[URgEnthalpy]

Will PCl₃ "have access" to H₂O molecules from different hydrates of an AlCl₃?
Also are AlCl₃ hydrates stable when suspended in a little bit wet aprotic solvents (benzene, STP)?

I am concerned if I should post the question in the Inorganic Chemistry Forum, anyway I chose this one because of major influence of organic medium as a solvent and the fact that lewis acids are commonly used in Organic Chemistry.

[pgk]

1). Yes, during the reaction procedure (heating, etc.), so you need to preliminary dehydrate AlCl3. Besides, hydrated AlCl3 is not an effective Lewis acid, neither an effective Friedel-Crafts catalyst.
2). No, they are not.

[URgEnthalpy]

Are there really procedures to dehydrate AlCl₃  Even if possible who would e.g. waste PCl₃ to dehydrate AlCl₃ as both are Lewis acids.

I rather considered a situation where AlCl₃ is added to a little bit wet substrate (benzene) and then PCl₃ is added. Will PCl₃ be the Lewis acid in this rxn, or it'll just regenerate AlCl₃ "stealing" its' water molecules?

[pgk]

1). By melting AlCl3 under vacuum or by dissolving it in an organic solvent, followed by azeotropic distillation.
2). AlCl3 is slightly slightly soluble in benzene and toluene, soluble in halogenated solvents and very soluble (and ionized) in water, which means avoiding wet aprotic solvents.

[Borek]

I seem to remember reading about a procedure involving liquid ammonia, the idea was when you eliminate oxygen there are no problems with Al₂O₃.

[URgEnthalpy]

2). AlCl3 is slightly slightly soluble in benzene and toluene, soluble in halogenated solvents and very soluble (and ionized) in water, which means avoiding wet aprotic solvents.

Assuming I'm acylating aprotic solvents, there is no way to omit this, or is it? AFAIK FC acylating benzene etc. requires only aryl-like substrate, acyl halide/ anhydride and a catalyst, what's more AlCl₃ is dirt cheap, I really don't want to minimalize waste. TBH I don't want to acylate anything.

Just theoretical consideration of the following reaction.

AlCl₃ +   nothing happens as there is no acyl halide
AlCl₃ + H₂O  AlCl₃·H₂O    (always present in some quantities)

PCl₃ + 3AlCl₃·H₂O    H₃PO₃ + 3HCl + 3AlCl₃ or this reaction doesn't proceed?

[pgk]

1). Something happens because if you filter the suspended AlCl3.H2O, you get a little lower amount than the initial added one, due to its low solubility in the wet benzene.
2). If you add alkyl halides or acyl halides or acid anhydrides, a little or nothing will happen because the Al empty 3p orbitals are occupied by a free electron pair of the co-crystallized water molecule and therefore, AlCl3.H2O cannot catalyse Friedel-Carfts reactions, neither form the Cl3P-AlCl3 complex (PCl3 is a Lewis base).
3). Hydrolysis by co-crystalline water cannot occur and thus, hydrolysis of PCl3 by AlCl3.H2O in benzene cannot proceed at 1/3 molar ratio, even by heating because dehydration of AlCl3.H2O demands temperatures, quite higher than the boiling point of PCl3 and benzene (76 and 80oC, respectively). Except if adding a large excess of PCl3 that dissolves AlCl3.H2O and heat it up to 75oC.
But all that change by refluxing in solvents with boiling point, higher than 100-110oC.
PS: The most common AlCl3 hydrate is the hexahydrate form: AlCl3.6H2O.

[URgEnthalpy]

1. Dissolution is a physical process no chemical reaction, this is a chemical forum, so I was right saying that nothing happens.
2. I know that hydrates CAN NOT catalyse FC acylation.
3. AFAIK you CAN NOT dehydrate AlCl₃ hydrates
"Heating this solid does not produce anhydrous aluminium trichloride, the hexahydrate decomposes to aluminium hydroxide when heated:
Al(H₂O)₆Cl₃ → Al(OH)₃ + 3 HCl + 3 H₂O" - or you wanted to say about hydrolysis catalyzed by PCl₃ in higher temp?

Finally the most interesting part. Does PCl₃ creates complexes with AlCl₃? Both are Lewis Acids so I don't see a reason why it should. Also AFAIK AlCL₃ forms the dimer in STP and it doesn't disturb the FC acylation.

I saw a video where P₄O₁₀ catalyzed FC rxn, how?

[pgk]

Let’s try to put everything in order:
1). P has an electronic structure: 1s2 2s2 2p6 3s2 3p3 3d0, which means that PCl3 has empty 3d P orbitals and consequently, it is Lewis acid.
But also, PCl3 has a free electrons pair and consequently, it is Lewis base, too and can form complexes with strong Lewis acids, such as AlCl3.
2). PCl5, POCl3 and P2O5 (P4O10) still have empty 3d P orbitals but not a free electrons pair and consequently, they are Lewis acids only.
3). As being Lewis acids, POCl3 and P2O5 (P4O10) can be effective catalysts (alone or as a co-catalysts) in particular Friedel-Crafts type reactions (e.g. Vilsmeier formylation, Fries rearrangement, etc.).
4). AlCl3 is a salt of a weak amphoteric base (Al(OH)3) with a strong acid (HCl) and consequently, it has acidic properties in aqueous solutions due to the partial hydrolysis to Al(OH)3 and HCl. Higher temperatures increase the equilibrium constant and the rate of this hydrolysis.
5). Due to different co-ordination energy levels, water molecules do not escape at the same temperature in multi-hydrate complexes. Thus, dehydration of monohydrates usually occurs at ≈ 100oC, dihydrates at ≈ 120-130oC, trihydrates at ≈ 140-150oC and so on…and polyhdrates may reach up to very high temperatures.
6). At these high temperatures, the liberated water vapor quickly hydrolyse the oligohydrate or anhydrous AlCl3. However, this can be avoided by simultaneous elimination of the liberated water (e.g. vacuum in lab, azetropy in lab or industrial scale, using water scavengers in industry, etc.).
7). In other words, you can melt AlCl3 hydrate under vacuum, in a flask with a weak-medium flame and after dehydration and cooling, you can add the necessary reagents and solvents and continue with a Friedel-Crafts reaction. But this procedure demands adequate experience because a stronger flame can quickly form Al2O3 by simultaneous dehydration/ dehydrochlorination. Besides, this method is recommended only for “in situ” dehydration purpose and cannot be applied as a preparation method of anhydrous AlCl3.