Sciencemadness Discussion Board

Easiest way to obtain Lucas Reagent?

sveegaard - 11-7-2010 at 02:43

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

JohnWW - 11-7-2010 at 03:34

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]

unionised - 11-7-2010 at 04:42

I'm pretty sure that drying down a solution of ZnCl2 won't give you the anhydrous material because it will lose HCl.

bbartlog - 11-7-2010 at 05:56

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.

Contrabasso - 11-7-2010 at 10:30

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.

aonomus - 11-7-2010 at 10:39

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)

DJF90 - 11-7-2010 at 14:16

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.

Mister Junk Pile - 12-7-2010 at 09:57

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





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]

unionised - 12-7-2010 at 11:45

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..

bbartlog - 12-7-2010 at 11:57

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...

bbartlog - 12-7-2010 at 12:12

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.

sveegaard - 13-7-2010 at 03:31

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 :D

ziqquratu - 14-7-2010 at 21:10

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.

BlackDragon2712 - 6-11-2013 at 10:38

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