Sciencemadness Discussion Board - Fresh Look at Sulfur Halide Rxn w/Acyl Salts

Fresh Look at Sulfur Halide Rxn w/Acyl Salts

Sauron - 5-9-2007 at 21:48

A page in the Rhodium archive refers to a Chemistry & Industry article in the 40s taken from the Russian literature, detailing a lab prep of acid anhydrides from anhydrous alkili and alkiline-earth salts of carboxylic acids. These are reacted with sulfur chloride (S2Cl2) or sulfur bromide (S2Br2) either preformed, or prepared in situ. The reaction is fast and the yields high. Obviously the sulfur halide serves to convert part of the salt into acid chloride which then reacts with more acid salt to form the anhydride.

However, looking over the procedures it is clear that the reaction is more complex, at least in some of the examples. Nowhere is complete stoichiometry given, and in the very first example, a large excess of bromine is employed relative to both sulfur and sodium acetate.

So what exactly is going on here?

22 g sulfur (0.65 gram-atom)
320 g Br2 (4 gram-atoms or 2 mols of diatomic bromine)
440 g NaOAc anhydrous. 5.5 mols This is described as fused at 320 C. I believe this is an error, as NaOAc loses all its water at 125 C and starts to decompose above that.

The sulfur is dissolved in the bromine first, and the NaOAc added last with stirring.

Obviously less than two thirds of a gr-atom of S will only consume the same amount of Br2 to form a little less than 1/3 of a mol of S2Br2 or about 75 g worth. This reacts with the same quantity of NaOAc to form AcBr and NaBr. The byproduct is SO2 and sulfur according to the equation

2 S2Br2 + 2NaOAc -> 2AcBr + 2NaBr + SO2 + 3 S

AcBr + NaOAc -> Ac2O + NaBr completes the overall reaction.

That balanced equation appears nowhere in the Rhodium page (attached). I do not know whether or not it appears in the original Russian publication or in the Chem&Ind translation which I have not seen. I worked it out for myself.

The sulfur will of course form more S2Br2 as long as bromine remains. Whether or not SO2 reacts with bromine under these conditions I do not know. Normally a catalyst is required to form sulfuryl halides (e.g., GAC or camphor.) So we will assume that sulfuryl bromide does not form.

Another procedure describes the use of preformed S2Cl2, and in this case SO2 is vigorously liberated, which was not noted in the above procedure with sulfur and excess bromine.

I believe this adds to the understanding of this reaction. It would be interesting to try the reaction of SO2Cl2 with anhydrous NaOAc to see if this works. Perhaps sulfuryl bromide can form without a catalyst, I will check and see. But even if so it is a minor participatent in this reaction.

PS Mellor indicates that SO2Br2 is unlikely to exist, at least above liquid SO2 temperatures, so I think we can discount any such involvement.

Propionic Anhydride

40 g anhydrous sodium propionate 0.42 mol
2 g sulfur 0.063 gr-atom
22 g Bromine 0.3 gr-atom

Yield 25 g

Those numbers track well with the acetic anhydride prep, except that the bromine is less. (shrug) To get a direct comparison multiply by 11

440 g sodium propionate
22 g sulfur
242 g bromine

So one gr-atom bromine is missing and the excess of salt has been reduced, the FW of sodium propionate is 96 and that of sodium acetate 82. The ratio of salt to bromine is about the same in both cases.

--------------------

In another example, NaOAc is reacted with a stoichiometric amount of S2Cl2 preformed reagent and then distilled to give Ac2O, NaCl, 3 S and SO2. The authors however propose that the reaction procceds as:

2 NaOAc + S2Cl2 -> AcO-S-S-OAc + 2 NaCl

2 AcO-S-S-OAc -> 2 Ac2O + 3 S + SO2

No reference is given so I can't tell is the original investigators supported their proposed mechanism in any way. I will have to look and see is diacetyl disulfide has been isolated and characterized. Anyway, even if not it ought to be possible to halt this procedure before distilling (pyrolyzing) the supposed sulfide and isolating it, if it exists.

In which case this reaction may not involve an acyl halide at all. Wouldn't that be interesting? It would imply that this diacyl disulfide might be stable enough to store and allow preparation of Ac2O or other acid anhydrides as needed. How interesting.

PS Apparently diacetyl disulfide does indeed exist. It turns up as an impurity in commercial thioacetic acid, and can be regarded as the dimer of that. (AcS)2 from 2 AcSH I am still hunting for its preparation. Thioacetic acid, if I recall, is nasty stinky toxic stuff,

PPS The general method of converting thiols to disulfides is to use I2 as a catalyst. So thiolacetic acid, prepared per Org.Syn. by treating acetic acid with H2S, is reacted with I2 giving diacetyl disulfide. If the Russian authors being quoted in Rhodium are correct, this substance on distillation will be quantitatively converted to acetic anhydride.

Thiolacetic acid is a toxic material of very persistent stench. Thus far I have not found a commercial source for the diacetyl disulfide. All in all, this sounds like a great way to turn one's lab into a real skunk works. If the reaction of sulfur, bromine and sodium acetate (anhydr.) was not noted to be particularly stinky, then I suspect that diacetyl disulfide may not have been produced - unless it is unlike thiolacetic acid in its nasty smelling nature.

[Edited on 6-9-2007 by Sauron]

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Sauron - 6-9-2007 at 08:43

As mentioned, there's an Org.Syn. procedure for preparing thioacetic acid (thiolacetic acid) Ac-SH from H2S and acetic anhydride. The great August Kekule, however, prepared this from glacial acetic acid and P2S5 (P4O10) in 1854 as described in Ann., 90, p 309. I will post this shortly as it may be more convenient, and it is less circular than procedures requiring Ac2O or AcCl.

Also I do not have a cylinder of H2S, but I do have phosphorus pentasulfide!

Thioacetic acid is available from Aldrich (expensively) or Acros (half the Aldrich price.)

A procedure for oxidation of thioacetic acid to diacetyl disulfide is described in a US patent (attached below) as Example XI. The oxidant is DMSO and catalyst is conc HCL with a trace of I2 or HI. Yield 75% Diacetyl disulfide is a low melting solid mp 8-15 C which may well explain the odd description of the reaction mass in the Rhodium article. ("Semisolid semiliquid")

I am not seriously proposing thioacetic acid and diacetyl disulfide as a prep route to Ac2O. But it would be very interesting to prepare diacetyl disulphide by the patent route and then distill it to see if it pyrolyzes to Ac2O, S and SO2.

My feeling is that the best variation on this reaction to prepare Ac2O is to use preformed S2Cl2 which is available as byproduct from chlorination of CS2, on the way to making CCl4. This makes use of a byproduct, rather than consuming large quantites of bromine in the making of S2Br2 in situ.

But others may have different priorities.

Kekule's 1854 paper proposes preparation of thioacetic acid by distilling it from phosphorus pentasulfide.

5 AcOH + P2S5 -> 5 AcSH + P2O5

300 g glacial acetic acid
222 g P2S5

380 g thioacetic acid (theoretical yield)
142 g P2O5

In practice I'd use excess AcOH and reflux till the yellow pentasulfide has been converted to white pentoxide.

The bp of thioacetic acid is 93 C so efficient fractionation will be necessary to seperate it from the excess AcOH.

So far I have not noticed any yield stated by Kekule.

[Edited on 7-9-2007 by Sauron]

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Sauron - 6-9-2007 at 20:14

And here's the Kekule paper from Ann., vol 30 p 309. (1854)

It turns out P2S5 is not as expensive as I recalled, I must have been cheated on the Merck product.

Acros sells a Kg of phosphorus pentasulfide for less than $50.and this is enough to convert acetic acid to almost 2 L of thioacetic acid so maybe 1.5 Kg diacetyl disulfide and therefore 1 Kg or more of Ac2O.

So this is competitive with other methods of preparing Ac2O and cheaper than some.

[Edited on 7-9-2007 by Sauron]

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Ozone - 7-9-2007 at 18:13

Although the (Rhodium) equations are not balenced, e.g. 2 moles of the 1,1'-disulfanediylbis(oxy)diethanone will yield 2 moles of acetic anhydride, three of S and 1 of SO2. Thermodynamically this look like it will work.

Pondering:

But, in order for this to occur, we would need to see a concerted reaction involving 2 or more equivalents of the diethanone (or the molecule would probably fall appart whilst liberating SO2).

done pondering.

Unless I am missing something, though, the bisdithioketone from thiolacetic acid is not equivalent and should not yield acetic anhydride via the same mechanism. Perhaps this might work with a large excess of air or some hydrogen peroxide (and heat)--might get frisky!

Please let me know if I missed something (patents are a pain when your in a hurry).

Cheers,

O3

[edit] Although my German is terrible, this, from Kekule, looks interesting and on the right track:

"Reaction zeigt , erhalt man wohl ein intermediares Anhydrid
von der Formel :
C2H3O-S-OC2H3"

On the attached figure I don't like the "peroxyanhydride" bit that the Chemoffice naming algorithm coughed up. I meant to remove it, but forgot. oops.

[Edited on 7-9-2007 by Ozone]

thiolacetic acid_01_small.jpg - 22kB

Sauron - 7-9-2007 at 21:49

Well done, I overlooked that difference.

Thanks for the correction.

SO I will look for an alt prep of (CH3C(O)OS)2.

C4H6O4S2

I just drew that in MDL ISIS Draw 2.5 w/AutoNom and the latter barfed on naming this, gave an undefined structure error message. I ran Clean molecule and still get same error.

So do you think this compound unlikely as intermediate? Unlikely to exist?

If so then my original hypothesis (halogenation by S2X2 of NaOAc to acetyl halide followed by attack on more NaOAc) is more likely.

Kekule indeed reported that acetic anhydride treated with P2S5 gives acetyl sulfide MeC(O)OSOC(O)Me

If this compound pyrolizes to Ac2O (extruding sulfur ) then indeed I'd say this is a possible intermediate. But whence comes the oxygen?

---------------

If S2Cl2 (or S2Br2) form (AcOS)2 as reported by the Russian investigators and Rhodium, then it follows that SCl2 (there is no SBr2) ought to give (AcO)2S.

We are taught (by the Russians) that the reaction of S2X2 with NaOAc gives upon pyrolisis, Ac2O. And we are taught by Kekule that we can make the (AcO)2S from acetic anhydride and P2S 5. Can it too be pyrolized with or without P2O5?

[Edited on 8-9-2007 by Sauron]

chemrox - 28-1-2008 at 14:39

I wanted to follow-up on this thread with some related information and a question. Also at Rhodium and next to the process cited above is this one:

Propionic anhydride. To 40 g. fused and powdered sodium propionate in a flask of 250 cc. capacity a solution of 2 g. sulphur in 22 g. bromine was added while stirring. The temperature was kept at about 50 C. When the operation was completed, the anhydride was distilled off in vacuo. The crude product (25g.) was fractioned under normal pressure, and the fraction 155-156 C was collected. Yield, 23 g. propionic anhdride of 90% purity = 85%..."

This looks a lot more attractive but there's a point (con)fusing me. Why is the acid salt "fused?" Is it assumed it was just made from the acid and thus could have water in it? Could there be another reason? If one buys reagent Na-prop could he proceed without the fusion?

smuv - 28-1-2008 at 15:20

sodium propionate is deliquescent, it is best to make sure your salt is anhydrous before the reaction. The same goes for trying to make say, acetic anhydride from sodium acetate and acetyl chloride, any water present will really kill your yields so it is best to fuse the salt to minimize loss of product due to water.

chemrox - 31-1-2008 at 11:24

I was evidently mislead with a consequent waste of material and lessons learned. The TCT method makes a-chloro-acids not acid chlorides.

Solo placed a pdf in refs. I copied it below.

[Edited on 1-2-2008 by chemrox]

Nicodem - 31-1-2008 at 14:37

Quote:

Originally posted by chemrox
The TCT method makes a-chloro-acids not acid chlorides.

No it does not. That would make for a redox nonsense. Count the electrons.
BTW, your link points to no post relevant to cyanuric chloride. Please correct it or provide the PDF.

a-Cl-acids via TCT paper

chemrox - 1-2-2008 at 20:06

@Nicodem- a usually reliable source may want to re-think

PS_ I would feel relieved to learn I had misread this or reinterpreted

CRX

[Edited on 1-2-2008 by chemrox]

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PainKilla - 1-2-2008 at 20:51

a-Chlorination of Carboxylic Acids using Trichloroisocyanuric Acid (TCCA) is not using Cyanuric Chloride (TCT)

Déjà Vu!... Another case of abbreviations causing mayhem...

[Edited on 1-2-2008 by PainKilla]

oh s**t

chemrox - 3-2-2008 at 12:08

Thanks Painkilla, apologies to Nicodem and others. I'm still confused by this reagent and will shortly report an experiment using it to make acid Cl. Caught a flu that caused a hiatus and may explain some of the confusion.

failed anhydride experiments

chemrox - 9-4-2008 at 17:53

Experiment 1 was a scale up from the sulfer/bromine method and it failed in the sense that most of the acid Na salt remained unreacted. Two possible factors; inadequate drying-the salt sat a long time after fusing and powdering, the other factor, inadequate stirrring. I found it impossible to move the stuff inisde the flask sufficiently to get good mixing.

The other failure was also a scale-up with some of the same issues. The method was Na salt plus acyl halide. The addition of the halide did not produce a lot of heat right away. When the addition was complete it looked like twice as much halide could have been added. Distillation of the resultant mix started at 65 degC and ended at ~160 degC
acyl halide bp 80; acid bp 141 and anhydride bp is 168 go figure. Azeotropes? There was a lot of leftover material to recover after all was done.

len1 - 9-4-2008 at 18:57

I am very surprised by this method. It is essentially identical to the time-honoured preparation except all in one pot and with one exception, there is no SO3. The normal chain is

S + Cl2 -> SCl2 + SO3 -> SOCl2 + AcOH -> AcCl + AcONa -> Ac2O.

Here all the ingredients are mixed in one pot (normally this lowers yield) minus the SO3. So if this works SOCl2 overkill and S2Cl2 should do in the above sequence. Somehow I doubt it. Ac2O, AcOCl are all highly hydroscopic. As a rule synthesis of such reagents in good yield requires a dehydrating step or a dehydrating agent, such being SOCl2 and SO3.

[Edited on 10-4-2008 by len1]

chemrox - 9-4-2008 at 20:05

refer me to the one you're citing if you don't mind. I still need a supply of about 300 ml acid anhydride and I'm getting tired of spending precious lab time this way. Thanks! PS I edited because I'm very confused by what you're telling us and thinking I miscommunicated. As I understand it, SOCl2 is one way of making the acid Cl. I have the acid Cl from another method. So my "one-pot" is reacting the acid Cl with the Na salt as you indicated. I don't see how you break that one down further. Please clarify. Or have I done?

[Edited on 9-4-2008 by chemrox]

len1 - 9-4-2008 at 20:38

Im sorry, I think there has been a misunderstanding. I was referring to the method at the top of this page, with Ac2O from S, Br2, and NaOAc directly. If you have the AcCl from another source this does not apply to you. As far as Im aware reacting this with anhydrous NaOAc + AcCl->Ac2O + NaCl should work as ive seen that reaction documented in many places (but have never tried it myself).

Sauron - 9-4-2008 at 22:20

Len1:

That reaction you cited may look fine on paper but in practice things are different

While you can at risk of life and limb employ SO3 to prepare SOCl2 there is a better safer way that has been described on this site more than once.

Phthaloyl chloride is easily prepared from phthalic anhydride and benzotrichloride per procedure of Thomas Kyrides, byproduct is benzoyl chloride, also very useful.

Phthaloyl chloride reacts with SO2 (dry) and catalytic amound ZNCl2 to give SOCl2.

HOWEVER

AcOH and SOCl2 does NOT produce a practical yield of AcCl. This has ALSO been stated many times on this forum, and can be found in Vogel's chapter on acid chlorides. The bp of SOCl2 is too close to that of acetyl and propionyl chlorides to allow facile separation by fractionation. This results in a highly impure product. Vogel recommends using thionyl chloride for C4 acids and higher.

The practicable methods for preparing acetyl chloride (or propionyl chloride which is chemrox's target) are benzoyl chloride (method of H.C.Brown, posted here many times); phthaloyl chloride (method of Kyrides, posted here many times); TCT in acetone with Et3N and extracting the acid chloride from pptd cyanuric acid with DCM; or use of oxalyl chloride (method of Roger Adams, posted here many times) which can also directly prepared the anhydride from the acid.

The two methods of those four most likely to produce best results are benzoyl chloride and TCT, the phthaloyl and oxalyl chlorides being relatively expensive..

All of the methods involving anhydrous sodium acetate or propionate are best done NOT by dehydrating sodium acetate trihydrate (which has to be done twice) but by purchasing the best grade anhydrous NaOAc available such as Merck super-dry product, and using it directly from freshly opened bottle. Gattermann recommends fusing even such anhydrous product, once. But everyone I know of who has tried this gets a lot of charring. Apparently it is rather technique intensive and scale sensitive. IMO, seek other methods.

[Edited on 10-4-2008 by Sauron]

Nicodem - 9-4-2008 at 23:26

Quote:

Originally posted by len1
I am very surprised by this method. It is essentially identical to the time-honoured preparation except all in one pot and with one exception, there is no SO3. The normal chain is

S + Cl2 -> SCl2 + SO3 -> SOCl2 + AcOH -> AcCl + AcONa -> Ac2O.

I don't understand what makes you think it is similar (surely it can not be identical) to what you described?
The reaction discussed in this thread is about the thermal decomposition of the very unstable AcOSSOAc (I guess one can call it "diacetyl sulfoxylate") and related intermediates to acetanhydride.
Of course the reaction can only give yields above 0% if the sodium acetate contains less than 10% water which is not easy to achieve. But then again that is the problem with all routes to acetanhydride starting with sodium acetate anyway.

len1 - 10-4-2008 at 00:20

The similarity is in the starting materials and the product - the reactions suggested could well be different, but the start and end point is the same. What is surprising is that the essence is a huge shortcut in the chain I described and in the absence of a strong dehydrating agent. Ac2O generally requires a strongly dehydrating environment in order to form - I was expressing my doubts that the reaction presented represents such an environment, its another way of looking at things. I could be wrong, the test is in the pudding.

@Sauron I was using SOCl2 as a reagent capable of forming acid chlorides, of this there is no doubt. Separability is not at issue, rather reactivity is. As a practical method for AcCl SOCl2 has the disadvantage you mention. Kyride's reaction, which I see you have been a big fan of is interesting in its own right though.

[Edited on 10-4-2008 by len1]

[Edited on 10-4-2008 by len1]

Sauron - 10-4-2008 at 00:22

I'd buy the Merck super-dry grade NaOAc, in a packing size that I could use all at once, and see what sort of yield I could get right out of the bottle, no fusion. If yield is depressed, I'd do a single fusion with next batch and see if charring can be avoided and what the yield is. I have a forced-air, controlled temperature drying oven by Memert, that ought to do it. I believe it heats to 250 C. The required temperature is not that high. (It is stated incorrectly in Rhodium). NaOAc starts to decompose >125 C.

Sure, the anhydrous salt costs more than the trihydrate but your savings are ephemeral when you are paying for 3H20/mol and then charring a lot when trying to drive that off.

Nicodem - 10-4-2008 at 07:55

Quote:

Originally posted by len1
The similarity is in the starting materials and the product - the reactions suggested could well be different, but the start and end point is the same. What is surprising is that the essence is a huge shortcut in the chain I described and in the absence of a strong dehydrating agent. Ac2O generally requires a strongly dehydrating environment in order to form - I was expressing my doubts that the reaction presented represents such an environment, its another way of looking at things. I could be wrong, the test is in the pudding.

But this particular reaction involves no dehydration. Actually all the reactions (that I can currently think of) leading to either acid chlorides or acid anhydrides involve either a rearrangement of heteroatomic functions (for example: RCOOH + SOCl2 => RCOCl + SO2 + HCl and all similar ones) or acylation of a carboxylate (for example, RCOO- + RCOCl => (RCO)2O + Cl-

.
If carboxylic anhydrides could be prepared by dehydration, then plain old H2SO4 would suffice since it would give a thermodynamically allowed equilibrium, yet it appears it does not work even though H2SO4 is more than enough strong acid to form the RCO+ required for the acylation of RCOOH.

len1 - 10-4-2008 at 17:37

This is going along the lines of our previous argument about decarboxilation to pyridine. You are arguing about reaction mechanisms, I say one has little confidence is theorising about these with certainty - careful and tedious experiments have to be done. Thermodynamics is something with simpler and more certain results. Two things about thermodynamics:

1) It only needs the start and end point - doesnt give two hoots about reaction mechanisms - hence my comment about the similarity of the processes -

2) Thermodynamics can only predict if a process is possible not if it will occur - and thats where your comment about H2SO4 falls in - it is thermodynamcally allowed but does not occur. Thats standard.

Heres what thermodynamics says.

The starting and end points are:

AcOH -> Ac2O + H2O delH = a

so whaever the steps, this is a dehydration of AcOH, because water is released on the RHS.

a is positive so the reaction wont occur by itself, we need a dehydrating agent to drive the equilibrium. Here are the differences I was referring to:

1) SOCl2 + H2O -> SO2 + 2HCl delH = b for the standard process

2) S2Cl2 + H2O -> 1/2SO2 + 2HCl + 3/2S delH = c

It does not matter whether these are or are not the actual reactions occuring at the molecular level. They correspond to the start and end products. Thermodynamically that is full stop.

Formation enthalpy SOCl2 ~ -246 kJ/mol
Formation enthalpy SO2 ~ -298 kJ/mol
Formation enthalpy S2Cl2 ~ -60 kJ/mol

So delH for second reaction is 30kJ/mol more negative. This means S2Cl2 is actually a stronger dehydrating agent than thionyl chloride, contrary to what I was expecting. So yes there is no thermodynamic reason why reaction cant occur. Doesnt mean it will actually take place though. I find it strange though that people use SOCl2 in the dehydrations if S2Cl2 will do.

To be fair we should actually use delG rather than delH. That is however quite dependent on reaction conditions. Nonetheless because 1) releases 1/2mole more of gas this narrows the gap between the two processes.

[Edited on 11-4-2008 by len1]

Sauron - 11-4-2008 at 00:13

As the thread author and someone who knows something about the subject of acyl chlorides and anhydrides I would just like to reiterate that thionyl chloride has nothing whatsoever to do with the subject of this thread.

Thionyl chloride is one of a number of reagents for making acyl chlorides. That is taught is every taxtbook or organic chemistry.

The authors of those texts are often uninterested in the fine print and exceptions. THOSE teach us that SOCl2 is unsuitable for aliphatic acid chloride preps lower than C4.

Both acetyl chloride and propionyl chloride are < C4.

Furthermore, Nicodem is perfectly correct. While for pedagogical purposes the first year texts present formation of acid anhydrides as union of two mols of R-COOH into 1 mol of RCO-O-OCR with abstraction of 1 mol H2O - that is simply neither how it happens nor how it is done.

Here are examples of carboxylic anhydride formation:

NaOAc + AcCl -> Ac2O + NaCl.

Where's the water? There was no water in the reactants and there's no water in the products.

To find any of the elements of water we's have to go back to when the AcOH was turned into a salt and the other AcOH was turned into an acyl halide.

The first produced NaOAc and H2O. The second, probably HCl.

Two seperate reactions both offstage.

Similarly
2
NaOAc + S2Cl2 - AcOSSOAc + 2 NaCl

No water. None on either side of equation.

Pyrolyze that and you get Ac2O and still no water.

How about reagents that proceed to Ac2O from AcOH in one pot, one step?

Like Oxalyl chloride.

Roger Adams teaches us in JACS 90 years ago that AcCl is formed first and then this reacts with AcOH, products are HCl and (from the oxalyl chloride) CO and CO2.

Even in the cases of dicarboxylic acids of the right chain length to form cyclic 5 and 6 membered anhydride rings, the mechanism is not a simple abstraction of water.

As Nicodem correctly pointed out, were it otherwise you could prepare anhydrides by use of powerful dehydrating agents like H2SO4 or P2O5 (P3O10).

This however does not work.

The mechanism is if not everything - almost everything.

[Edited on 11-4-2008 by Sauron]

len1 - 11-4-2008 at 01:04

Sauron - 11-4-2008 at 02:23

Correction

Actually, in the special case of some dicarboxylic acids, e.g., those with C4 or C5 chain lengths such as succinic and maleic acids, phthalic acid, etc. it is possible to prepare the anhydride by direct abstraction of water, such as by heating the dicarboxylic acid in an inert high boiling solvent (Vogel suggests tetrachloroethane).

The formation of such cyclic anhydrides by reaction of the open chain dicarboxylic acid with acetic anhydride can also be regarded as a dehydration reaction.

But with acetyl chloride, although the reaction still proceeds to an anhydride and AcOH, this is not formally a dehydration.

Anyway this is merely a special exception, these procedures do not apply to intermolecular anhydride formations, only intramolecular ones. I just wanted to be accurate and fair in my remaarks.

[Edited on 11-4-2008 by Sauron]

benzylchloride1 - 26-4-2009 at 21:04

I prepared some acetic anhydride from sodium acetate, bromine and sulfur today. 20ml of bromine was placed in a 100ml beaker and 3.5g of sulfur powder was added slowly while stirring with a glass rod. 66.5g of anhydrous sodium acetate was placed in a 500ml rbf. The sodium acetate was not fused. A pressure equalizing funnel was attached and the sulfur bromide- bromine solution was added to the funnel. The S2Br2/Br2 solution was slowly added to the sodium acetate. The mixture fumed and an orange liquid refluxed on the walls of the flask. After the addition the funnel was removed and the mixture was stirred with a glass rod. The mixture turned into a suspension of white crystals in a clearish liquid. A distillation set up was assembled and the mixture was heated with a heating mantle to distill over the product. The product distilled between 120 C-125C @ 580mm Hg. The distillation was stopped because of charring of the flask contents. A considerable amount of liquid remained in the flask. Approximatly 16g of a clear liquid was obtained. A small quantity of o-toluidine was acylated with the acetic anhydride in aqueous solution. Upon quencing the reaction mixture with water, white crystals were obtained. The derivative was recrystallized from water and formed white needles. The derivative is drying and a melting point will be taken.

panziandi - 27-4-2009 at 03:01

Nice work well done! Be interesting to get the mp.

Sauron - 27-4-2009 at 04:03

Use a batter pump and distill at c.25 mm Hg not 580 mm on a water bath (a rotav is ideal) and you will get no charring and a much better yield

Then redistill the crude Ac2) at ordinary pressure.

benzylchloride1 - 27-4-2009 at 20:31

Another experiment was conducted today using S2Cl2 instead of S and Br2. 100g of anhydrous sodium acetate was placed in a 500ml RBF equipped with a Claisen adaptor, reflux condenser with a CaCl2 tube and a additon funnel. 38ml of S2Cl2 was placed in the addition funnel and slowly added to the mixture. The mixture became very hot. After the mixture cooled some what, it was stirred with a glass stirring rod. The mixture was heated with a heating mantle and periodically stirred with the glass rod. Magnetic stirring will not work because of the semi-solid nature of the reaction mixture. The mixture was yellowish in color, but melted to a liquid with a tan colored solid suspended in it. After about 45 minutes of heating, the mixture was allowed to cool and a distillation head attached and the mixture was vacuum distilled using a water pump. About 30- 40 ml of a clear colored liquid was obtained that acetylated o-toluidine in aqueous solution. The crude acetic anhydride will be distilled from potassium permanganate to remove sulfur compounds. The anhydride will then be weighed so as to obtain the percentage yield. I scaled down this procedure 4 times due to the fact that I did not have 400g od sodium acetate and enough S2Cl2 today. I made the S2Cl2 today as well. The synthesis of acetic anhydride can be conducted in one day from sodium acetate, sulfur, hydrochloric acid and trichloroisocyanuric acid. The vacuum distillation really improves the yields, thanks for the advice. This method works extremely well and is suitable for the preparation of acetic anhydride by amateur chemists that have a fair amount of equipment. A properly functioning fume hood is essential for making the disulfur dichloride and the anhydride due to the poisonous nature of the S2Cl2 and chlorine. I wonder if a method could be adapted from this for synthesizing benzoyl chloride. I am looking for a cheaper source of the anhydrous sodium acetate, since I pay around $10 per pound with shipping.

[Edited on 28-4-2009 by benzylchloride1]

[Edited on 28-4-2009 by benzylchloride1]

Sauron - 27-4-2009 at 22:42

Well done!

You were smart to buy the anhydrous NaOAc as this is the point so many others got stuck on, trying to dry the trihydrate.

Benzoyl chloride is readily prepeared:

1) from TCT and the acid in acetone and in presence of TEA

2) from benzotrichloride and phthalic anhydride using ZnCl2 catalyst at 120 C for 20 hrs reflux. You get a nearly quantitative mixture of phthaloyl chloride and benzoyl chloride. Both very useful. You can use the phthaloyl chloride to make SOCl2 from SO2 gas.

You can make the trichloride from toluene or benzyl chloride.

benzylchloride1 - 1-5-2009 at 14:27

The sulfur bromide/ bromine method gives a cleaner product then the disulfur dichloride method, but is considerably more expensive due the amount of bromine required. I obtained a 56% yield of acetic anhydride boiling between 110 C and 130 C, upon distilling from 0.5g of potassium permangante to remove sulfurous contaminants. I live at a very high altitude and boiling points are way lower then those at 760 mm. My thermometer is not very good. The heating was conducted with a heating mantle and aluminum foil was used as insulation. All of the distillate had the characteristic odor of acetic anhydride. The product acetylates amines at room temperature. The acetyl toludine derivative melted at 145 C, indicating that it is the para derivative. The toluidine was a mixture of ortho and para isomers obtained from the reduction of crude mono nitrotoluene. I now have a vacuum distillation setup, I nitrated around 150g of toluene and separted the isomers by fractional vacuum distillation and then freezing the residue to obtained the crude p-nitrotoluene. I now will have some pure o and p-toluidines to work with once I run the reduction reaction. I plan on running the acetic anhydride synthesis full scale when I get some more anhydrous sodium acetate. I have a 2L reaction kettle with a removable flange head. This will be ideal because I can mix the disulfur dichloride and sodium acetate with a glass stirring rod and then place the flange head on the kettle with a reflux condenser protected with a CaCl2 tube. The heating and distillation will be conducted with a Buchi water bath because I do not have a mantle for the reaction kettle. Now we have a confirmed method of producing acetic anhydride of reasonable purity from readily available chemicals.

[Edited on 1-5-2009 by benzylchloride1]

Magpie - 1-5-2009 at 15:38

Very nice, and thanks for sharing this with the forum.

In one of your earlier posts you said that you distilled your product at reduced pressure using a water pump (aspirator). Since S2Cl2 and Ac2O are sensitive to water I was wondering if the ~25mmHG vapor pressure of water from the pump causes any problem?

Sauron - 1-5-2009 at 20:45

Tjis is my 3rd trt at posting this, I will copy this tome before hitting the button.

IMO there's a reason why you get a cleaner product with Br than Cl. Your S2Cl2 is contaminated with SCl2 and S. I am assuming that you made the S2Cl2 from the elements and that you did not perform heroic procedures to purify it.

It is very hard to obtain pure S2Cl2. See Vrauer's monographs on S2Cl2 and SCl2. The former is stable, the latter is now.

2 SCl2 (equilibrium) S2Cl2 + Cl2

S2Cl2 is a great solvent for S and if you add more Cl2 to try to react all the S you wll make more SCl2 instead.

Why not same hassle with Br2?

Br2 forms only S2Br2. The lit. is divided on whether SBr2 exists at all, the consensus seems to be either no or it is so unstable that for practical purposes it is not there. See Mellor, Gmelin and ATBOIC.

This reaction requires the S2X2 which has a disulfide structure X-S-S-X in order to metathetize with sodium acetate to AcO-S-S-OAc.

SCl2 does not work, and if you had some you could prove it.

There's your source of contamination.

The S + Br2 procedure was first published in a Russian journal and reprinted in late 40s in Chemistry & Industry (London) so it really needed no confirmation.

Members here got all hung up trying to do the double fusion of NaOAc.3H2O which is tricky. You wisely cut the Gordian knot by purchasing commerial anhydrous NaOAc. Tell me, did you use it as is or did you fuse it once? If you used it out of the bottle, fusing it once at 135 C then powdering the melt and using it at once, may boost your yields.

benzylchloride1 - 1-5-2009 at 21:09

I did not fuse the anhydrous sodium acetate before reacting it with the S2Cl2. I currently do not have a suitable drying oven for that purpose, but I plan to buy one in the next few months. The S2Cl2 was directly prepared from sulfur and dry chlorine. The flask containing the sulfur was heated with a mantle to a temperature slightly above the melting point of the sulfur and chlorine was passed into the mixture. After an initial lag period, the S2Cl2 distilled as orange colored drops. The sulfur chloride in the receiving flask was reddish in color due to SCl2. The sulfur chloride in my opinion is more unpleasant then either chlorine or benzyl chloride.The fume hood does a good job at taking care of the chlorine gas during the chlorination phase, but the odor of sulfur chloride is barely noticable when the apparatus is disassembled. During the reaction between the S2Cl2 and sodium acetate, the odor is worse because the mixture is exposed to the air during the stirring process. A warning to others; do not repeat this experiment without a properly functioning fume hood. The residue in the flask after distilling the acetic anhydride can be dissolved with water, much brown colored collodial sulfur is present that wash formed during the reaction. I should place a CaCl2 tube between the aspirator and vacuum adaptor to prevent the ingress of moisture.

Polverone - 2-5-2009 at 09:12

The section on acetic anhydride in the 1921 edition of the first volume of Thorpe's Dictionary of Applied Chemistry, available here, is quite interesting. Earlier and later editions do not have such a wealth of information about processes employing sulfur chlorides and oxychlorides, but this was apparently the dominant industrial mode at the time of this edition and Thorpe's devotes a corresponding amount of space to the topic. There's also information about purification and avoiding side-products that may be useful. Sulfur monochloride and dichloride have both been employed, as has sulfuryl chloride. The chlorides may be generated in situ.

Magpie - 29-5-2009 at 16:39

After reading of the success of benzylchloride1 I was anxious to try the S2Br2 synthesis of Ac2O.

Working with Br2 is exciting enough for me but add in the preparation of a vicious looking reagent like S2Br2 and the stage is set for excitement. Lacking a detailed procedure, not understanding the stoichiometry, and not able to find an MSDS for S2Br2, added to the anticipation. I prepared carefully and wore full safety regalia: safety goggles, mid-arm rubber gloves, long-sleeved shirt, lab apron, and above all: hood fan ON.

I followed BZ1's procedure exactly, except I had the benefit of a newly purchased, but not yet used, overhead stirrer.

I had reason to believe that my NaOAc was of good quality so did not fuse it. I prepared my own bromine from NaBr and washed it with con H2SO4. Sulfur was flowers of sulfur, USP grade.

Addition of the sulfur to the bromine was uneventful. I really saw no evidence of a reaction other than there was no sediment in the beaker after stirring. This was placed in a pressure equalizing funnel.

The mixer was turned on and the S2Br2 + Br2 slowly added to the 500 ml RBF. Although the mixer was working well it only stirred the material within the diameter of the blades. So most of the NaOAc just sat there as an annulus. But I kept adding the liquid until it was all in. Then I removed the addition funnel and poked the unreacted material into the center using a glass stirring rod and the side-ports of the RBF. There was some minor refluxing going on but not much. All of a sudden (within a few seconds) the NaOAc began reacting and the whole mass turned into a creamy liquid that stirred well with the mixer alone! The temperature rose to possibly as high as 80C. I did not measure it but just felt the flask by hand. After 15-20 minutes of stirring it had cooled off.

Tommorrow I will vacuum distill to recover the putative Ac2O.

Here's 2 pictures. The first is before the breakup of the solids while the liquid was still being added:

Attachment: phpsApV52 (97kB)
This file has been downloaded 1949 times

[Edited on 30-5-2009 by Magpie]

The second is of the creamy product following the sudden breakup of the solids:

[Edited on 30-5-2009 by Magpie]

[Edited on 31-5-2009 by Magpie]

Magpie - 31-5-2009 at 17:08

My preparation of Ac2O via S2Br2, started 2 days ago, was completed today.

The reaction product formed previously was vacuum distilled at 170-180 mmHg with distillate temp as high as 70C. This went smoothly with grey, slightly charred solids remaining in the pot. Crude distillate volume was about 30mL.

This was then redistilled at atmospheric pressure, obtaining 2 cuts. 1st cut: 128-135C, ~15mL; 2nd cut: 135-143C, ~10mL. I think a fractionation could serve well here instead of the simple distillation I used.

To confirm identity of the Ac2O I mixed 1 mL of cut 2 with 2 mL of aniline, added 15 mL of water, and shook vigorously. There was an immediate and massive formation of white crystals: putative acetanilide. These crystals were redissolved (by heating) and the solution then filtered. The filtrate was cooled to recrystalize. The mp of the crystals was determined to be 113-114C in agreement with my handbook value for acetanilide.

Disscussion

Some things that I think would facilitate this preparation:

1. Figure out some way to add the solid material (NaOAc) last, a little at a time. This would greatly aid in stirring, thus preventing the difficulties both chemrox and I experienced. It would also keep the reaction under control. Just before my reaction suddenly took off I thought I had a failed reaction, then, in 5 seconds, I thought I had a runaway.

The good thing about adding it first is that you have good containment Once you get a whiff of S2Br2 you are glad of that!

Note: Because of the sudden and dramatic breakup and reaction of the NaOAc chunk I'm thinking Sauron may well be right about the formation of an acyl halide that triggers the reaction. This also seems consistent with the requirement for initial Ac2O in one of the Rhodium procedures.

2. Acquire enough crude Ac2O to allow a fractional distillation, thus improving purity of the product.

3. Employ CaCl2 cartridges during the distillations to prevent or reduce moisture absorbtion from the atmosphere.

4. By all means have a fume hood, and consider your neighbors. Time it for when they aren't around.

Conclusions

Overall I like this synthesis - mostly because it gives you the goods!

Thanks to Rhodium, Sauron, and benzylchloride1 for leading the way.

[Edited on 1-6-2009 by Magpie]

benzylchloride1 - 31-5-2009 at 18:02

Excellent work Magpie! The sulfur bromide solution almost looks like bromine, if the sulfur is added to fast, a slightly vigerous reaction occurs. The reaction takes a while to initiate and then the mixture turns into a liquid that is easy to stirr. What type of stirrer shaft and bearing did you use? I have 2 mechanical stirrers; one working and the other non working. I have been using rubber stoppers with glass tubing and rubber tubing to seal. I think that these would have a hard time with the sulfur halides. I am planning on running the full scale synthesis involving sulfur chloride and sodium acetate in my 2-liter reation kettle; this should yield around 250g of acetic anhydride

Magpie - 31-5-2009 at 19:08

Thanks BZ1.

The stirrer I have is a 2-bladed, folding prop type. Both the prop/shaft and the bearing are made of PTFE. I bought these through an eBay store, where the prices are much more reasonable than those of the scientific supply houses.

I make my Br2 using NaBr, 6% H2O2, and sulfuric acid. I'm still working on optimizing my procedure, but here is my method for the last batch:

2NaBr +H2SO4 + H2O2 --> Br2 + Na2SO4 + 2 H2O

1. Mix 51.5 g of NaBr with 125mL of 6% H2O2 in a 600 mL beaker. Pour this into a 250 mL RBF.
2. Assemble the RBF for simple distillation, no thermometer needed.
3. Add 16 mL of con H2SO4 and close up the system. Ice cold cooling water ON. No heat needed.
4. Turn hood fan ON. Some Br2 will escape through the vacuum adapter tubulation.

The bromine will be generated profusely. In fact it somewhat overhwelmed my 19/22 condenser.

Conclusion: The acid should not all be added at once so as not to overwhelm the condenser. Yield looks to be about 7mL, not that great. Placed Br2 in a narrow-necked bottle with PTFE lined cap. Placed in freezer for short term storage. When rethawed most of the floating water can be withdrawn with a Pasteur pipette. Right before using dry the Br2 by washing it with con H2SO4 using a sep funnel.

benzylchloride1 - 31-5-2009 at 20:38

The method that I use produces bromine in high yield; around 90%. I use an excess of sulfuric acid and a slight excess of 35% hydrogen peroxide for the bromine synthesis over the amount of sodium bromide used. The NaBr is dissolved in a 50% sulfuric acid solution and heated with a mantle. The H2O2 is slowly added, and the bromine distills. The receiving flask is chilled in an ice bath, hardly any of the bromine escapes. The reaction mixture is carefully heated to boiling and boiled until most of the bromine has been expelled out of the distillation flask. Most of the bromine distills during the addition of the H2O2; the reaction can get out of hand and contaminate the product. This happened to me the first time, add the H2O2 slowly, around 2mL per minute. I typically use about 100g of sodium bromide, I typically get around 70g of bromine after drying with sulfuric acid. The bromine can be distilled if desired, I did not distill it. I use a 500ml distilling flask and a 24/40 distillation setup. I produced around 200g of bromine in one afternoon; I used 150g of it for a 1-bromonaphthalene synthesis and the other 50g of it for the acetic anhydride synthesis. Magpie, could you U2U me the name of the Ebay seller that sells the stirrer setup; I am interested in purchasing one.

[Edited on 1-6-2009 by benzylchloride1]

[Edited on 1-6-2009 by benzylchloride1]

Sauron - 31-5-2009 at 21:06

At least no one os hung up on drying sodium acetate anymore.

Recommending the use of anhydrous NaOAc (commercial) seems to have been one of my better pieces of advice, though it took some time to take hold.

Magpie - 1-6-2009 at 14:19

from benzylchloride1:

Quote:

I am planning on running the full scale synthesis involving sulfur chloride and sodium acetate in my 2-liter reation kettle; this should yield around 250g of acetic anhydride

Wow - that is definitely full-scale. I would think that you would need good agitation so that you never had anything approaching a runaway. It also might be smart to have that kettle semi-immersed in a water bath with plenty of ice cubes handy! You even might want a reflux condenser.

I know some members have used dry powder feeders. It seems like that would be ideal, as good stirring and a controlled reaction could then be achieved by adding the NaOAc slowly and in small portions.

I wonder how much trouble it would be to make one's own dry powder feeder - sealed to the reaction vessel, of course. You don't want S2Br2 and Br2 vapors running loose, even in a good hood. If someone could tell us about these and/or show a picture of one I would appreciate that.

Edit: It's amazing what a little searching will bring:
http://www.sciencemadness.org/talk/viewthread.php?tid=9623#p...

[Edited on 2-6-2009 by Magpie]

benzylchloride1 - 2-6-2009 at 21:04

The reaction kettle will be placed in a Buchi water bath and chilled with ice. I plan on placing the sodium acetate into the kettle and slowly adding the S2Cl2 through an addition funnel. I will need to purchase a 3-way Clasien adaptor because the reaction kettle lid has only one 24/40 neck. I use a Fredreichs condenser with a CaCl2 drying tube. The S2Cl2 has a very foul odor and is toxic. A self contained powder addition funnel would help, but they go for a lot of money on Ebay. The original procedure mentions preparing individual small batches of the reaction mixture, but this would be wasteful due to losses during transfer and also exposes the chemist to large amounts of S2Cl2