DFliyerz
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Anhydrous Aluminum Bromide
As you may already know, a much more easily made alternative to anhydrous aluminium chloride for use as a catalyst in reactions such as Friedel-Craft
is anhydrous aluminium bromide, made by the reaction of aluminum metal and bromine. This reaction is very well documented, but personally I have only
seen it documented for demonstration purposes. I'm wondering what the best method for making anhydrous aluminium bromide that is either pure or could
be easily purified is in a relatively small home lab setting.
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byko3y
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Lerner's book "small scale blah-blah-blah". It all pretty much direct reaction of elements, the distillation si also simple - unlike AlCl3 it has
melting point at atmospheric pressure.
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chemrox
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Read up on vacuum sublimation. Your product will be Al2Br6 a strong Lewis acid and excellent Friedel-Krafts catalyst.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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AJKOER
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As you may know, you will need a pure Aluminum source (soda cans are in the 99% range as I recall).
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blogfast25
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Quote: Originally posted by byko3y  | | Lerner's book "small scale blah-blah-blah". It all pretty much direct reaction of elements, the distillation si also simple - unlike AlCl3 it has
melting point at atmospheric pressure. |
Few substances are easier the re-sublimate than AlCl<sub>3</sub>. It's not a problem at all.
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MrHomeScientist
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Not quite. My research turned up three alloys used in soda can manufacture:
Aluminum alloys used for the walls of the can
3104-H19:
http://www.matweb.com/search/DataShee...
3004-H19:
http://www.matweb.com/search/DataShee...
Aluminum alloy used for the lid
5182-H48:
http://www.matweb.com/search/DataShee...
(this is for H32 - all 5182's appear to be same composition)
These place the cans at around 95 - 98% Al.
I've done the Al + Br<sub>2</sub> demonstration, and it's pretty spectacular. I don't know how you'd control that to yield a useful
product, though. Many additions of tiny amounts of Al in a sealed container?
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gdflp
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Essentially yes, but adding the Br2 slowly instead of the aluminum. Here's the procedure from len1's book Small Scale Synthesis
of Laboratory Reagents with Reaction Modeling
| Quote: | Thirty-three grams of aluminum 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 condenser is cooled by freezing water
from a large ice water reservoir with a small (~2 W) recirculating pump. The top of
the reflux condenser is protected from moisture by a CaCl2 tube, and prior to commencement
of 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 aluminum 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°C–220°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 Al2Br6 in its middle section. The temperature at the end of the reaction 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 receiving flask. Boiling commences at about 120°C–
140°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. If a multiple feed adaptor is not used and the receiver flask needs to be
replaced, the system must be flushed with nitrogen prior to reconnection. Toward
the end of the distillation, some Al2Br6 “icicles” may be observed sticking to the
walls and possibly plugging the adaptors. The glass can be gently warmed at that
point with an air gun until the product melts and drops into the receiver flask.
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 desiccator, 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 the bromine.
drops to just below 180°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 CaCl2 and an empty guard flask to the aspirator
vacuum; the other neck is connected to a nitrogen source through an inlet |
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byko3y
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Didn't want to create a new thread - answering to this one.
As you might know, one of the ways to prepare anhydrous AlBr3 is to suspend an activated aluminium into aprotic solvent, like ether, nitrobenzene,
xylene, dibromomethane (dichloromethane reacts with AlBr3, leading to AlCl3 and dibromomethane).
I've seen only few successfull reports about this kind of preparation, nitrobenzene and xylene was used. Demethylation of vanillin
Since I want to live a long life, I decided not to use nitrobenzene.
I don't think I've had any success with xylene, maybe because my bromine is not dry enough, or because I added it too fast. I've used copper activated
aluminium. In the end it probably started reacting with xylene forming god only knows what (bromoxylene and xylene bromide?) and fuming like crazy
(those were not bromine fumes). In the end I've got 2 layers: top transparent orange and bottom dark brown with aluminium residue in it (wtf? it's not
bromine because there's no bromine fumes). It's like 70% of stochiometric amount of aluminium reacted, but as far as I can see after neutralizing the
solution with dillute HCl (plain water leads to unseparable dispersion of Al(OH)3) - the bottom layer is just a liquid tar, liquid but not soluble
enough in xylene to stay in the solution.
Pure AlBr3 in xylene is colorless Color of Solutions of Aluminum Bromide in Benzenoid Hydrocarbons, while in practice the solution is indead yellow Solid Complexes of Aluminum Bromide with Aromatic Hydrocarbons, but mine was more like orange-yellow while researchers got yellow with xylene.
Some people told that it's actually simple to prepare aluminium halides by mixing elements in inert solvents, although I can't find any success
stories. At least I tried.
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UC235
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Sounds like a mess byko3y. You've made a bromination and Friedel Crafts catalyst in-situ with a solvent that is electronically activated toward ring
bromination and susceptible to benzylic bromination. Any benzylic brominated products will go on to alkylate other molecules of xylene with AlBr3 as
catalyst. Ring and benzylic bromination both produce copious amounts of HBr which is the fuming gas you observed.
Additionally, AlBr3 in the presence of HBr causes FC-dealkylation/realkylation which will tend to migrate methyl groups into 1,3,5-arrangement on
non-ring-brominated benzene centers.
I think the only solvent you could safely use to mediate AlBr3 formation without major side-reactions is a saturated hydrocarbon like hexane (and in
the dark to minimize radical bromination). Attempts to run the reaction in halogenated hydrocarbons like DCM or chloroform forms tar as the aluminum
metal reacts with them too.
I have prepared a small amount of AlCl3 using DCM but can't recommend it due to difficulty in removing the high boiling dark colored liquid from the
crude product and the hours of time needed to produce a mere 12g or so.
With regards to anyone hoping to run a FC acylation with the product, do not use an acid anhydride if you can help it. Standard "catalyst" loading for
anhydride is 2.2eq of AlCl3 while for acyl chloride, it is 1.2eq.
Elsewhere on the forum, I believe Magpie has demonstrated acetyl chloride from acetic anhydride in good yield. I suspect this conversion is less
tedious than doubling your batch size of aluminum halide.
[Edited on 18-1-2016 by UC235]
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byko3y
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You correctly pointed that saturated hydrocarbons actually react with bromine. Once I was stupid enough to store bromine in a tightly closed bottle
with hexane, in the morning the stopper was blown up like half a meter high and fumes started to come out rom the vessel, while some dark oily residue
could be observed at the walls of the bottle.
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careysub
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BTW, aluminum FOIL is 99.35% aluminum (alloy 1235).
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