Sciencemadness Discussion Board

Making 33% w/w HBr in GAA

Pavel_ChemGeek - 20-7-2019 at 09:55

I need some 33% HBr in GAA but i find it fairly expensive
So i was thinking to make it myself.
I found one supplier but 220$ for 1L is expensive. and i only need 100-250mL Maximum.
So i did some research.
And i find i must start with a Bromide salt ( What is Best KBr or NaBr ?)
Next i cant' use H2SO4 because it'll produce some bromine too, same for the method with H2 and Br2, ( and i have to produce BR2 first))

IMO my best choice is KBr + H3PO4 with a drying tube ( what drying agent should i use ?) then react the Anhydrous HBr with GAA. It's a one pot reaction who seems easy to do. Apparently using an excess of H3PO4 produce more HBr

What are your thoughts about this ?
If you have some advice feel free to help me.

Assured Fish - 20-7-2019 at 15:43

So anhydrous HBr, this is a pain in the ass.
You can use either the potassium or sodium salt, it will make little different besides stoichiometry.

Conc H2SO4 will oxidize HBr to bromine, thus the phosphoric acid approach is the only real option if your opting for direct protonation.
Now H3PO4 and its salts are actually quite hydroscopic as it is so if this is as concentrated as you can get it then your stream of HBr should already be relatively dry.

I think CaBr2 should work quite well, you will want to probably avoid CaCl2 as the displacement is an equalibrium reaction and with a constant flow of HBr taking any generated HCl with it, it will push the equilibrium in favour of HCl irrelevant of the electronegativity.
Another drying agent and undoubtedly the best would be phosphorous pentoxide, but i doubt you have this.
Im also thinking maybe anhydrous magnesium or sodium sulfate, there may be an interation to a very minimal extent resulting in the production of a small amount of H2SO4 but this should negligible.
Neither salt should oxidize the bromide either.

There is also a few other approaches to making dry HBr, one involving tetralin and Br2 and then of coarse there is reaction phosphorous tribromide with H2O.
There are a couple links down the bottom of this page to some threads that discuss this.

If you opt to make your own CaBr2 then your in for a hell of a time as the hydrates can decompose to calcium hydroxide on heating.
I don't actually know how you could do this cleanly except maybe doing it in incremental steps at steadily rising temperatures and using a vacuum.

draculic acid69 - 20-7-2019 at 18:27

What bout br2 and a ketone? I know haloketones are tear gas but as far as making DRY HBr in what I assume is gunna be a makeshift/homemade or very basic minimal glassware it would actually be the simplest way. It's a one pot rxn.or if you have access to cheap tetralin that might be the best way but if you gotta buy a litre of tetralin at a premium price plus delivery just pay for the hbr in gaa and skip the bullshit.or make some bromo benzene and capture the hbr.the idea of making wet hbr (h3po4 and nabr require a few hundred degrees c to produce hbr making it not as doable as it appears on paper)and then trying to successfully dry it is not as easy as it seems in real practice.dos your application require anhydrous hbr in gaa or can H2O be present?

Boffis - 31-7-2019 at 11:43

Practically anhydrous HBr can be produced from sodium bromide and sulphuric acid. There is a patent; no. US 2705670, that describes the commercial production of HBr from NaBr solution and diluted sulphuric acid. There is a range of concentration within which the oxidation to bromine is negligable and the evolved gas is essentially anhydrous HBr. I have tried this and it works pretty well when I am making HBr in ether. This process can tolerate a few % water vapour so I make no attempt to dry it.

nimgoldman - 31-7-2019 at 12:54

Unless you need absolutely anhydrous solution, I would go for the patent mentioned above.

At small laboratory scale, you can make HBr by bromination of tetrahydronaphthalene (Tetraline) with bromine - I was able to obtain tetraline from one chem supplier. However, the maximum theoretical yield is only 50% based on bromine.

If you need high concentration of HBr for certain brominations (I was once thinking about bromination of safrole), preparing concentrated HBr solution might be viable by simply bubbling wet HBr gas trough a chilled hydrobromic acid. At freezing temperatures, you can dissolve 221 g of HBr in 100 mL water (more than 100% !).

Another solution would be simply bubbling wet HBr through GAA. This might still work for the application you are interested in.

AJKOER - 31-7-2019 at 13:18

'Photochemical Reaction of Bromine with Hydrogen at Room Temperature' in
J. Chem. Phys. 49, 1155 (1968);
John H. Sullivan at .

The abstract:

"The rate of the photochemical production of hydrogen bromide was measured in a hydrogen–bromine system irradiated with 5780‐Å light. The rate was found to be proportional to the square of the concentration of bromine atoms. Absolute light intensities were determined and third‐order rate constants for the equation d(HBr) / d t =2k[Br]2[H2] were determined at 0° and 25°C. The rate constant is given by k = [ (4.8 ± 4) × 10^5*exp(0∓800/RT) liter2mole−2·sec−1. The data of previous photochemical experiments at room temperature, usually but erroneously considered to be consistent with the chain mechanism, are shown to be consistent with the third‐order reaction."

[Edited on 31-7-2019 by AJKOER]

woelen - 31-7-2019 at 23:08

I myself have had success with H3PO4 and KBr (or NaBr). I started with 85% H3PO4. I heated this to appr. 150 C to make its concentration even higher than 85%, but I do not really know its concentration after this treatment. Maybe it was 90%, but I doubt it was more than that.

I let the acid cool down and then I added the KBr. On gentle heating and slowly increasing the temperature, HBr escapes. You need quite some H3PO4 to have this working smoothly. Unfortunately this reaction is much slower than the reaction with H2SO4 and also is less complete. If you just need a small amount of HBr, then this method is useful though.

The advantage of the use of H3PO4 is that you get really colorless HBr. With H2SO4 of moderate concentration it still is easy to have a little Br2 in your product. Some too strong heating or having the concentration a little too high leads to formation of Br2 and the end-product always is a little yellow/brown.

draculic acid69 - 1-8-2019 at 00:19

Still no answer as to whether it needs to be anhydrous.if it does that's different to if it doesn't.would making a relatively dry hbr in gaa solution and then use molecular sieves to fully dry it?

[Edited on 1-8-2019 by draculic acid69]

rockyit98 - 1-8-2019 at 01:22

Glacial acetic acid isn't totally water free(less than 1%).so if you dissolve some Br2 in it and then bubble some SO2 you will be able to get 33% w/w HBr in GAA.only down side is sulfuric acid by product.
and don't worry H2SO4 is only oxidize in hot or concentrated solution.
i think you could get rid of H2SO4 ny adding Anhy. Sodium Acetate .totally get rid of water use Acetic anhydride.

draculic acid69 - 1-8-2019 at 01:44

Gaa + br2+so2=hbr+h2so4 somehow, where does the hydrogen come from without water present?if it came from the gaa that would leave a whole heap of free radicals floating around.i know this is a well known rxn but without water I don't get it