4
$\begingroup$

In the Friedel - Crafts acetylation of bromobenzene, how is it possible to obtain a black-coloured product?

When the acetic anhydride was slowly added to the reactants - $\ce{AlCl3}$, DCM, bromobenzene - a small, hard, yellow lump formed.

Would this be $\ce{AlCl3}$?

When the acetic anhydride was added, the reflux was initiated. The lump appeared to disappear, leaving behind a dark red solution.

However, when the reflux was turned off, the solution appeared black. I'm trying to think of potential products, such as ortho and para bromobenzene or acetic acid or $\ce{Al(OH)3}$ but why so black?

Does anyone have any ideas?

$\endgroup$
1
$\begingroup$

I assume "DCM" represents dichloromethane, if so...

There is an old paper that describes the formation of a low molecular weight polymer from the double Friedel-Crafts alkylation of benzene by dichloromethane. That's probably what's happening in your reaction as a minor pathway. The dissolved polymer and tar is what's producing your black solution.

I actually ran that reaction once, I wanted to prepare that polymer. It didn't work for me, but at the end, after the aluminum chloride had been removed, I did have a dark solution.

enter image description here

$\endgroup$
  • $\begingroup$ Thanks ron . Can you link that paper to me? When you say ''tar'', what do you mean? $\endgroup$ – Edward Jan 31 '15 at 20:50
  • $\begingroup$ @Edward I looked for a link before I posted and couldn't find one, the paper may be to old for an on-line citation to exist. I'm guessing it was Journal of the Amer. Chem. Soc., maybe the 1940s or 50's. If someone searches Chemical Abstracts with the terms Friedel-Crafts, dichoromethane, benzene, polymer, I think you'd find it. When I rotovap'd the black liquid down, up wound up with a tar-like solid. I wasn't able to isolate a clean, low-molecular weight polymer (oligomer) like the paper described. I figured the tar was just a mess of higher MW polymers. $\endgroup$ – ron Jan 31 '15 at 21:06
  • $\begingroup$ Okay. I was confused by a further two things. The addition of half-saturated NaCl - does that reduce the surface tension thereby allowing the organic droplet to ''fall'' as such? And washing with water can remove AlCl3 by way of Al(OH)3 , H+ and Cl- ions...but why is HCl gas not produced? $\endgroup$ – Edward Jan 31 '15 at 21:47
  • $\begingroup$ And the bridge between the two benzenes? Would that be the culmination of SN1? $\endgroup$ – Edward Jan 31 '15 at 22:14
  • 1
    $\begingroup$ The bridge is the result of CH2Cl2 undergoing 2 Friedel-Crafts alkylations with 2 aromatic rings. While it is not a true SN1, the Friedel-Crafts alkylation does involve a carbocation-AlCl3 complex. $\endgroup$ – ron Jan 31 '15 at 22:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.