Sciencemadness Discussion Board

Pyrolysis of aluminum acetate?

bbartlog - 4-2-2010 at 14:29

The prior discussion about acetic anhydride as a possible product of thermal decomposition of zinc acetate started me thinking about some of the patterns in the end products that result from pyrolysis of metal acetates. In general it seems that

- if the metal forms a fairly stable carbonate (alkali metals but also for example iron), then ketones are the largest molecule that is likely to form in pyrolysis; thus the acetates will be turned into carbonate and acetone (and smaller molecules as the temperature gets higher).
- if the metal is easily reduced, then acetic acid will be produced, as in the pyrolysis of verdigris or silver acetate
- if the metal is not easily reduced, but the carbonate is not very stable, then acetic anhydride might be possible.

Thus zinc, with a carbonate that starts to decompose to the oxide beginning around 200C, but which is not readily reduced to elemental metal, looks like a plausible candidate (and was mentioned earlier in this thread). But even more attractive on the face of it would be aluminum: the carbonate isn't at all stable while the oxide is very stable. Thus aluminum, if forced to divest itself of two (CH3COO), should be more likely even than zinc to keep hold of one oxygen atom and allow the formation of CH3CO(O)COCH3.
The downside of this is that the triacetate of aluminum is not easily made. The preparation in Brauer *starts* with acetic anhydride. Similarly there is a US patent (2141477) which, while using far smaller quantities, still starts with acetic anhydride to prepare neutral aluminum acetate. Wikipedia suggests barium acetate plus aluminum sulfate, but I can't find whatever primary reference this is based on (if indeed it isn't just pure fabrication).
In addition to this, the decomposition of basic aluminum acetate (aka aluminum diacetate) in vacuum *seems* like it might afford the anhydride (albeit with poor atom economy), thus:
2Al(OH)(CH3COO)2 -> Al2O3 + 2CH3COOH + CH3CO(O)COCH3
...just because there isn't enough hydrogen available to release all the acetate as acetic acid, and the decomposition temperature of 370C shouldn't be enough to immediately decompose the anhydride. Unfortunately the references I find on this pyrolysis are all much more interested in the alumina product than the vapors that are given off; but anyway no AcOAc is mentioned.
So a few questions:

- can anyone substantiate the claim in Wikipedia (now cloned elsewhere) that barium acetate and aluminum sulfate will yield aluminum triacetate?
- does anyone have a reference that shows the decomposition products (and quantities thereof) of basic aluminum acetate?
- does it seem plausible that the triacetate would yield acetic anhydride on heating?

(edit: sorry, had meant to post this in the stickied acetic anhydride thread. Please move as convenient).

[Edited on 4-2-2010 by bbartlog]

kmno4 - 4-2-2010 at 16:08

I have read many articles about thermal decomp. of M(I, II, III) acetates (hydrated and anhydrous) in different conditions.
There was no article reporting Ac2O as product of pyrolysis.

[Edited on 5-2-2010 by kmno4]

bbartlog - 4-2-2010 at 17:26

If you look at p21 of the acetic anhydride thread, there is a paper linked there that indicates that acetic anhydride is produced as a product of the pyrolysis of zinc acetate (second page of paper, equation (4)). However, given the lack of support elsewhere and the fact that the authors of that paper were concerned with the production of the basic zinc acetate and not the anhydride, I do think that they may just have written that as a shorthand to account for all the atoms lost to pyrolysis, and perhaps not to indicate the actual product. Since I have zinc chloride and sodium acetate I suppose I should really just do the experiment, though I don't think I can pull a vacuum as good as theirs...

User - 4-2-2010 at 17:46

Somewhere from sciencedirect.
Quote:

Abstract

Zinc hydroxy acetate, Zn5(OH)8(CH3CO2)2·4H2O, has been prepared by the precipitation method. It has been demonstrated by FTIR analysis that, contrary to previous reports, the interaction of the acetate anion with the matrix cation is ionic. TG analysis, mass spectral analysis of the evolved gases, and in situ variable temperature PXRD and FTIR analysis have shown that decomposition of the material to ZnO involves the formation of Zn5(OH)8(CH3CO2), Zn3(OH)4(CH3CO2)2 and anhydrous zinc acetate (Zn(CH3CO2)2) as some of the acetate-containing intermediate solid products. The acetate anion is finally lost, at temperatures below 400 °C, as acetic anhydride, (CH3CO)2O.


This might also work with leadacetate i guess.
Anyone willing to do some testing ?


[Edited on 5-2-2010 by User]

bbartlog - 4-2-2010 at 18:25

Watts' Dictionary from 1888 suggests that lead (II) acetate decomposes to acetic acid, CO2 and acetone above 280C, leaving finely divided metallic lead. Too easily reduced, in other words. I guess decomposition under vacuum might yield different results, or for that matter lead (IV) acetate (not that easy to obtain either...) might do the job.

kmno4 - 4-2-2010 at 23:29

Quote: Originally posted by User  
Somewhere from sciencedirect

Solid State Sciences
Volume 11, Issue 2, February 2009, Pages 330-335

This observation is based on MS spectra. No amount of Ac2O were isolated, no yield given. Also formation Zn(Ac)2 is comfirmed as "probably".

"Although a number of authors have investigated the thermal
decomposition of zinc hydroxy acetate, there are differences in the
interpretation of the decomposition profiles and identification of
the solid phases and gaseous products released at different temperatures"

Really, there are many articles about it. Cited article is the first one (I know) claiming Ac2O as main product of acetate decomposition.

Nicodem - 5-2-2010 at 04:50

Zinc acetate could be considered a mixed anhydride of two acids (CH3COOH and Zn(OH)2). As such it could in principle decompose to the two anhydrides, that is (CH3CO)2O and the nearest stoichiometry of basic zinc acetate, provided the products are being separated in the reaction process (via vacuum distillation, etc.). The same goes for the aluminium salt. So, going just by theory, I would tend to believe that the formation of acetic anhydride is likely, but with some scepticism about this being the main thermolysis route (kmno4 is correct in pointing out that if something is detected as a product, but no quantitave data is given, it means little for those who are interested in preparative reactions).
Other acetates of fairly acidic cations (like Al, Fe(III), Cu(II), Sn(IV), Zr(IV)...) might also decompose to acetic anhydride, but I don't have a clue on how you prepare such acetates. For example, it is probably easy to make such acetates in their basic or hydrated form, but I would expect Fe(OAc)3 or Cu(OAc)2 to require acetic anhydride to dehydrate the corresponding hydrated/basic forms. The other problem is in that some cations have a relatively high oxidation potentional (Fe(III)->Fe(II) or Cu(II)->Cu(I)->Cu(0)) and this might cause redox reactions to prevail. Metals that do not have acidic enough cations (such as the above mentioned Pb), or are strong oxidants (Ag was also mentioned), or the (hydr)oxides of which are strong bases (like alkali (earth) metals), obviously do not fit here (and besides it is known their acetates decompose differently - there are threads dedicated to these reactions).

So if any such reaction is to be tentatively aplied preparatively it would be limited to heating a mixture of anhydrous sodium or potassium acetate with either excess ZnCl2 or AlCl3. Any other way would be impractical (not to say that dry distilling such a mixture is practical!)