SuperOxide
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Concentrating hydrobromic acid higher than the azeotrope?
Im in the process of making some hydrobromic acid right now, and I know that distilling it will set the concentration to ~47.6%, but I was curious if
there's a way to get it more concentrated than that?
For example - HCl will distill over at an azeotrope of ~20.2%, but you can get up to 37% if you bubble some gaseous HCl through the azeotropic HCl
acid while chilling it down.
Is it possible to do something similar with HBr? Bubbling dry gaseous HBr through a solution of HBr acid? And if so - Whats the highest concentration
that can be achieved?
I'm not sure I need a higher concentration of HBr, but I couldn't find much on it, so this made me a bit curious.
Thanks.
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clearly_not_atara
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https://www.osti.gov/etdeweb/biblio/8148785
HBr likely displays typical zeotropy with the boiling point continuously decreasing above 48% HBr and the vapor mix having a higher proportion of HBr
than the liquid.
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Keras
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Apparently, the maximum you can reach is about 65 % at 20 °C w/w (solubility 1930 g/L).
Either you can boil an excess amount of azeotropic HBr, which will release moist HBr. You can then use reflux, which will condense the water, or pass
the gaseous mixture through a calcium chloride trap (or both), and you get relatively dry HBr that you redissolve in your main solution.
Or you can get dry HBr by reacting sodium bromide with 85% phosphoric acid. Contrarily to sulphuric acid, phosphoric acid is not an oxidant, so you
should be able to get reasonable amounts of HBr gas using this path (please test before, though, I'm somewhat perplexed by the use of phosphoric acid
to get HBr, but it seems to work fine).
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Tsjerk
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@keras: when you distill an azeotropic solution, you can't selectively condense water from the vapor, that would break the azeotrope somehow. Then you
could condense water from 96% ethanol and condense the ethanol a little later.
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Fery
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you can also generate HBr gas as a side product of bromination of arenes (e.g. benzene/toluene/naphthalene + Br2 + Al/Fe/AlBr3/FeBr3 catalyst)
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unionised
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You can break an azeotrope by distillation at a different pressure.
You can also generally do so by adding a suitable solute.
For example, if you take the (47.6% at normal pressure) azeotrope and dissolve KBr or NaBr in it, then distil it you will get a different
concentration in the condensate.
However, if you try to produce more than about 65% HBr, the condensate will be a mixture of HBr gas and the saturated solution.
You can do a bit better by cooling the material further (for example, by running ice-water through the condenser).
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woelen
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I once saw HBr sold commercially as 62% solution. That concentration is somewhat comparable in handling and storage to a 36...37% solution of HCl. It
will fume quite a lot at room temperature and these fumes will corrode metal objects nearby, just like HCl-fumes.
I once tried making more concentrated HBr from NaBr and H3PO4 and leading the cooled down gas through 48% HBr (which can be purchased commercially,
e.g. at chemcraft.su). This process works quite fine, but you really must use a long tube for the HBr gas you make from H3PO4 and NaBr, otherwise the
absorbing liquid becomes too hot and you lose a lot of fume. Also beware of suck-back.
What I did was first boiling my H3PO4 (85%) to drive off a little more water, and then I added solid NaBr. This produces HBr. One nasty issue I had
was strong foaming and the foaming was so bad that it almost went out of the flask into the tube, which leads to the liquid into which HBr is
absorbed. You must heat slowly and gently. It might be that your H3PO4 does not have this issue, it might be that my acid had some impurity, but
anyway, be prepared for this. It is also wise to add an additional bottle between the flask in which you generate HBr and the flask in which you
absorb HBr.
I think that I did not really reach 62%, but the acid I had definitely had a higher concentration than the 48% solution I started with. The 48% acid
hardly shows any visible fumes, while the liquid I had clearly fumed, albeit much less than 37% HCl.
I also was afraid of etching my flask in which I heated the H3PO4 + NaBr, but I could not observe any etching. My experience is that H3PO4 only etches
glass at a noticeable rate, when it is very hot (over 150 C) and already lost quite some water.
The process of making HBr gas is surprisingly clean. No formation of Br2 at all.
Unfortunately, on storage, the HBr turns yellow/brown fairly quickly. After a week or so, it had a clearly visible yellow/brown color. This color did
not become stronger after that. I think that a highly concentrated solution of HBr in water is oxidized by oxygen from the air fairly easily. For this
reason it is important to store the acid in glass bottles (which are non-porous) with a very good seal. On the other hand, the color of dissolved
bromine is quite intense. Even a tiny amount (only 0.1% or even less of all HBr is oxidized) gives a clearly visible yellow/brown color.
[Edited on 15-10-21 by woelen]
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teodor
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I can't imagine any benefit of 65% HBr over 47% HBr. I know that 38% HCl has benefits e.g. in alkyl chlorides synthesis because of lower water context
but considering higher reactivity of HBr, the water contents in it probably doesn't matter for such type of synthesis.
On contrary, getting HBr in non-aqueous solvents would be interesting.
The dissolving of HCl in methanol is not so exothermic comparing to water, at least I was able easy control this process by ice-bath cooling and
regulating the HCl stream (to prevent the suck-back).
I am not sure about the possibility to store HBr in methanol (again, because of higher reactivity) but an ethereal solution should be possible.
Keeping this solution will be a pain in the ass (just imagine HBr fumes coming together with ether fumes) but you will be able to get any HBr/water
mixture concentration just by mixing water and ethereal HBr in the required proportion and let the ether evaporate.
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woelen
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A benefit of higher content HBr (and also HCl) is that in those liquids there is a considerable amount of non-dissociated molecules of HBr (or HCl).
The fuming of solutions of HCl is due to the presence of molecules of HCl in water. Dissociated HCl (present as solvated H3O(+) and Cl(-) ions) cannot
leave the liquid. So, strong fuming of the liquid also shows that there is a high concentration of non-dissociated molecules. These non-dissociated
molecules are more reactive and can be used in organic chlorination reactions (replacing hydroxyl groups by Cl).
With HBr the effect is even stronger, because non-dissociated HBr is more reactive than non-dissociated HCl.
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unionised
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In some cases this stuff- HBr in acetic acid- might be useful.
https://www.sigmaaldrich.com/GB/en/product/sigald/248630
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S.C. Wack
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Quote: Originally posted by woelen  | | I also was afraid of etching my flask in which I heated the H3PO4 + NaBr, but I could not observe any etching. |
I suspect that it depends on how much HF is in one's acid and whether under the conditions it leaves the flask as such or as SiF4 or hangs out and
forms fluorosilicic acid and is hydrolyzed and so on.
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SuperOxide
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| Quote: Originally posted by Keras | Apparently, the maximum you can reach is about 65 % at 20 °C w/w (solubility 1930 g/L).
Either you can boil an excess amount of azeotropic HBr, which will release moist HBr. You can then use reflux, which will condense the water, or pass
the gaseous mixture through a calcium chloride trap (or both), and you get relatively dry HBr that you redissolve in your main solution.
Or you can get dry HBr by reacting sodium bromide with 85% phosphoric acid. Contrarily to sulphuric acid, phosphoric acid is not an oxidant, so you
should be able to get reasonable amounts of HBr gas using this path (please test before, though, I'm somewhat perplexed by the use of phosphoric acid
to get HBr, but it seems to work fine). |
Interesting. I would have tried to use H2SO4, so thanks for the heads up.
Im not sure I need the 85% conc, but it might be worth trying in the future. I haven't seen anyone use such a concentrated HBr.
Im going to use mine to make ethyl bromide, which I think the concentration doesn't matter too much for.
Hm.. That's interesting. Wonder what that could be used for?
[Edited on 16-10-2021 by SuperOxide]
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clearly_not_atara
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As I recall, HBr/GAA can catalyze some reactions where conc H2SO4 would give oxidation products. It's also an interesting system in other ways.
In particular, it converts propylene glycol to 1-bromo-2-acetoxypropane, allowing for distinct 1,2 functionalization. I am not sure if this can be
replicated with other glycols.
I am not aware of dealkylations of aryl ethers with HBr/GAA, though it certainly seems plausible.
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teodor
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| Quote: Originally posted by S.C. Wack | | Quote: Originally posted by woelen | | I also was afraid of etching my flask in which I heated the H3PO4 + NaBr, but I could not observe any etching. |
I suspect that it depends on how much HF is in one's acid and whether under the conditions it leaves the flask as such or as SiF4 or hangs out and
forms fluorosilicic acid and is hydrolyzed and so on. |
"Etching" of glass with H3PO4, according to studies I have read, happens only when the polyphosphoric acids form. H3PO4 doesn't impact glass until the
point it starts to lose water. So, it is in full accordance with woelen's observations.
Also, the way it "etches" glass is not usually visible. But it makes the glass very fragile. So, after H3PO4 evaporation it is possible to crack the
flask with a small impact.
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S.C. Wack
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| Quote: Originally posted by teodor | | "Etching" of glass with H3PO4, according to studies I have read, happens only when the polyphosphoric acids form. |
Make your own acid from sodium phosphate + HCl, keeping it in glass, (even though any HF is presumably distilled with the excess HCl) and see how that
goes. I suspect that cheap acid does show F on analysis, because the manufacturer doesn't want to release it into the air or dispose of it as
hazardous waste.
I've familiarized myself with quite visible unintentional chemical glass etching of all kinds, including this. Which is why I brought it up.
On topic (apparently) obviously the main internet interest would be for bromosafrole and such, and there's no need to make neat HBr if all one is
going to do with it is make ethyl bromide...or reason to ask if there's no reason to ask.
There's some out there, and also with PTC.
[Edited on 17-10-2021 by S.C. Wack]
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karlos³
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Did that, they are usually quite dirty reactions though.
verrückt und wissenschaftlich
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