Domo_Kun
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Aluminum bromide from HBr
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?
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Mixell
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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).
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Domo_Kun
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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]
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Mixell
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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.
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Lambda-Eyde
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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?
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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.
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Domo_Kun
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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?
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Domo_Kun
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Lambda-Eyde , that would be awesome!
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Lambda-Eyde
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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
CaCl<sub>2</sub> 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
210<sup>o</sup>C-220<sup>o</sup>C. 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
Al<sub>2</sub>Br<sub>6</sub> in its middle section. The temperature at the end of the reaction drops to just below
180<sup>o</sup>C; 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 CaCl<sub>2</sub> 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 Al<sub>2</sub>Br<sub>6</sub>. This is also removed by the aspirator without condensing in
the receiver flask. Boiling commences at about 120<sup>o</sup>C-140<sup>o</sup>C, 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 Al<sub>2</sub>Br<sub>6</sub>
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 Al<sub>2</sub>Br<sub>6</sub> 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 )!
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Ozone
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see: http://www.youtube.com/watch?v=UTbXKcD8ngM&feature=relat...
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Sedit
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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.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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Lambda-Eyde
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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]
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Sedit
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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.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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Lambda-Eyde
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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<sup>-8</sup> 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.
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Domo_Kun
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Thank you Lambda-Eyde !
I think I might just buy that book 
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