Sciencemadness Discussion Board - Aluminum bromide from HBr

Aluminum bromide from HBr

Domo_Kun - 13-11-2011 at 05:56

I have some 48% HBr and I am wondering if I can make aluminum bromide that way:

6HBr + 2Al -> AlBr3 + H2

I ask because the wikipedia page on AlBr3 says "reactive" as its solubility with water. Is it just because its hygroscopic or will I loose my product if it reacts with water?

Mixell - 13-11-2011 at 06:03

You will have Al(H2O)6 3+ and Br- in solution. With some time and effort you can make a solid AlBr3(H2O)6 from it. The anhydrous form cannot be liberated from the solution (except boiling with thionyl chloride of drying under a stream of HBr gas).

Domo_Kun - 13-11-2011 at 06:08

Thank you for your response.
I'm assuming that is not the best way to make anhydrous Al2Br6 then?
reaction Al with elemental bromine is too intense right? Is there a way to have HBr gas to react with aluminum in anhydrous conditions?

[Edited on 13-11-2011 by Domo_Kun]

Mixell - 13-11-2011 at 06:24

The reaction with gas requires high temperatures.
Woelen had made PBr3 from red P and Br2, I think its based on the same principle. I think it is manageable if you do it step by step in tiny quantities.

Lambda-Eyde - 13-11-2011 at 07:16

Quote: Originally posted by Domo_Kun

Thank you for your response.
I'm assuming that is not the best way to make anhydrous Al2Br6 then?
reaction Al with elemental bromine is too intense right? Is there a way to have HBr gas to react with aluminum in anhydrous conditions?

Our very own len1 has described a suitable procedure in his book, "Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling". I have the book at home. If you're interested, I can describe the procedure when I get home.

Domo_Kun - 13-11-2011 at 07:17

Ok so if I try to react Br2 with Al, I have to condense the Al2Br6 on some surface over the reaction and then quickly scrape it off and store it air tight right? Is there a better way, with larger yeilds of say 10g at a time?

Domo_Kun - 13-11-2011 at 07:33

Lambda-Eyde , that would be awesome!

Lambda-Eyde - 17-11-2011 at 17:02

First of all, I hope Len doesn't object to me sharing his procedure on the forum - I view it as a teaser for his book, which I recommend that everyone buy.

Warning! This procedure has the potential of killing an untrained chemist not taking the proper safety precautions! The synthesis involves reacting neat (!) bromine with aluminium granules. Bromine is volatile and highly toxic and the reaction is extremely exothermic, which, of course, is a bad combination. Use of a fume hood, lab coat, appropriate heavy gloves, goggles and a face shield is mandatory! Copious amounts of aqueous sodium thiosulfate should be at the ready in case of an accident.

From Leonid Lerner's Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling, CRC Press (2011) (some non-essential parts are ommited):

Experimental

Thirty-three grams of aluminium granules is placed in a flat-bottom, 1-L, three-neck flask with a thin, ~1 cm layer of glass wool protecting the bottom. The flask is equipped with a thermometer, a double-surface reflux condenser, and a dropping funnel with pressure equalization containing 185 g bromine, previously dried by shaking with concentrated sulfuric acid. The top of the reflux condenser is protected from moisture by a CaCl2 tube, and prior to commencement of the reaction, the setup is flushed with dry nitrogen.

The reaction is started by allowing 10-15 drops of bromine to run into the flask, and waiting up to several minutes for the evolution of white fumes, indicating a reaction. Following this, a few more drops are cautiously added, observing the gradual rise in temperature. The bromine reacts with the aluminium in sporadic fashion, with the addition frequently accompanied by flashes and sparks lasting several seconds; at other times the addition produces no obvious sign of reaction. With the bromine added in bursts of 5-10 drops, after about 10-15 min the temperature in the flask should reach 210oC-220oC. After about half the bromine has been added, vigorous refluxing commences in the bottom portion of the condenser. This provides cooling and allows the addition rate of bromine to be increased, controlled by the reflux rate. Provided this regime is maintained, there should be no noticeable escape of bromine at the top of the condenser or deposition of solid Al2Br6 in its middle section. The temperature at the end of the reaction drops to just below 180oC; however, no external heating is required. The entire process takes about 2 h.

At the end of the reaction, the insides of the flask are dark red, while white-red flakes are condensed on the flask walls. The reflux condenser is now removed, and the dropping funnel replaced by a downward-sloping distillation head leading to a 250 mL, three-neck receiver flask. One neck of the flask is connected through a small U-tube containing CaCl2 and an empty guard flask to the aspirator vacuum; the other neck is connected to a nitrogen source through an inlet adaptor with a shut-off valve. Vacuum is applied to the system and the reagent flask heated. Initially, bromine vapor fills the apparatus and is removed by the aspirator. This is followed by some liquid bromine that distills when the flask temperature rises above the mp of Al2Br6. This is also removed by the aspirator without condensing in the receiver flask. Boiling commences at about 120oC-140oC, and the first fraction collected is of reddish color. After about 10 g of product has passed, the distillate becomes perfectly colorless, and this fraction is gathered.

When all product has been collected, the vacuum is disconnected and the apparatus filled with nitrogen. The receiver flask is now disconnected while the product is still liquid and its contents are rapidly emptied into a preheated mortar contained in a nitrogen-filled dessicator, making an effort to spread the liquid onto the mortar walls as much as possible. When the mortar has cooled, the Al2Br6 can be ground under nitrogen and the resulting powder stored in a glass-plugged bottle. The yield is about 180 g, or 85 % based on bromine.

Comments (by me):
The layer of glass wool is important in order to protect the flask from cracking. The flask should be situated over a through (sp?) to contain the reaction in case of a catastrophical failure. Considering the nature of bromine and Al2Br6 at >200 degrees, plastic might not be the best idea... The experimental doesn't mention the particle size of the aluminium used, only that it is "granular". I think using >400 mesh pyrotechnics grade aluminium would be a bad idea, and I think I won't have to explain why. And again, I have to stress how important a good fume hood and protective equipment is. From personal experience I have found it unbearable to transfer 15 mL liquid bromine from a flask to a dropping funnel without adequate ventilation. Also, bromine attacks tissue with vengeance (quote Magpie

Ozone - 17-11-2011 at 17:25

see: http://www.youtube.com/watch?v=UTbXKcD8ngM&feature=relat...

Sedit - 17-11-2011 at 17:46

I have told people before the aluminum halide hydrates CAN be liberated by adding it to DMSO. This precipitates as complex that can be dried till its anhydrous. Stronger heat decomposes this back to DMSO and AlCl3 (the chloride was what the patent was for) and the anhydrous Aluminum trihalide will sublime out of the mixture leaving behind DMSO.

Also there was some talk of using Aluminum Sulfate and NaCl in the past, this heated at high temperatures converts to sodium Sulfate and AlCl3 would distill out of the reaction. I presume NaBr would behave in a similar fashion. I don't know of anyone performing this reaction yet however.

Lambda-Eyde - 17-11-2011 at 20:27

Quote: Originally posted by Ozone

see: http://www.youtube.com/watch?v=UTbXKcD8ngM&feature=relat...

Not really a preparative procedure, is it?

Quote: Originally posted by Sedit

I have told people before the aluminum halide hydrates CAN be liberated by adding it to DMSO. This precipitates as complex that can be dried till its anhydrous. Stronger heat decomposes this back to DMSO and AlCl3 (the chloride was what the patent was for) and the anhydrous Aluminum trihalide will sublime out of the mixture leaving behind DMSO.

I'm aware of the patent you speak of, but I have yet to see anyone on SM actually do it. Not that I doubt the patent, it's just that I'd like to see an amateur pull it off. IIRC the patent calls for heating of the metal halide-DMSO complex under high vacuum (<1 torr), which is out of range for most of us. For those who have pumps that can pull such a high vacuum, the water and DMSO vapors present a larger obstacle. At least for me, both dry ice and teflon diaphragm pumps are out of the question. However, it would be interesting to see if it could be pulled off with a lower vacuum. Edit: now that I think of it, dry ice isn't needed for the condensation of neither DMSO nor water. A salt-ice bath would suffice.

But that's missing the point anyways, because aluminium tribromide is quite different from its little sister aluminium trichloride. The tribromide fumes like crazy in air, owing to the rapid hydrolysis giving HBr and aluminium oxides/hydroxides. Aluminium chloride also hydrolyses, but not as fast as the bromide. Keeping the solution acidic with HCl slows the reaction with water.

[Edited on 18-11-2011 by Lambda-Eyde]

Sedit - 17-11-2011 at 20:58

I will double check but IIRC there was no vacuum and DMSO has a relatively high boiling point meaning that the AlBr3 or whatever will sublimate out well before DMSO begins to fume. I have seen the appearance of success on small scale but the smell of DMSO sickens me.

Lambda-Eyde - 17-11-2011 at 21:24

I double checked it for you, and the last step in the process (removing the DMSO from the complex) involves heating under a high vacuum. The first example calls for a vacuum of 10-8 torr. I'm assuming we're both talking about US Patent 3471250.

Now that I read it, I saw that the patent mentioned aluminium tribromide hexahydrate, so it could be that I'm wrong in assuming that the bromide doesn't form a stable hydrate. I just don't see how it could happen, however.

Domo_Kun - 19-11-2011 at 06:24

Thank you Lambda-Eyde !
I think I might just buy that book