Sciencemadness Discussion Board

HBr synthesis from BCDMH

Brominerain - 25-3-2023 at 08:12

I want to produce hydrobromic acid to synthesize brominated alkanes(C 1-3) for a Grignard reaction. However, I can't access NaBr or KBr easily, it is usually unreasonably expensive. My only other alternative option is the BCDMH pool bromination agent. I am not willing to deal with elemental bromine, so my idea was to make a suspension of BCDMH in water and reduce it with ammonia to make NH4Cl(ammonium chloride), NH4Br(ammonium bromide), and N2(nitrogen gas). Afterward, I could boil down the solution and then separate the bromide from the chloride with acetone and then follow the dilute sulfuric acid method.

Is my improvisation/method flawed??
Is there any better method to reduce BCDMH that you are aware of?

Thank you in advance for your response!!!

[Edited on 25-3-2023 by Brominerain]

clearly_not_atara - 25-3-2023 at 11:04

I have been thinking that you could separate a mixture of NH4Br and NH4Cl by simple heating. The chloride sublimes at 337 C, while the bromide boils at 452 C. You probably don't have to actually hit 337 C since that is the atmospheric equivalent vapor pressure and evaporation will start below this temperature. The difference in volatility is probably related to the higher acid strength of HBr.

However, I've never tried it or heard a report of doing so, so I'd suggest starting with small quantities and good fume control if you want to give it a shot.

AvBaeyer - 25-3-2023 at 14:29

Where do you get the idea that ammonia will react with BCDMH to give ammonium halides? If you are proposing a direct reaction you will get nitrogen halides (eg NCl3, etc.) Do you have any supporting information for your proposal? By the way, the MSDS for halohydantoins list mixing with ammonia or ammonium salts as a hazardous reaction.


Brominerain - 25-3-2023 at 15:09

I'm sorry that I didn't provide any further Information on the reaction between the ammonia and BCDMH.
The reaction equation was found on woelen's website
( )

Edit: the equation its self is not mentioned, but since it is said that the reaction is similar to that of TCCA with ammonia, I assumed it would be (2BCDMH + 2NH3 = 2dimethylhydantoin + 2HCl + 2HBr + N2) (but since HCl and HBr may react with the BCDMH an excess of ammonia shall be used)

[Edited on 25-3-2023 by Brominerain]

[Edited on 25-3-2023 by Brominerain]

j_sum1 - 25-3-2023 at 16:27

I honestly think your best bet is to go via Br2.
There are established methods for getting bromine from BCDMH. Bromine can be handled safely with care. There is no risk of unexpected reactions, for example producing NCl3.

I think the hazards of reacting with ammonia are greater than those arising from working with bromine. To me it is a no-brainer.

Brominerain - 26-3-2023 at 01:12

I am probably going to make bromine and then reduce it with elemental sulfur since the danger of NCl3 is considerable.
However small amounts of BrCl will also be produced. So I'm afraid it will react with sulfur and water to make either HCl and HBr or some sort of sulfur halide. (I am probably wrong, I was not able to find any publications on BrCl reactivity)
If I happen to be correct how do I separate them, to end up with relatively pure constant boiling hydrobromic acid?

[Edited on 26-3-2023 by Brominerain]

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j_sum1 - 26-3-2023 at 01:28

Bromine disproportionates to Br- and BrO3- in NaOH. Selective crystallisation separates. Bromate is a useful oxidiser side product and you get NaBr for your HBr synthesis. I would go that route rather than muck around with sulfur.

Fantasma4500 - 27-3-2023 at 02:25

PbBr 0.4g/100mL
PbBrO3 = 8g/100mL 20*C
Pb ClO3 2 = 144g/20*C 100mL
PbCl 0.67g/100mL 0*C

PbCl + PbBr + H2SO4 + EtOH = PbSO4 + HCl + HBr
HCl + HBr + EtOH = EtBr

this is some of the stuff ive been writing about in my text document named "bromine monster" due to funny happenings of some type of teargas when trying to react it with ethanol
i guess finding something to take away the HCl would make sense, distill that out as HBr + HCl? ah, dump that into NaOH- fractionally crystallize out the NaBr

i do think reacting this bromine thing with ammonia is begging for a little happy accident, it could form hypobromite as well, ammonium bromate is also extremely unstable

PbAc may be made by CuAc + Pb metal, then you can avoid using nitric acid to get watersoluble lead salts

Fantasma4500 - 12-9-2023 at 05:11

okay, updating since i went ahead and tried EtOH + bromine tabs again, this time im having success

first i crudely crush the tablets to fit into 24/40 flask, then i add in enough warm water to cover them
i have a 2 necked flask where air is pumped into the flask, a drip funnel attached at top dripping ethanol into the flask with tabs, the flask is on a hotplate and before ethanol is added i let it heat up well
the flask is connected to another 2 necked flask by a distillation bridge, second flask is in a waterbath and second exit from the flask leads to a PVC hose going into a tall glass tube with NaOH solution, at the end of the hose theres a melamine sponge which causes the gas bubbles to come out with more surface area

once its hot, the air flow is turned on and ethanol is added, with just 5 drops of ethanol the bromine tabs will rapidly produce a lot of bromine gas, the reaction is hot and also produces bromoethane which can be collected in second flask along with liquid bromine condensing

in the NaOH solution you get a mix of NaBrO and NaClO, the bromine is the first to leave the tabs but some bromine will still be in solution, it can be pushed out by boiling the liquid as bromine boils at about 60*C

the NaBrO/NaClO mixture may be removed by acetone where bromine is lost as CBr3 unless you recover it, or degraded into NaBr + NaBrO3 using either heat or a UV lamp

supposedly the bromine/EtBr/EtOH mixture can be refluxed until it all becomes EtBr/EtOH, then fractionally distilled, or reacted with NaOH to yield NaBr + EtOH and NaBrO

since bromine starts to sting at already 1ppm its adviced to have some ammonia in the air sorrounding the apparatus and naturally ventilation going over the NaOH solution that catches the bromine gas

clearly_not_atara - 12-9-2023 at 06:04

Quote: Originally posted by AvBaeyer  
Where do you get the idea that ammonia will react with BCDMH to give ammonium halides? If you are proposing a direct reaction you will get nitrogen halides (eg NCl3, etc.) Do you have any supporting information for your proposal? By the way, the MSDS for halohydantoins list mixing with ammonia or ammonium salts as a hazardous reaction.


What?! No, you just e.g. reduce it with magnesium and then precipitate all the Mg by adding ammonium hydroxide or carbonate or oxalate or something. Or you reduce it with zinc and precipitate zinc as the... um... something.

I only meant to address the question of separating Cl and Br, not the whole process! You should definitely not combine BCDMH with ammonia. I'm sorry if I seemed to imply that.

However, having had some time to consider it, I think the better option would be to exploit the low solubility of KBrO3 (3.1% w/w at 0 C). To do this you need to precipitate all of the hydantoin while keeping the bromine (and probably chlorine) in solution. Then heating with a stoichiometric amount of hypochlorite should give predominantly bromate with a little chlorate:

(Br+) + (Cl+) + ClO- + 4 OH- >> BrO3- + 2 Cl- + 2 H2O

KBrO3 can then be purified by recrystallization from water without too much loss and decomposed at leisure to KBr by simple heating.