Sciencemadness Discussion Board

AcCl + ZnO ---> ??

turd - 7-8-2006 at 13:39

In J.Org.Chem. Vol. 69, 6953-6956(2004) iranian researchers present a high yielding aromatic acylation catalysed by ZnO:
Quote:
An aromatic compound (1mmol) was added to a mixture of ZnO (powder, 0.04g, 0.5mmol) and acid chloride (1mmol) at room temperature and stirred with a magnetic stirrer. [...react for 5-20min...] The solid mass was then eluted with dichloromethane (CH2Cl2) (20mL) [...wash, dry...]. Evaporation of solvent furnished practically pure the corresponding product.

Sounds too good to be true?
Well, I tried this at 100mmol, but when adding AcCl to ZnO, there was an highly exotermic reaction giving a nasty dark red liquid and a solid sticking to the wall of the flask. Adding a solid aromatic substrate gave an ugly dark red gunk, impossible to stir. Extraction with DCM gave a red organic layer, I didn't bother to work it up.

What happened?

Nicodem - 7-8-2006 at 22:34

I would assume ZnO + 2AcCl is able to form some ZnCl2 on the surface of ZnO, but the reactions in the paper include even a FC on chlorobenzene which would not never work with ZnCl2 itself.

Too bad you didn’t try to work up the product. Color and viscosity do not say much about the reaction occurrence.

Try again, perhaps with half less ZnO, cool in an ice bath while introducing AcCl and if the mixture becomes unstirable dilute with some CH2Cl2. If it does not work, then just face it that a large part of letters, communications and notes are bullshit. If they were not why would the authors not write a true article? It takes some more analyses but at least the paper is well worth something.

PS: The paper is attached for others to see.

Attachment: jo0494477.pdf (74kB)
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not_important - 8-8-2006 at 00:34

Yeah, tossing stuff out when you don't know what it is is a poor way to learn, and it likely leaves everyone else guessing.

I wonder if your ZnO is to blame. What was its origins and history? Right now I suspect it was either wet, had absorbed base from its formation, or had some organic component.

Likewise, what was the source of the AcCl - commercial, homebrew, if the latter then by what method?

turd - 8-8-2006 at 12:19

Hey now, I didn't toss it away, I just didn't work it up.

In a second experiment I did it in an open vessel, stirring and grinding with a spatula. I got 10% conversion (1H-NMR) and very minor impurities.

I don't like this method - that's not chemistry. At least I can reuse the obtained product/reagent-mix with a decent method. :/

AcCl is good, ZnO is technical grade.

not_important - 8-8-2006 at 14:12

Interesting. There's an old reaction where 2 moles of an acid halide react under the influence of anhydrous ferric chloride to give the halide of a beta keto acid, a bit similar to the acetoacetic condensation Hamonet, Bull. soc. chem. (2) 50, 355 (1885); (3) 2, 334 (1889).

It's possible that ZnCl2 can promote the same reaction, if so workup with water should result in some acetone - easy to detect.

I'd say try a bit of zinc oxide in a test tube, heat it to 150-200C, loosely cork and let cool, then add a cc or so of AcCl; to see if the ZnO had picked up water. Sure sounds as if had and they reacted and boiled, sending oxide onto the flask walls.

Of course the authors of that report are Iranian, and one appears to be female, so there's a good chance that they published a bogus report just to strike a blow at the AngloAmerican Empire.

edit to add - looks as if someone else did a this reaction acylating pyrrol. TiO2 has been used similarly under microwave irradiation.



[Edited on 8-8-2006 by not_important]

turd - 13-8-2006 at 10:48

More of the same (same authors):

Synthesis;13;2165-2168(2004)
Quote:
To a mixture of graphite (powder type; 0.1g, 8mmol) and MeSO3H (0.3ml, 4mmol) at 80°C in an oil bath, aromatic ether (1mmol) and carboxylic acid (1mmol) were added. The reaction mixture was kept at 80°C with occasional shaking for a certain period of time (Table 2) as required to complete the reaction (monitored by TLC). The reaction mixture was extracted with CHCl3 (2x20ml). The organic layer was washed with sat. aq NaHCO3, dried (Na2SO4). Evaporation of solvent gave practically pure product.


For anisole + acetic acid they give 98% in 20min. Note that this doesn't even need an acid chloride. If it works that is.

[Edited on 13-8-2006 by turd]

Vitus_Verdegast - 16-8-2006 at 16:08

A similar paper from the same authors:

Solvent-Free catalytic Friedel-Crafts acylation of aromatic compounds with carboxylic acids by using a novel heterogeneous catalyst system (p-toluenesulfonic acid and graphite)

What is interesting is that they claim obtaining some yield for the Friedel-Crafts acylation of anisole (with benzoic acid) using this method only when a solventless system is applied. Using CH2Cl2 or CHCl3 as the solvent gives only trace amounts of product.

They also tried using H2SO4 at 80°C for 3 hours, but only a trace amount of product was obtained.

I remember for Friedel-Crafts using carboxylic acids that alongside of the well-known polyphosphoric acid also p-TsOH could be used succesfully with a Dean-Stark trap.
But this was a long time ago and a patent IIRC...

dawt - 14-4-2021 at 11:43

Since this reaction has been brought up several times in other threads as well, I thought I'd report my failure here.
Attempted the reaction with toluene and acetyl chloride as described in the paper and was left with unstirrable gunk as in the post above, also working at a 100 mmol scale. Then I tried using an excess of toluene and dripping in the AcCl under stirring, which caused the ZnO to form a hard crust on the side of the flask. There was also a slight exothermic reaction at the end of which the solution had turned yellowish, so I heated the mixture to reflux, at which point the mixture turned dark, until no more HCl escaped the apparatus through the drying tube, and did the workup as described in the paper. Yielded a small amount of dark oil (with a faint smell which might just have been residual toluene, not sure) wich I attempted to purify via the bisulfite adduct, but no solids separated.

Edit: Just to address some of the points brought up in this thread: The ZnO was dried over a flame before use and the AcCl was from a fresh bottle.

[Edited on 2021-4-14 by dawt]

zed - 15-4-2021 at 17:12

Works with AlCl3. Claimed not to work with ZnCl2. Some folks claim it works with Zn powder.