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Lab preparation of AlCl3: reduction of zinc chloride with aluminium powder

For the laboratory preparation of anhydrous aluminium chloride (CAS 7446-70-0) several routes are available including chlorination of aluminium powder or turnings with elemental chlorine or dry hydrogen chloride. Here we present an alternative method based on the use of anhydrous zinc chloride powder as a chlorinating agent.

The reduction reaction:

ZnCl2(s) + 2/3 Al(s) → Zn(s) + 2/3 AlCl3(s)

… has Standard Heats and Free Energy Changes, ΔH and ΔG, of about – 50 kJ/mol (both), suggesting this reaction would proceed with modest generation of heat. Initial experiments showed that this was indeed the case and that the self-sustaining reaction delivers enough heat to sublime the aluminium chloride (sublimation point 180 C).

For the lab preparation of anh. AlCl3 the apparatus below was used (prior to reagent charging):

A large borosilicate boiling tube (OD about 25 mm, length 145 mm) is horizontally clamped up, in such a way that its neck penetrates into a 250 ml Simax wide necked reagent bottle, which will act as a condenser flask.

A freezer bag containing some ice and a bit of water is then draped over the reagent bottle (see second photo).

2.4 g aluminium powder (a small excess resp. of stoichiometry) and 15.6 g anh. ZnCl2 are weighed and quickly ground together (the ZnCl2 powder was slightly lumpy). ZnCl2 is hygroscopic, so work fast. The reagent mixture is then loaded into the boiling tube and clamped as above. The reaction is initiated by strongly heating the side of the charge that is closest to the condenser flask. Once initiated and fumes of AlCl3 can be seen to sublime off, the reaction is self-sustaining. The AlCl3 vapours collect in the condenser flask and condense (with minimal losses). The flow of AlCl3 can be regulated somewhat by adding gentle heat from a medium hot Bunsen burner or by withdrawing it.

The anh. AlCl3 is obtained as a white mass. Immediately stopper the condensation flask hermetically because AlCl3 is very hygroscopic.

It cannot be excluded that some contamination of the product by mechanically entrained ZnCl2 occurs. The apparatus allows easy re-sublimation of the AlCl3.

Here’s the apparatus post-reaction:

In the tube is left crude Zn powder. Condensed AlCl3 can be seen in the Simax reagent bottle.

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