Originally posted by Nicodem
If I would be in a desperate need of anhydrous AlCl3 and only had OTC chemicals, I would try to make it from Al foil and trichloroethylene.
I never did this as I was never in such a need of AlCl3 to bother so much, but I see no reason why it couldn’t be done. However this reaction is not
as straightforward as the MSDS of trichloroethylene seams to imply when warning of its violent reaction with Al and some other metals [1]. Obviously
in real experience C2HCl3 is one hell of an inert solvent (personal experience).
The reaction of trichloroethylene is, accordingly to [2] from which most of the present information is taken, something like this:
Al + CHCl=CCl2 ==> AlCl3 + [CCl2=CH]2 +[CHCl=CCl]2 + similar oligomeric vinylchlorides
Once some AlCl3 forms, it is able to cause a “self-accelerating, 'autocatalytic' dehydrochlorination of the solvent”.
In case anybody is to make such experiments here are some advices (which might be completely wrong since I never bothered to make a thorough research
on it). Also beware that trichloroethylene can produce toxic fumes like acetylenedichloride in some reactions and possibly other nasty side products
(fume hood!).
One problem is that commercial trichloroethylene contains some stabilizators that inhibit its reaction with metals [3]. In fact they are so efficient
that trichloroethylene can be safely used to clean aluminum and other metals (it is its main use in fact). Simply distilling it once might not be
enough since many inhibitors have a low boiling point. Maybe washing it with conc. sulphuric acid before distillation might help. Adding any other
solvent (toluene, DCM…) helps in further disinhibiting trichloroethylene.
Another problem is the inertness of the Al metal surface passivated by the oxide layer. The reaction is autocatalytic and starts only when some AlCl3
(or possibly other acids) forms on the metal surface. Therefore once it starts it also runs to the end (exothermally!). Hence, besides the removal of
the stabilizators, an activation of the Al foil surface is necessary. This should be possible in two ways: either by amalgamation or by etching the
surface by a reagent which would form a Lewis acid. The amalgamation should be possible by adding a drop of mercury in the Al/C2HCl3 mixture followed
by intense mixing. The other possibility would be to add some iodine crystals instead, in order to form some AlI3 on the surface. Reflux and anhydrous
conditions are a must for the reaction to start (a CaCl2 trap on the top of the condenser should be used). In order to prevent the exothermic runaway
of the reaction, only a small amount of aluminum should be added until the reaction starts and then further amounts of it added in portions. It is
best to use a considerable excess of C2HCl3 in order to minimize the formation of polymeric vinylchlorides, tar or carbon.
The formed AlCl3 probably forms a complex with the solvent (it forms a red complex with Me-CCl3, for example). Maybe it would be necessary to
precipitate it by addition of petrolether or naphta. The precipitate should be then vacuum filtered after the reaction is finished and cooled. The
polymeric side product should be washed off by some dry petrolether. If the product is a complex of AlCl3 with the solvent it might be possible to
obtain AlCl3 by heating it in order to destroy the complex. AlCl3 is extremely hygroscopic and therefore all this should be done in anhydrous
conditions.
A similar reaction should be possible with other chlorinated solvents like 1,1,1-trichloroethane, CCl4 and even methylenechloride.
[1] http://www.bocgases.ca/newsite_eng/gases/pdfengli/G217.pdf (“May react violently with aluminum, titanium, magnesium or their alloys.”)
[2] http://www.eurochlor.org/chlorsolvents/publications/digest20...
[3] patents on the inhibitors and stabilizators: GB617572, GB994167, US4368338, EP0033782, US4287003, GB940494, US2803676 and many other (there is
even a patent on how to remove stabilizators from trichloroethylene, but I can’t find it anymore) |
