I understand that anhydrous zinc chloride is a Lewis acid and as such can only work in anhydrous conditions. Once hydrated, it will
lose it's catalytic properties (e.g. for alkylations, electrophilic substitutions) and cannot be dehydrated back merely by heating (as it will react
with the water and decompose to hydroxy-chloride):
ZnCl2.H2O -- heat --> Zn(OH)Cl + HCl
I don't understand though why many preparations call for zinc chloride catalyst, but also work in aqueous HCl (e.g. Blanc Chloromethylation, Lucas' Reagent). Wouldn't the aq. HCl make zinc chloride immediately irreversibly hydrated?
Maybe zinc chloride does not have such a high affinity for water as e.g. AlCl3 and might exist in equilibrium with its anhydrous counterpart (?):
ZnCl2.H2O + HCl <----> ZnCl2 + HCl.H2O
This would mean that simply dropping zinc metal or zinc chloride hydrate in conc. HCl will suffice rather than having to prepare the anhydrous
material first?clearly_not_atara - 6-5-2023 at 10:03
The goal of the Lucas reagent is to increase the reactivity of HCl. It also works to a lesser extent with CaCl2, IIRC. But zinc chloride is more
reactive. Anhydrous ZnCl2 is preferred because it minimizes the amount that you need to use, and professionals can simply buy it.
In fact you can make HCl (g) by adding concentrated HCl to anhydrous metal chlorides, including CaCl2. I don't know if HCl (g) will react with
ethanol, but as I recall it is effective for the chlorination of ethylene glycol.
Adding zinc to hydrochloric acid will not work, since you get a weaker solution than you started with. Adding ZnCl2*6H2O might work, but it dilutes
the solution. What does work, however, is that HCl (g) may be added to a solution of ZnCl2, giving a "recyclable" Lucas reagent:
This variant avoids the preparation of anhydrous ZnCl2 and gives a full-strength reagent.
[Edited on 6-5-2023 by clearly_not_atara]nimgoldman - 6-5-2023 at 14:57
Quote:
What does work, however, is that HCl (g) may be added to a solution of ZnCl2
I see. This would explain why some writeups on Blanc chloromethylation (esp. those that use aqueous HCl and formalin instead of paraformaldehyde) call
for saturating the solution with hydrogen chloride.
The Blanc chloromethylation reaction forms benzyl alcohol which is subsequently converted to benzyl chloride with the help of ZnCl2 (there is an
intermediate zinc complex).
I am trying this reaction but making anhydrous ZnCl2 for it seems like a huge ordeal (heating zinc to hundreds of degrees under stream of HCl...). I
hope that hydrated zinc chloride, hydroxide or chloride hydroxide followed by saturating solution with HCl(g) will do. From books it seems zinc is not
very reactive towards HCl...
I will read the paper on recyclable Lucas' reagent and hopefully find more details.clearly_not_atara - 6-5-2023 at 20:03
When gassing HCl into an aqueous solution, you must be careful about suckback. Make sure you are prepared for this. SyntheticFunk - 23-5-2023 at 23:38
One method I came across and tried was the bubbling of dry HCl gas into a flask containing DCM and zinc metal. A sulfuric acid bubbler is used to keep
the HCl gas anhydrous and as was already pointed out make sure you have a trap in case of suckback.
I never got around to actually testing the water content/general effectiveness of the produced ZnCl2 solution, but it did produce HCl upon contact
with water and the zinc pieces *almost* entirely disappeared throughout the attempt. As long as it is kept very strictly anhydrous I see no reason why
I would not work.
