2 and hot Ac2O vapors are also noxious. Therefore, this procedure must be conducted outside or in an efficient fume hood.

Full safety gear is recommended, ie: eye goggles, thick rubber gloves, long-sleeved shirt, and a lab apron.

Procedure

A. Equipment Set-Up
Secure an angled 3-neck 500 mL round bottomed flask (RBF) to a ringstand. Attach a pressure-equalizing addition funnel to a side-neck. Install a reflux column equipped with a CaCl2 tube in the other side-neck. If available, install a mechanical stirrer in the center neck. If not, install a plug. All glassware is to have ground-glass fittings, coated with a thin film of silicone grease, and clamped.

Provide cooling water for the reflux column.

B. Reagent Preparation
Weigh out 66.5g (0.81 moles) of anhydrous sodium acetate and set aside in a covered beaker. Weigh out 3.5g (0.11 moles) of sulfur in a beaker and set aside. Have available 20 mL (0.39 moles) of dried Br2. (Note 1)

Note that bromine is the limiting reagent. The sulfur, except that released as SO2, is recycled in situ.

C. Ac2O Generation
Temporarily remove the center plug of the 500 mL RBF and pour in the anhydrous sodium acetate using a powder funnel. Reinstall the plug. Turn on the cooling water.

Pour the 20 mL of dried bromine into a 100 mL beaker. Slowly mix in the sulfur with the bromine using a stirring rod. When the resultant S2 Br2 + Br2 is homogeneous, pour it into the pressure-equalizing addition funnel (valve closed!) and install the plug in the fill port.

Slowly dribble in the S2 Br2 + Br2 onto the sodium acetate powder while stirring continuously mechanically, or intermittently by use of a stirring rod inserted though the center neck. Replace the plug after any manual stirring. Achieving a homogeneous mix may be somewhat difficult. (See Discussion below.) Keep stirring after all of the S2 Br2 has been added. If necessary, remove the pressure-equalizing funnel and manually stir via the side port. Close the port with a plug when not in use. When the reaction finally takes off the products will turn into a creamy slurry. Considerable heat will have been generated, bringing the slurry temperature as high as 80°C. Continue to stir for about 20 minutes. The light yellow color of the slurry is due to the presence of the elemental sulfur byproduct and indicates that the Ac2O formation is nearing completion. Continue stirring as the slurry cools to room temperature.

D. Vacuum Distillation
Set up for vacuum distillation of the Ac2O using the 3-neck 500mL RBF. Place an insulating blanket over the RBF. With a vacuum of 23”Hg (absolute pressure = 178 mmHg) the Ac2O comes over at about 72°C. Yield of the crude Ac2O will be ~30 mL.

E. Simple Distillation
Set up for simple distillation of the crude Ac2O at atmospheric pressure. Use a 50 mL RBF as pot. Collect 2 cuts. The first cut (~15 mL) will come over at about 128-135°C. The 2nd cut (~10 mL) will come over at 135-143°C. Literature value for the bp of Ac2O is 140.0 °C. Yield, based on the 10mL of cut 2 and the amount of Br2 charged, is ~27%.

Cleanup
Cleanup of the 500mL RBF should be straightforward with very little strong sulfur smell. There will be NaBr crystals and some minor char in the bottom of the flask. This residue will wash out easily with hot soapy water.

Confirmation of Identity
There are several qualitative tests that can be used for the confirmation of acid anhydrides (Note 2). One that is particularly dramatic is the formation of acetanilide upon mixing of Ac2O with aniline. The acetanilide can then be re-crystalized and a melting point determined (113-114°C).

Discussion
The reactants can initially be difficult to mix following the addition of the S2Br2 to the dry NaOAc. This appears to be why some variants of this method, with either Br2 or Cl2, also use additional amounts of the anhydride product to thin the mix of reactants. Do it on a small scale, then take the crude anhydride and use it as a 'solvent' for a larger run.


Notes
1. The use of anhydrous reagents is important. S2 Br2 decomposes in water. If the dryness of the NaOAc is in doubt it should be fused per the directions found in Vogel’s Practical Organic Chemistry, 3rd Ed, Section II, 50, 9, p. 197.
2. See: http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.h...

References
1. Thorpe’s Dictionary of Applied Chemistry, vol. 1, Revised Edition, 1921, p 28. (Thanks to Polverone for this interesting and obscure information.)
2. “Laboratory Method for the Preparation of Organic Acid Anhydrides,” by Jehuda Orshansky and Eilahu Bograchov, Chemistry & Industry, 44, November 4, 1944, p. 382. (Thanks to Sauron for pointing this out in the Rhodium archives.)
3. Thanks to not_important for the tip about adding preliminary product anhydride to the NaOAc as solvent to facilitate reactant mixing.

[Edited on 9-12-2010 by Magpie]

[Edited on 11-12-2010 by Magpie]




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[*] posted on 10-12-2010 at 15:12


Good work! So this method is confirmed as working, though with relatively low yield.
The sulfur dissolves rapidly in the bromine, doesn't it? I tried this out myself a long time ago, but was discouraged by the very broad boiling range of the crude product.
Are you planning to try out the Na acetate + SO2 + Cl2 method as well?

Did you make the anhydrous sodium acetate yourself? If yes, tell me more.




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[*] posted on 10-12-2010 at 16:18


Quote: Originally posted by garage chemist  
Good work! So this method is confirmed as working, though with relatively low yield.
The sulfur dissolves rapidly in the bromine, doesn't it? I tried this out myself a long time ago, but was discouraged by the very broad boiling range of the crude product.
Are you planning to try out the Na acetate + SO2 + Cl2 method as well?

Did you make the anhydrous sodium acetate yourself? If yes, tell me more.


Yes, this method has been confirmed by me several times and I believe by benzylchloride1 also.

Yes, the S2Br2 formation is rapid and without challenge.

I have no plans to try out any other method as I was only interested in getting small amounts for other experiments.

I bought the anhydrous NaOAc off eBay. But I understand entropy51 has a facile method for making his own.

The real challenge is just getting everything to continue mixing after the S2Br2 is added to the NaOAc. I have a mechanical mixer with ptfe bearing, shaft, and prop. But I still have to dig in with glass stirring rods from the side ports. Once the reaction takes off and the mix goes creamy there's no more problems.




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[*] posted on 10-12-2010 at 16:40


The mixing problem appears to be why some variants of this, with either Br2 or Cl2, also use additional amounts of the target product to thin the mix of reactants. Do it on a small scale, then take the crude anhydride and use it as a 'solvent' for a larger run.

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[*] posted on 10-12-2010 at 19:28


Quote: Originally posted by not_important  
The mixing problem appears to be why some variants of this, with either Br2 or Cl2, also use additional amounts of the target product to thin the mix of reactants. Do it on a small scale, then take the crude anhydride and use it as a 'solvent' for a larger run.


I agree, although I have not yet tried this. I will add this note to the procedure.




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[*] posted on 20-12-2010 at 02:03


Congratulations on your succes! This is a very useful reagent! It's of no use for me because I have plenty of Ac2O, but I still like to see that it worked.

What is the first fraction of 15mL that came over?
I asume this is also acetic anhydride, maybe some azeotrope?
Test this material by adding a mL to a mL of water. It should not mix and form two layers, but react when very quickly when heated to boiling. Or do the test by reacting it with aniline.
It also might be acetic acid. What kind of sodium acetate did you buy? From a supplier selling plastic bags of the stuff for use in the experiment 'Hot Ice'? Maybe he mentioned it as anhydrous, but it might be partially or fully hydrated, and not truly anhydrous as the anhydrous material is not needed for the Hot Ice experiment and is more expensive, so it might be a typo on eBay. Or is it a reagent grade material?
You can maybe test how much water it contains by putting a weighed amound in the oven, and weighing it again after it has fully dehydrated in the oven.


[Edited on 20-12-2010 by Jor]
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[*] posted on 20-12-2010 at 07:14


Thank you Jor. No, I have not done any research on the nature or usefulness of the discarded fraction. I was only interested in getting small amounts of Ac2O for some planned experiments.

The NaOAc was, as you say, obtained off eBay in a plastic bag. The seller claimed it to be anhydrous. It is very white and free-flowing. I can't remember if it was advertised as "hot ice" material. Much of it is.

[Edited on 20-12-2010 by Magpie]




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[*] posted on 21-12-2010 at 01:38


Small note for anyone who needs dehydrated NaAc.

Any remaining water can be removed using microwaves.
The material doesnt heat up when there is no water left.




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[*] posted on 21-12-2010 at 06:55


Good idea; sure beats fusing the stuff.

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[*] posted on 28-12-2010 at 10:50


Quote: Originally posted by User  
Small note for anyone who needs dehydrated NaAc.

Any remaining water can be removed using microwaves.
The material doesnt heat up when there is no water left.


NaOAc. Very different than NaAc (Which I'm not even sure would be a stable compound).

Are you sure that it ceases to heat? I can tell you from experience that CaCl2 and MgSO4 both do continue to heat. The MgSO4 got so hot that the glass dish it was in melted on the bottom.




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[*] posted on 29-12-2010 at 10:10


Sorry for being unclear, I tested this with NaOAc (sodium acetate).

Yes I can confirm this from multiple tests, the material when dry did not heat whatsoever, tests where performed in a keramic dish.
Also no charring occured.
Specific runtimes/wattage I cannot remember for I dont have my notes nearby.
My guess would be that I fired it up in 30 to 60 sec cycles.

Be aware that when it is heated and still contains water superheating can occur.
Also it will start to bubble and thus grow in size dramatically.

Don't hessitate to ask in case I forgot anything.




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[*] posted on 31-12-2010 at 00:02


Can acetic acid be used instead? Then it might be easier to recover the bromine by electrolysis.



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[*] posted on 4-3-2011 at 16:57


Waffles SS recently made a comment about the the poor yield of this procedure, ie, ~27%.

Orshansky and Bograchov (see reference 2 of my procedure) reported a yield of 87.5%. However, their procedure was significantly different than mine in 2 respects, ie:

1. Their scale was 6.7X that of mine.
2. They used 50g of acetic anhydride as solvent.




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[*] posted on 4-3-2011 at 22:17


Did you try S2Cl2 method?

[Edited on 5-3-2011 by Waffles SS]
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[*] posted on 5-3-2011 at 08:42


Quote: Originally posted by Waffles SS  
Did you try S2Cl2 method?


No, for the same reason that Orshansky and Bograchov chose not to: bromine is easier to work with than chlorine.

[Edited on 5-3-2011 by Magpie]




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[*] posted on 5-3-2011 at 22:24


May you give us some pictures of this method?
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[*] posted on 6-3-2011 at 08:40


Quote: Originally posted by Waffles SS  
May you give us some pictures of this method?


See: http://www.sciencemadness.org/talk/viewthread.php?tid=9096&p...




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[*] posted on 14-11-2012 at 08:14


could a swap work here, using easily made propionic anhydride, and glacial aceetic?
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[*] posted on 14-11-2012 at 08:24


you'll end up with the mixed anhydride as a serious byproduct.
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