Jor
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recrystallisation of pyridinium tribromide
Today I attempted the synthesis of pyridium tribromide (containing the Br3(-) anion). I can't find a synthesis in literature, but I do know that
sigmaaldrich sells the 90% tenchincal grade, wich tells me that it is hard to isolate in pure form.
I added to 2mL of about 42% HBr about 0,5mL of pyridine and 0,3mL bromine (not measured precisely) and at once a red precitipate is formed, and not
much liquid/solvent remains, just a red solid. This was put on filter paper to absorb most moisture, and it stained the paper red. I put the still
somewhat red solid in a beaker, and then went to the computer to type this! 
I don't have enough material to test different solvents to recrystallisation, not do i have much pyridine, so I rather dont waste a lot of pyridine on
making more. What would be a good solvent to recrystallise this from? I cannot find much information on this.
Hot GAA dissolves ALOT of the stuff. I know this because I read in a paper that someone added 178g pyridium tribromide in 100ml hot GAA. But then I
don't know the solubility in cold GAA.
[Edited on 11-7-2009 by Jor]
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chemrox
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I think you have Pyridine/HBr complex and Br2 in aqueous solution. See:
J. Chem. Phys. 121, 10467 (2004); DOI:10.1063/1.1809577
Issue Date: 1 December 2004
The nature of the complex formed between pyridine and hydrogen bromide in the gas phase: An experimental approach using rotational spectroscopy
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Jor
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No, I think I have the desired complex, as no red bromine fumes can be observed and smelled at all.
Besides pyridine.HBr is white
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Klute
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You could try slurrying the complex with ether, and adding GAA dropwise until complete dissolution, then adding more ether to precipitate the complex.
I have some at work, which was made by a student a few months ago. It is a light orange powder.
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bfesser
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From Macroscale and Microscale Organic Experiments, Third Edition, Williamson, p.276:
| Quote: |
(E) Bromination with Pyridinium Hydrobromide Perbromide
(C5H5N+HBr3-).1 This substance is a crystalline, nonvolatile, odorless complex of high molecular weight (319.84), which, in the presence of
a bromine acceptor such as an alkene, dissociates to liberate 1 mol of bromine. For small scale experiments, it is much more convenient and agreeable
to measure and use than free bromine.
Add 80 mg of the reagent to a reaction tube, and add 0.5 mL of acetic acid. Swirl the mixture, and note that the solid is sparingly soluble. Add
20 mg (0.025 mL) of cyclohexene to the suspension of the reagent. Swirl, crush any remaining crystals with a flattened stirring rod, and if after a
time the amount of cyclohexene appears insufficient to exhaust the reagent, ad a little more. When the solid is all dissolved, dilute with water, and
note the character of the product. By what property can you be sure that it is the reaction product and not the starting material?
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| Quote: | Note for the instructor
1. Crystalline material suitable for small-scale experiments is supplied by the Aldrich Chemical Co. Massive crystals commercially available should
be recrystallized from acetic acid (4 ml/g). Preparation: Mix 15 mL of pyridine with 30 mL of 48% hydrobromic acid and cool; add 25 g bromine
gradually with swirling, cool, and collect the product with use of acetic acid for rinsing and washing. Without drying the solid, crystallize it from
100 mL of acetic acid. Yield of orange needles, 33 g (69%). |
According to the Sigma-Aldrich MSDS, it should be stored at 2-8 degrees C and is light sensitive. No mention of color, only that it's a solid.
Attachment: pyridinium_hydrobromide_perbromide_MSDS.pdf (27kB)
This file has been downloaded 750 times
Sorry, the scan of the page from Williamson didn't quite turn out.
[Edited on 7/13/09 by bfesser]
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Nicodem
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Jor, if you are further interested in pyridinium polybromides of the type PyHBr(Br2)n, you can check Can. J. Chem., 52, 2560–2565 (free full text access). See also Journal of Molecular Structure, 17, 329-340.
This is a reference on its use as reagent: Use of pyridinium bromide perbromide for brominations. Journal of Chemical Education,
26 (1949) 613-614.
If you are interested in the iodo versions of these complexes, check the attached paper:
Periodides of pyridine
P. F. Trowbridge
J. Am. Chem. Soc., 19 (1897) 322–331.
Attachment: Periodides of pyridine.pdf (503kB)
This file has been downloaded 753 times
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Jor
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Thanks a lot for all the attached papers everyone.
Unfortuantely I cannot open them right now, as I have no PDF on this computer.
Slurrying in ether didn´t work too well, as the complex is soluble in ether, giving a dark red solution. But I left the solid in contact with the
atmosphere for almost a day, and it had become a wet slurry, so I will try this as well when I have prepared it again.
Thank you for the synthesis bfesser. I washed the solid with very dilute (a few %) of HBr-solution (as I think plain water will destroy the complex,
because pyridium-ion will protonate water at neutral pH) instead of the GAA. After all I was just experimenting a bit. Next time I will attempt the
synthesis you posted.
Yes Nicodem, I wanted to prepare the iodo-complex as well, but the found out I had no HI. I have tried to make some by boiling KI in 85% H3PO4, and
lead the gasses into water, but
this was not succesfull. I guess I will need to use some red P for this purpose, or use H2S, but I rather not use the latter.
And no I am not making controlled materials, after all I live in Europe  
Quite interesting that pyridium-ion stabilises all these polybromide anions.
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not_important
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or even better get a larger USB flash drive and visit http://portableapps.com/apps to pick up not only Sumatra PDF but PDFTK Builder, FireFox, Thunderbird, 7zip, FTP and Telnet clients, and Open
Office in portable form. Just pop the USB memory into a Windows machine and you can run those programs without having to install them, plus have
access to your bookmarks, and not worry about leaving files behind on the PC.
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ptlmayank
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hi jor use dichloroethane(EDC) for crystallization as well as solvent for pyridine bromide perbromide. never use acetic acid as solvent becz it emit
some harmful gas which burns the eye......EDC is safe and no bromine vapour comes from reaction mass....heat just 60-70 and cool to get
crystals..........
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