sveegaard
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Easiest way to obtain Lucas Reagent?
Hello everybody 
I need some Lucas reagent, but it seems impossible to obtain anhydrous ZnCl2 in Denmark 
Do you think it could be produced by adding a little zinc to conc. HCl?
I have some AlCl3-solution, could this be used instead as a Lewis acid? Or does this have to be anhydrous too?
Cheers
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JohnWW
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You could obtain ZnCl2 by dissolving some Zn or ZnO or ZnCO3 in an excess of HCl, and slowly evaporating the solution to near dryness to crystallize
out the ZnCl2, which is very soluble. (I once made some ZnCl2 this way). ZnO is obtainable from pharmacies, being mixed up with olive or peanut oil
and applied to the skin as a sun-screen.
However, AlCl3 cannot be so easily obtained, because it is much more liable to hydrolysis by water. Anhydrous AlCl3 is obtainable only by direct
reaction of Al metal with gaseous HCl or Cl2. Anhydrous ZnCl2 could also be made more quickly this way than by the aqueous method, if you have
equipment for handling gaseous HCl or Cl2.
[Edited on 12-7-10 by JohnWW]
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unionised
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I'm pretty sure that drying down a solution of ZnCl2 won't give you the anhydrous material because it will lose HCl.
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bbartlog
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You can't dry ZnCl2 by evaporating to dryness (nor by other trivial means, as noted). But you don't need anhydrous ZnCl2 to make Lucas' Reagent -
you're adding it to an aqueous solution of HCl, it's not going to matter if it has a bit of water. I would expect adding zinc to HCl as you proposed
would work fine.
I don't know whether AlCl3 would also work, but whatever solution of it you have is surely not aqueous, would likely not be miscible with conc. HCl,
and the AlCl3 would react with the water to form some kind of chlorohydrate. Likely not a good idea.
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Contrabasso
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The ZnCi2 available from Timstar in the UK is £10 for 500g and supposedly assays to 97% -they are a supplier for school labs. www.timstar.co.uk and search their site thoroughly for a link to their downloadable catalogue.
Getting supplies to a foreign country could be costly though if they do it correctly, but they do indicate their ADR commodities well in their price
list.
How anhydrous does it really need to be? It's a test reagent so it shouldn't be severely sensitive to water, cos water is everywhere in the
atmosphere.
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aonomus
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Would zinc powder react with HCl gas bubbled into MeOH perhaps? You would avoid the solvation problem if you had anhydrous solvent. I believe this is
one of the ways anhydrous FeCl2 is prepared (iron in MeOH + HCl gas)
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DJF90
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Considering that Lucas's reagent is a solution of ZnCl2 in conc. HCl (i.e. there's water present), I would expect hydrous zinc chloride would work
sufficiently.
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Mister Junk Pile
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You know, that makes sense to me but just about every procedure I have read says to use "anhydrous zinc chloride". Perhaps it is so the exact mass is
known?
I've seen some people go to strange lengths to "be able to" use hydrated ZnCl2.
For example:
| Quote: |
this invention provides a process which comprises introducing gaseous anhydrous hydrogen chloride into an aqueous solution of zinc chloride under
conditions effective to produce a more efficient and/or useful Lucas reagent than Lucas reagent made in the conventional manner by reacting solid,
anhydrous zinc chloride with 38% hydrochloric acid. The conditions under which Lucas reagent is formed pursuant to this invention are readily
achieved. Thus the gaseous anhydrous hydrogen chloride is introduced into a water solution formed by dissolving at least 60 parts by weight of zinc
chloride (ZnCl2) in 40 parts by weight of water, and preferably at least about 70 parts by weight of zinc chloride in 30 parts by weight of water. The
solution can be saturated or even super-saturated with the zinc chloride, but preferably the solution is free of solids. During the introduction of
the anhydrous hydrogen chloride gas into the zinc chloride solution the temperature of the reaction mixture is maintained at one or more temperatures
in the range of about 10° to about 60° C., and preferably at one or more temperatures in the range of about 20° to about 40° C. at ambient
atmospheric pressure. The amount of anhydrous hydrogen chloride gas introduced into the zinc chloride solution should be in the range of slightly
above 1 mole (e.g., 1.05 moles) up to about 2 moles per mole of zinc chloride. Preferably the amount of HCl gas used is in the range of slightly above
1 mole to about 1.5 moles per mole of zinc chloride.
http://www.patentstorm.us/patents/5856597/description.html
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Why go through all that trouble if Zn(s) could just be dissolved in HCl? And why must the HCl be anhydrous? I must be missing something here.
Maybe it's just that the water of hydration on the ZnCl2 slightly dilutes the acid causing a less effective reagent? I would think that this effect
would be negligible though. Does anyone have direct experience with this?
Also, in what cases could Lucas' Reagent NOT be used as a substitute for thionyl chloride (for secondary and tertiary alcohols--and alcohols with
other functional groups as well).
[Edited on 7-12-2010 by Mister Junk Pile]
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unionised
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The reagent is used to turn alcohols into chlorides.
The by product is water.
If you add excess water then you drive the reaction backwards.
Anhydrous zinc chloride acts as a dehydrating agent; water is trapped as water of hydration round the ions. Then you can start to form chloro species
from the ZnCl2 and HCl which are more powerful acids and chlorinating agents..
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bbartlog
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Well, the Lucas' Reagent created via that patent *would* be more concentrated (have less water per unit of HCl) than one made by adding ZnCl2.H2O(x)
to 38% HCl. So it might well have superior activity. ZnCl2 can form very concentrated solutions (as the patent implies); I have a mason jar of 75%
ZnCl2 (by weight) in water, for example. If as the patent suggests it is possible to dissolve two moles of HCl per mole of ZnCl2 in such a solution,
the end product would be close to 3:2:1 (ZnCl2:HCl:H2O) by mass; such a ratio can't be achieved by adding ZnCl2 to 38% HCl (at least the HCl will
always be short).
This would be an interesting thing to try if I have another go at making isopropyl chloride; the prep seems to require that not too much water is
present, and I hadn't realized that an aqueous solution of ZnCl2 will retain this much HCl without outgasing horribly...
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bbartlog
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| Quote: | | Maybe it's just that the water of hydration on the ZnCl2 slightly dilutes the acid causing a less effective reagent? |
Well, yes. But based on my experiences trying to chlorinate isopropyl alcohol I think it's not a linear or simple effect. There seems to be some
breakpoint (of concentration, i.e. lack of water) beyond which the chlorination proceeds a lot more easily. It's not simply a case of driving the
reaction equilibrium by eliminating water; if that were the case, the fact that isopropyl chloride can be distilled off (removing it from the
equilibrium) would suffice. The 'chloro species' that unionised mentions are the important thing.
I do want to say: when you first mentioned it, I thought you wanted the reagent in its role as a test for alcohols. If you want to do preparative
chemistry with it then I think it definitely makes sense to try to eliminate as much water as is feasible. One thing I've done is to mix the aqueous
HCl with the target alcohol, then remove much of the water using MgSO4 and filtering, before adding the ZnCl2 solution. But if you can gas with dry
HCl you'd be in even better shape.
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sveegaard
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Quote: Originally posted by unionised  | The reagent is used to turn alcohols into chlorides.
The by product is water.
If you add excess water then you drive the reaction backwards.
Anhydrous zinc chloride acts as a dehydrating agent; water is trapped as water of hydration round the ions. Then you can start to form chloro species
from the ZnCl2 and HCl which are more powerful acids and chlorinating agents.. |
That's right. And as others say, the HCl is aqueous itself. I've got hold on some ZnO; I'll have a go with that.
Thanks for all your answers 
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ziqquratu
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Couple of points:
1) In sufficiently acidic solutions, Lewis acids remain in their halogenated form (using the acid of the same halide, of course!) and thus remain
active - hence why the Lucas reagent can work despite being an aqueous solution.
2) ZnCl2 can be dried by carefully melting (mp about 275 *C) with a torch or heat gun, and stirring carefully with a glass rod until no more bubbles
form. If you see white (HCl) fumes, you're heating it too hot! Once it's dry, pour the melt onto something heat-safe (enamel tray recommended, good
quality pyrex dish, perhaps?), let it cool, crush and store.
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BlackDragon2712
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The other day I purchased 1 L of butanol, the problem was that they didn’t told me if it was the primary or secondary type of alcohol so I decided
to make a lucas test but I only had 32.5% HCl. I did the test anyway using isopropanol and ethanol as references. I tested the unknown alcohol and no
reaction was observed, as well as in the ethanol. But in the isopropanol solution a white precipitate was observed, though not very notable but it was
on the bottom of the test tube (the white "cloud").
In conclusion the less water you have in your solution the best the test (or the easiest to see) will be, but it will work with not anhydrous zinc
chloride though not very well.
Also as ziqqurat says, zinc chloride can be dehydrated by heating it very carefully
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