Dope Amine
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Thoughts on the preparation of 30% HBr/Acetic Acid
It seems that I am not the only one who has considered the issue of preparation of HBr in acetic acid. (Yes, I've UTFSE)
Gassing glacial acetic acid seems like a hassle (and has been unsuccessful for some here).
The Wikipedia entry is often cited in stating that phosphoric acid is preferred over sulfuric acid because it won't generate Br2. I have
searched high and low for any proper literature on this preparation and it can't be found. The reality is that phosphoric acid will work but it needs
to be heated to ca. 100° C.
An attempt was made to create a reaction mixture containing 15% HBr by combining phosphoric acid 85% and a slight excess of lithium bromide in a
solution of glacial acetic acid and 88% formic acid. Upon heating to reflux (~105° C.) with a methoxy aryl ether and a bit of boric acid the
solution took on a slight orange color (a good sign?). It is unknown whether the desired demethylation took place but what is known is that the use
of lithium bromide was a bad choice because upon addition of methanol there was filtered a lithium grease-like substance that had a relatively low
water solubility similar to that of trilithium phosphate. LiBr also has a higher bond dissociation energy compared to NaBr because of lithium's more
covalent nature so the use of NaBr seems much more sensible.
Another idea for producing HBr/acetic acid solution of use is by simply adding NaBr and conc. sulfuric acid to glacial acetic acid. This has been
done previously in amounts to theoretically make a 40% w/w solution of HBr/acetic acid (with a slight excess of NaBr in the hope of consuming all of
the sulfuric acid). Surprisingly, no exothermia was noticed when all was dumped together at r.t. This solution was then cooled and maintained at 0°
C. while an alkene was added for a bromination reaction. The purpose in this instance of maintaining the solution at 0° C. was to prevent
the breakage of aryl alkyl ethers. The addition of the alkene caused a salt to crash out (hopefully sodium bisulfate) and the color to slowly change
to burgundy and then purple/blue. Hopefully this was from a reaction with HBr and not sulfuric acid?
Sorry for all of the detail. I really hope there will be people who care to ponder all of this.
1. So, does it seem likely that either of the above methods produced the desired HBr/acetic acid?
2. Do the other ions/salts in the mixture seem to pose a problem? I know that for the preparation of aqueous HBr one can add NaBr to diluted
sulfuric acid and then add acetone to crash out the sodium bisulfate and then boil off the acetone, thus removing the byproduct salt. Considering
that the addition of a non-polar organic molecule to the above NaBr/sulfuric/acetic acid caused the salt to crash out (again, hopefully
NaHSO4 and not NaBr) maybe the addition of acetone to this mixture might again cause precipitation?
Does it seem plausible that addition of NaBr to a solution of sulfuric acid diluted with glacial acetic acid would similarly prevent the oxidation to
Br2 as in the case with diluted aqueous sulfuric acid?
3. There has been mentioned the ability to produce HBr from glacial acetic acid, another non-oxidizing acid like phosphoric acid. This seems like it
would be very low-yielding, if occurring at all, considering what a weak acid acetic acid is. But if it were true then heating acetic acid with NaBr
could potentially produce some HBr all on its own, although the yield might limit the solution to less than 15% strength. Can Anyone comment on the
plausibility of the claim on making HBr using simply acetic acid and NaBr?
4. Can anybody comment on the likely % yields of HBr actually generated on any of these methods?
- AcOH+H3PO4+NaBr+heat
- AcOH+H2SO4+NaBr
- AcOH+NaBr+heat
5. Lastly, it seems that the oxidative production of Br2 can possibly happen at elevated temperatures (ca. 100° C.) even with a
non-oxidizing acid. Depending on the purpose, an optimized production of 15% HBr in acetic acid could likely require consideration of the ability to
be heated to such temperatures.
Thank you! 
[Edited on 18-1-2014 by Dope Amine]
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Dope Amine
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Another issue I was pondering was the idea making constant boiling aqueous HBr, diluting with acetic acid and then possibly drying out the water with
mag. sulfate. Although this would certainly be more of a hassle than other mentioned methods and I doubt mag. sulfate would work in this situation.
...and yet another consideration is the preparation of HBr with sulfuric acid and NaBr in DMSO...
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Dope Amine
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To continue this conversation with myself...
I found this bit to be particularly interesting:
"When acetic acid is the solvent, the most acidic species that can exist is the conjugate acid of acetic acid (that is, protonated acetic acid which
has a pKa of about –6. The pKa of HBr is –8 to –9.5. Consequently, the HBr–acetic acid system has an effective pKa of about –6. In aqueous
solution, HBr is dissociated to give H3O+, the conjugate acid of water, which has a pKa of –1.7; therefore the effective pKa of aqueous HBr is
–1.7. Hence, HBr in acetic acid is far more acidic than HBr in water because of the greater acidity of the conjugate acid of acetic acid, the
solvent."
So, going with this bit of mental masturbation I was emboldened to preform an experimental preparation of HBr in acetic acid using sulfuric acid and
heat (gasp!). This was done by first adding glacial acetic acid to the rbf, then conc. sulfuric acid with stirring (exotherm. from dissolution), then
NaBr. The amount of acetic acid was added such that it would theoretically produce a 7% HBr in acetic acid solution. The reason for this was
because of fear that the HBr might more forcefully escape the acetic acid solution upon heating. The rbf was then fitted with a reflux condenser and
then a stopper losely fitted on the top of the reflux condenser. The reason for the stopper was so that I could better assess the pressure built up
by this reaction system. As the solution began to heat into the 30 deg. C. range it took on a fairly intense yellow color but as it heated into the
50 deg. C. range it began to take on a more orange color that was indicative of bromine formation. This was confirmed by the visible presence of
bromine collecting in the joint grease. So, it has been confirmed that, like aqueous HBr, HBr generated from sulfuric acid in acetic acid also cannot
withstand heating. What is interesting is that even with the reflux going on in a closed system there was never any noticeable build-up of pressure
so it seems that HBr has a similar affinity to acetic acid as it does to water. Another bit of info is that, even with added acetic acid, the solid
never completely disappeared. The chunky texture of the NaBr crystal solids did change to a much more fine material but I cannot say whether this was
from partial dissolution of NaBr or from complete dissolution of NaBr but the formation of sodium bisulfate.
It is interesting to note that while a Chinese manufacturer describes their 30% HBr in acetic acid solution as being yellow/orange while other
manufacturers describe this same solution as being a faint yellow. I don't like bashing on the Chinese because I truly want their products to have
competitive quality as well as price but in this instance this supplier's quality looks suspect as we will see in the next experiment.
Experiment 2: AcOH + NaBr + 85% phosphoric acid. This time the acetic acid amount was such that it would produce a theoretical concentration of 24%
HBr or with a 50% yield of HBr generated a concentration of 14% HBr. Again, the AcOH was added and then the phosphoric acid with stirring and then
finally the NaBr. The rbf was setup similarly with the reflux condenser and the stopper on top to make a closed system The rbf was heated in an oil
bath all the way up to 115 deg. C. At a bath temperature of roughly 100 there began a condensation on the rbf that I can only describe as being
similar to intense condensation similar to a rain forest with many small droplets continually forming. This was also the approximate temperature at
which the solution began taking on a pale yellow color. The bath temperature continued up to close to 120 at which time my mineral oil began
popping so it was brought back down to approximately 115 and held there for approximately 2 hours with the hope of ensuring complete reaction. Again,
the chunky NaBr was converted to or replaced by a fine solid. And again, no pressure ever built up in this closed system even when the bath
temperature was well over 100!
The reaction bath has now cooled down to r.t. My intention is to carefully pour the HBr in acetic acid solution into another rbf containing my
material to be reacted, leaving the fine solids behind in the old reaction flask. What is most significant about these experiments is that this
proves that the affinity of HBr for acetic acid is such that heating to reflux does not cause HBr fumes to be driven off. This means that
demethylations of aryl methyl ethers with 15% HBr in acetic acid DO NOT REQUIRE HIGH PRESSURE REACTION VESSELS such as the examples done where a
hydrogenation vessel was used. The only pressure will be generated by the production of methyl bromide which can be vented through a drying tube. In
research labs it is often found that expensive equipment is used, not because of necessity, but simply out of prudence/lack of knowledge and also
because it justifies that purchase of said expensive equipment. I have discovered this to again be the case in this instance. The only caution I
should make is that I cannot say to what concentration this is the case because I do not know the yield of HBr from the reaction mixture. This may
only be true more 15% solutions or 25% solutions or it could be true all the way up to 33% solutions.
You're welcome!
[Edited on 21-1-2014 by Dope Amine]
[Edited on 21-1-2014 by Dope Amine]
[Edited on 21-1-2014 by Dope Amine]
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