Sciencemadness Discussion Board - Preparation of Acetic Anhydride

Preparation of Acetic Anhydride

Magpie - 9-12-2010 at 08:52

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):

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]

garage chemist - 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.

Magpie - 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.

not_important - 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.

Magpie - 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.

Jor - 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]

Magpie - 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]

User - 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.

hissingnoise - 21-12-2010 at 06:55

Good idea; sure beats fusing the stuff.

UnintentionalChaos - 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.

User - 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.

tnphysics - 31-12-2010 at 00:02

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

Magpie - 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.

Waffles SS - 4-3-2011 at 22:17

Did you try S2Cl2 method?

[Edited on 5-3-2011 by Waffles SS]

Magpie - 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]

Waffles SS - 5-3-2011 at 22:24

May you give us some pictures of this method?

Magpie - 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&a...

SM2 - 14-11-2012 at 08:14

could a swap work here, using easily made propionic anhydride, and glacial aceetic?

tetrahedron - 14-11-2012 at 08:24

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

x3110n - 10-3-2018 at 11:13

Quote: Originally posted by User

Be aware that when it is heated and still contains water superheating can occur.

I know that you mean this in the most positive way, but superheating will NEVER occur when a substrate like sodium acetate is present in that ratio to the water. The crystals privide more than enough porous surface to help the convertion in to steam at 100 degreess C.

Just saying

XeonTheMGPony - 24-3-2018 at 04:26

Quote: Originally posted by x3110n

Quote: Originally posted by User

Be aware that when it is heated and still contains water superheating can occur.

ORLY?

As some one who's don this more then once, and cleaned the microwave more then once super heating and mini steam explosions most certainly happen if you don't stop and stir the crystals.

May I ask what procedure you used to avoid this?

Feriman - 18-4-2019 at 11:02

what is The most accurate way to test acetic anhydride?
how can I seperate acetic acid and purify acetic anhydride?

Magpie - 18-4-2019 at 12:07

Make a derivative (as I explained above) and determine its mp.

clearly_not_atara - 18-4-2019 at 14:23

The separation of Ac2O from AcOH is the harder question. I can't think of anything but fractional distillation (bps 119 vs 140).

Waffles SS - 18-4-2019 at 21:35

Magpie,

I want to suggest GB299342 patent for making Ac2O.I think you made HPO3 from Phosphoric acid before.

[Edited on 19-4-2019 by Waffles SS]

Magpie - 19-4-2019 at 07:00

No, I have never made HPO3 but would like to. What I made with H3PO4 was SO3. I posted this on the forum.

Waffles SS - 19-4-2019 at 07:07

Heating Phosphoric acid to 316c lead to Meta-Phosphoric acid.

You did exactly that:

Quote:

Preparation of HPO3

1. Set up the iron ring w/ceramic triangle or wire gauze on a ringstand at the elevation suitable for the Meeker burner. Place 600 mL beaker on the ring.

2. Add 135 mL of phosphoric acid to the beaker.

3. Apply the strongest possible heat to the beaker with the Meeker burner on MAPP gas.

4. Intensely heat the beaker until the melt turns red or 1 ½ hrs have elapsed, whichever comes first.

5. While the HPO3 melt is still hot pour it into a 250 mL rbf (the pot)

http://www.sciencemadness.org/talk/viewthread.php?tid=78495#...

[Edited on 19-4-2019 by Waffles SS]

Magpie - 19-4-2019 at 09:49

So I did! Thanks Waffles.

Feriman - 26-4-2019 at 21:43

I prepared some Ac2O by S2Cl2 method, then repeated experiment by adding my crude Ac2O as solvent to increase yield and purity,and result was good because the yellow color in my crude Ac2O from first experiment disappeared and I had more, but by a mistake some water was poured to my product and made it a milky color, I want to know what is purpose of milky color?

And glad to get any comment and idea about my experiment.

(My mistake): I connected outlet water of distillation condenser to a water aspirator and connected aspirator to vacuum adaptor of my distillation apparatus, at the end of experiment I closed inlet water so some water was pulled back in apparatus

[Edited on 29-4-2019 by Feriman]

NZniceguy - 29-7-2019 at 03:37

So I tried this method, making doubly sure that everything was dry and Sodium Acetate was fused and even fised again before use....I only had ten mls of Br2 so went with half of everything eg 1.7gm Sulphur and 33 gm Sodium Acetate.
I vacuum distilled and got around 25mls of a yellowy orange liquid.
I then set up for simple distillation and collected 3 fractions. One from 120 degrees C to 130 C which was about 12ml, another from 130C to 138C which was about 3ml and a third fraction from 138C which went up to 157C! Which was about 5ml
All 3 fractions have a distinct yellow colour and do not smell like Acetic anhydride. Also the high temp (above 140C) is obviously not right.
Is this sulphur contamination? If so what can I do to clean it up?
If not, what went wrong? What should I try different next time?
Thanks in advance.
Ps I have photos of the 3 fractions but am unsure how to upload them.

[Edited on 29-7-2019 by NZniceguy]

NZniceguy - 30-7-2019 at 01:35

Quote: Originally posted by NZniceguy

So I tried this method, making doubly sure that everything was dry and Sodium Acetate was fused and even fised again before use....I only had ten mls of Br2 so went with half of everything eg 1.7gm Sulphur and 33 gm Sodium Acetate.
I vacuum distilled and got around 25mls of a yellowy orange liquid.
I then set up for simple distillation and collected 3 fractions. One from 120 degrees C to 130 C which was about 12ml, another from 130C to 138C which was about 3ml and a third fraction from 138C which went up to 157C! Which was about 5ml
All 3 fractions have a distinct yellow colour and do not smell like Acetic anhydride. Also the high temp (above 140C) is obviously not right.
Is this sulphur contamination? If so what can I do to clean it up?
If not, what went wrong? What should I try different next time?
Thanks in advance.
Ps I have photos of the 3 fractions but am unsure how to upload them.

[Edited on 29-7-2019 by NZniceguy]

I read that boiling a small amount of Potassium Permanganate in the combined fractions may get rid of the impurity. Does anyone have any idea if this is correct?

Weka - 7-4-2020 at 21:30

On magpies writeup, if the reaction taking place is 4NaOAc + 2S2Br2 --> 4NaBr + SO2 + 3S + 2Ac2O as per Thorpe then you would need 1 mol sulfur for 4 moles NaOAc (assuming the sulfur is recycled) whereas the original write up uses 1 mole for every 8 moles approx, which is more in line with the reaction proposed by Orshansky and Bograchov. I assume that this was decided upon based on higher yields for a 1:8 ratio. If you did achieve higher yields with a 1:8 ratio would that mean Thorpe may have proposed a more minor reaction path (It is made clear SO2 is released)?
Or was it due to some other problem that a 1:8 (1S:8NaOAc) molar ratio was used instead of a 1:4 (1S:4NaOAc) molar ratio?

Also, would the same reaction proceed with (Heavy) chlorine contamination in the bromine?

Jenks - 14-4-2021 at 10:22

I hope this inspiration can help. We know that HBr is easily oxidized, and it made me wonder... Instead of providing an equivalent of the nasty liquid bromine, might it be possible to find an oxidizing agent that would covert bromide back to bromine without oxidizing the sulfur? Then we would have the beauteous situation of supplying acetate, sulfur, oxidizer and a catalytic amount of bromide salt, and getting anhydride and SO2 as product. I wish we could just use iodide, as HI is even more easily oxidized, but apparently iodine and sulfur are reluctant to react.