zephler
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33% conc to fuming HBr in GAA in-situ?
would it be possible to simply pour in an equal volume of HBr to GAA, and then add an excessive amount of MgSO4 to simply remove the water and produce
the coveted 33% HBr in GAA?
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madscientist
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Thread Moved
20-12-2010 at 18:05 |
bbartlog
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Seems roundabout, but sure, it should work, as long as you don't need truly anhydrous conditions (and even if you do I guess you could use it as a
first step). However without vacuum filtration I expect a lot of your solution would end up stuck in the MgSO4 hydrate, which will pretty much fill
the liquid volume given the amount you'll need. To me it seems easier and more reliable to gas the HBr into the GAA, but YMMV.
Why is 33% HBr in GAA 'coveted'?
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zephler
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it's coveted because there is endless discussion about how to make it - since getting a cylinder of HBr is not really an option for some
experimenters, there are suggested pathways of NaBr + H2SO4 in GAA, and then the arguement that H2SO4 is not good because the H2SO4 will also form B2
- some claim Br2 is the major product with HBr being the minor product in this case, so then H3PO4 is suggested. DMSO has also been suggested instead
of GAA. Keep in mind these are routes designed to avoid handling the somewhat nasty HBr gas directly. Of course one could make a HBr generator with
H3PO4 and NaBr in a separate Erlenmeyer, bubbling the resultant HBr through GAA (using a trap of course!!!). Then there has also been discussion of
just using straight up 48% HBr (aq) and a phase transfer agent, and last but not least bubbling HCl through your 48% HBr \ GAA (glacial acetic acid)
mixture to dehydrate -(this is why I suggested MgSO4 or even MS - although it would take a truck load of em). So you see with so many possibilities,
everyone has horror stories of trying one and it did not work, while ppl who do get those particular ones to work end up defending them, so it goes on
and on...
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sonogashira
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I would suggest making HBr gas and bubbling it into the GAA. HBr gas is easy to make pure, and quite safe to make also since one can use a closed
system going from generator to GAA. You can also determine the concentration easily by weighing the GAA/HBr mixture. But if you have 48% HBr, surely
this may be used instead of GAA? Certainly for demethylation.
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zephler
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yes for demethylation, but not for the bromination of alkenes, is your idea of a HBr generator H3PO4, NaBr and heat? AS for close system, are you
suggesting that you don't have a vent on the top of the vessel with the GAA?
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