Sciocrat
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Diluted hydrobromic acid
I have run across a procedure which involves the bromination of a secondary alcohol. The mentioned procedure involves 48% HBr, which is difficult to
obtain in my area.
However, sodium/potassium bromide and sulphuric acid can be found easily, so I was thinking about making diluted hydrobromic acid by reacting a
aqueous bromide solution with sulphuric acid.
2NaBr + H2SO4 -> Na2SO4 + 2HBr
After the reaction, the solution would be cooled to the lowest possible temperature (min. I could achieve is about -20°C), and all the precipitated
sulphate would be removed by filtration.
Thereby, by adding one mole of concentrated sulphuric acid (98 grams) to 2 moles of NaBr (204g) dissolved in 300 ml of water, one should get 2 moles
of HBr (160 grams). Theoretically, in case of 100% yield, one should end up with a 34% solution with a small amount of sulfate still present in the
solution.
Would this work? Also, is such HBr actually useful for the mentioned type of reaction?
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DJF90
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Just use the sulfuric acid and bromide WITH the alcohol. This will yield the alcohol, by generating HBr in situ, although you would know this if you
had bothered to UTFSE or even look in the recent threads.
In some cases H2SO4 is added to a mixture of HBr and R-OH in order to help the reaction along.
[Edited on 6-5-2010 by DJF90]
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Sciocrat
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Thank you.
I already found information about generating HBr in situ, but was not sure if and how would H2SO4 and the sulphate(s) effect the reaction and the
product. However, I didn't find much info about making and using diluted HBr, so I was also hoping for some thoughts about that too.
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12AX7
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You will get NaHSO4, not Na2SO4.
Tim
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DJF90
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That'd depend on the stoichiometry of the reaction would it not?
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Magpie
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IIRC 48% aqueous HBr is an azeotrope. If so, this should be easily obtainable by distillation.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Rich_Insane
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Wait... I read somewhere that you can't add H2SO4 to a bromide salt to get HBr because it was oxidized to Br2. I always thought that you could add
H2SO4 to KBr or something to get HBr, but I read somewhere that instead, it's oxidized to Br2. Is this for bromide salts or oxybromide salts
(bromates, etc)? Or is it downright false?
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DJF90
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It is oxidised to bromine. However under optimised conditions it is entirely possible, and no bromine is formed.
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Magpie
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See Brauer in the forum library. My notes indicate that I kept the temperature below 40C while adding the acid to the aqueous salt solution. I used
1 mole of NaBr in place of 1 mole of KBr, as that is what I had on hand.
[Edited on 6-5-2010 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sedit
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All these questions such as NaHSO4 and Na2SO4 being one of the products and the mention that H2SO4 cant be used because it oxidises NaBr were fully
discussed in the Ethylbromine thread along time ago. Dilute H2SO4 does not perfom as an oxidising acid t an great extent and yes the formed sulfate is
determined by stoichiometry of the reagents.
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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UnintentionalChaos
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There's a paper linked in the ethyl/isopropyl bromide thread about making azeotropic HBr. I have used it several times now (With NaBr instead of KBr)
and it works a charm, except for the distillation. Even if I collect only liquid coming off the fractionating column at the boiling point of the
azeotrope, it manages to be much more dilute, roughly 37-39% depending what batch. This can be corrected by using an uninsulated fractionating column
and painfully slowly distilling off the water until the still head temp. reaches the boiling point of the azeotrope. The liquid in the pot weighs in
at about 46-47% HBr after that, though a small amount of HBr is lost.
The second acidic proton of H2SO4 is significantly less acidic than the first, so the equilibrium would not favor generating HBr from it. The above
paper uses an excess of H2SO4 relative to only going to the bisulfate and reports that less will decrease the yield.
I didn't get even a faint color of bromine and I was somewhat sloppy with the H2SO4 additions. I did stir well though, and let it chill in an ice bath
after half the acid went in.
See orgsyn for mixed HBr/H2SO4 conversion of alcohols to alkyl bromides. This works better, but only somewhat better, than mixed H2SO4 and bromide.
When you have a substrate that isn't isopropanol or similar mundane alcohol, the higher yield may be worth the extra effort.
[Edited on 5-7-10 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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entropy51
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HBr Prep that UnintentionalChaos referred to.
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