[Heory]

 
When I carried out the Friedel-Crafts acylation reaction, the reaction mixture turned black. The solvent, dichlorobenzene, was removed by vacuum distillation when the reaction was complete. Then acidic water was added and the mixture was extracted by ethyl acetate. Unfortunately, I obtained the disgusting black tar at last. I tried to isolate my product from the black mixture with all kinds of solvents, in order to find out one that would dissove either the desired product or the black impurities, but failed. The desired product is an acid so column chromatography maybe not a good choice.

Could anybody tell me how I could remove the disgusting gummy tar? Thanks in advance.

[tmartin]

I've never done a Friedel-Crafts reaction before... seems like it's on of those reactions you should do at least once.

Is it possible that your product is just a very viscous oil and you have some slight impurity in there adding the black color to it?  If that's the case, I would try using some charcoal to filter out the colored impurities, I've done it before and it seemed to work fairly well (and it's been suggested on the forums a lot before, too I believe), just be careful not to lose your stuff on it.  It is an acid, however; I think if you ran a short column, you could probably get it off the silica without losing any.  What does the TLC look like?  Just a big spot at the bottom, or is it a mix?  I've performed column chromatography a few times on some acids and it seems to work out alright, you can sometimes spike the solvent with a little acetic acid to help prevent streaking down the column.  Or maybe just a quick acid base extraction could also get rid of the impurities too, I do that when I'm feeling particularly lazy or my reaction is about spot to spot.

[nj_bartel]

I've made a few imidazolium salts, which all seem to give brown/black tar.  Tried pretty much everything except HPLC.  Got some isolation with trituration but with really crappy yields.  Ended up running future reactions "dirty" with the tar without a problem, so you may try that.  You could run, obtain new product, and try to purify that instead.

[Heory]

My product should be a white solid and I think there are much black impurities in it. I will try an acid base extraction, hoping it would work. Thank you.

[Heory]

I've made a few imidazolium salts, which all seem to give brown/black tar.  Tried pretty much everything except HPLC.  Got some isolation with trituration but with really crappy yields.  Ended up running future reactions "dirty" with the tar without a problem, so you may try that.  You could run, obtain new product, and try to purify that instead.

I have done F-C acylation reactions for several times. With the black tar I  ran future reactions (Wolff-Kishner reduction and then ring closure by PPA) and I got the tetralones in very low yields. So this time I want to purify the product of this step.
How do you think about an acid-base extraction?

[OC pro]

Several options:

1. Acid-base extraction
2. Vacuum Distillation (less than 1mbar is needed I suppose)
3. Column chromatography (My favor). The product should run in petrol ether/ethyl acetate 1:1 mixture with 2 drops of glacial acetic acid in the TLC chamber (you should see a clear spot at Rf 0,4-0,6 I think). For the column eluent you simply add 1% acetic acid (means 10 ml in 1L eluent). You will have absolutely no problem with the column. The compound will not stick to the silica gel. Even without acetic acid it would work (gives a broader spot on TLC). Visualization of acid with UV and bromocresol green (especially for acids, EtOH solution, gives a green spot).

[demoninatutu]

Is it not more likely that you're getting polymerisation and turning all your reactant into black tar? That would certainly explain the low yields. I wasted months on a similar problem with maleic anhydride only to find the solution was to run the reaction 20ºC cooler.

In this publication:

P. Wipf and J. Jung J. Org. Chem. 2000, 65, 6319-6337

they add the reactants on ice and let it warm up to room temperature over 3 hours. If you're refluxing the dichlorobenzene, that would certainly be a problem.

[Heory]

thank you very much! I ran the reactoin 40ºC cooler. as a result, I got rid of most of the tar and the yield increased a lot.

[demoninatutu]

Very glad to hear it!