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Author: Subject: Anhydrous AlCl3 from Dean-Stark apparatus?
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[*] posted on 9-11-2020 at 10:22
Anhydrous AlCl3 from Dean-Stark apparatus?


I'm pretty new to using dean-stark traps, and I'm wondering, could a toluene-water (or similar) immiscible azeotrope be exploited to dehydrate aluminum chloride hexahydrate for Friedel-Crafts reactions? Why or why not?
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macckone
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[*] posted on 9-11-2020 at 10:27


No, because the water is chemically bonded.
The ligands are not as free as in a normal hydrate.
Upon heating it releases HCl and converts to the hydroxide.

[Edited on 9-11-2020 by macckone]
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[*] posted on 9-11-2020 at 10:28


I think you would end up with a suspension of aluminum hydroxide in toluene.
https://en.wikipedia.org/wiki/Aluminium_chloride#Reactions_w...




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[*] posted on 9-11-2020 at 10:47


Probably some kind of aluminum hydroxide/basic aluminum chloride mixture. It is a common misconception that compounds such as AlCl3 FeCl3 etc are structurally related to their hydrates. I think that it is misleading nomenclature to describe aluminum chloride hexahydrate as AlCl3(H2O)6 because AlCl3 is actually a covalent compound (technically a dimer, Al2Cl6) while the hexahydrate contains aluminum ions coordinated to 6 water molecules, in an octahedral formation, with three chloride counterions. It would more accurately be written as Al(H2O)63+ + 3 Cl-

Saying that "AlCl3" is the anhydrous version of "AlCl3(H2O)6" is about as unhelpful and inaccurate as saying that acetic anhydride is the anhydrous version of acetic acid. There may be a difference in their formulas that corresponds to the loss of water, but they are fundamentally different compounds.

As I said about FeCl3 in another thread, while I know there's no hope of it catching on, I think using different names for these two very different compounds would go a long way towards helping people appreciate the difference. I personally prefer aluminum trichloride among the accepted names, since it sounds more covalent than just aluminum chloride, but why not go farther and call it trichloroaluminum, or hexachlorodialuminum to completely distance it from salty naming conventions? Similar names are accepted for boron analogs. But I digress.




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[*] posted on 9-11-2020 at 17:38


Quote: Originally posted by Texium (zts16)  
It is a common misconception that compounds such as AlCl3 FeCl3 etc are structurally related to their hydrates. I think that it is misleading nomenclature to describe aluminum chloride hexahydrate as AlCl3(H2O)6 because AlCl3 is actually a covalent compound (technically a dimer, Al2Cl6) while the hexahydrate contains aluminum ions coordinated to 6 water molecules, in an octahedral formation, with three chloride counterions. It would more accurately be written as Al(H2O)63+ + 3 Cl-

Saying that "AlCl3" is the anhydrous version of "AlCl3(H2O)6" is about as unhelpful and inaccurate as saying that acetic anhydride is the anhydrous version of acetic acid. There may be a difference in their formulas that corresponds to the loss of water, but they are fundamentally different compounds.


Fascinating! I was unaware that they were different compounds.

I suppose I'll go looking for a source of anhydrous aluminum chloride then. And if necessary, I can always burn a piece of Al foil in dry chlorine.
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[*] posted on 10-11-2020 at 20:51


I like aluminium trichloride, I will start calling it that from now on.
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[*] posted on 11-11-2020 at 03:58


Quote:
Quote: Originally posted by Texium (zts16)  
...

Saying that "AlCl3" is the anhydrous version of "AlCl3(H2O)6" is about as unhelpful and inaccurate as saying that acetic anhydride is the anhydrous version of acetic acid. There may be a difference in their formulas that corresponds to the loss of water, but they are fundamentally different compounds.

As I said about FeCl3 in another thread, while I know there's no hope of it catching on, I think using different names for these two very different compounds would go a long way towards helping people appreciate the difference. I personally prefer aluminum trichloride among the accepted names, since it sounds more covalent than just aluminum chloride, but why not go farther and call it trichloroaluminum, or hexachlorodialuminum to completely distance it from salty naming conventions? Similar names are accepted for boron analogs. But I digress.


Hear, hear. As in the name “sodium tetrahydridoborate” / “sodium borohydride” for NaBH4.
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