We all want one of those dehydrating thingies. Unfortunately, stuff that reacts with water has a nasty tendency to... react with everything around us.
So this one is made using a sealed vessel (a bomb) at 200˚ C, which both provides plenty of energy to make it and keeps it separate from all the
stuff it would like to react with. Sound fun yet? Cyanogen bromide is produced as a byproduct.
What I'm saying is, please don't hurt yourself; I don't normally post on public fora for this reason. This method could be very very useful to someone
who knows what they're doing, because the methods and reagents are all very accessible, but it could be very dangerous to someone who doesn't.
Yields of up to 55% of cyanuric bromide are obtained by heating bromine with potassium ferrocyanide at 200˚ C for about seven hours
Well, at least bromine and potassium ferrocyanide aren't that scary. The logistics of the reaction are dictated by the density of gaseous bromine,
which is about 3 grams per liter at 200˚ C and 1 atm. For each additional atmosphere of pressure, you can get 3 more grams of bromine, which
means only 2 grams of (CN)3Br3 per atmosphere per run.
Or one can use a high-boiling solvent: bromine will have a higher effective density at a given temperature/pressure in solution, allowing you to use
much lower pressures; remember, it's the partial pressure of bromine that matters, not the overall pressure. With the right solvent, the whole thing
might just happen at scale in a RBF; candidates include methylsulfonylmethane, iodoform, and trichlorobenzene. This is probably the best way to make
any significant quantity.
The purification described in the book is a pain. Cyanogen bromide is almost certainly present in some amount as a byproduct, although the presence of
(Lewis acidic) Fe3+ ions should promote trimerization. The reaction presumably generates CNBr in situ which quickly trimerizes. So while you're free
from having to actually work with CNBr, you absolutely must have a fume hood to do this.
Cyanuric bromide is pretty convenient as dehydrators go, though. It's less volatile than the chloride and much safer than thionyl chloride/phosgene.
It may be able to convert sulfonic acids to sulfonyl bromides, e.g. tosyl bromide. And of course it can dehydrate acetic acid. With some tweaking the
preparation could be made more practical.
The book is Chemistry of Heterocyclic Compounds vol 13: s-Triazines and Derivatives, and it cites these German papers which I cannot read:
Anywho, yeah, hope you guys like this. And remember: at least it's not ketene
vanishes
[Edited on 4-11-2013 by clearly_not_atara]
[Edited on 4-11-2013 by clearly_not_atara]woelen - 5-11-2013 at 03:16
The reagents are not a real problem, but the reaction conditions are (200 C, for 7 hours). Even more troublesome is the purification/isolation of
C3N3Br3, requiring repeated vacuum sublimation at 250 to 300 C. This is not something for the average (home) lab, only well-equipped labs can do this
kind of things.
This would be a very interesting compound though. It allows you to make certain acyl bromides (e.g. acetyl bromide, propionyl bromide) and it can be
used for many brominating reactions, even the ones which are hard with other reagents.
Do you have practical experience with the synthesis and isolation of this?vulture - 5-11-2013 at 06:34
Vacuum sublimation is not very hard if the required vacuum is within reasonable ranges, i.e. not lower than 1 Torr.
