Sciencemadness Discussion Board

Recycling bromide from Grignard reactions

michalJenco - 27-12-2019 at 10:39

Hi fellow chemist,

I am currently on a Grignard reaction spree and I wonder if it possible to recycle the Mg(OH)Br for preparing further alkyl bromides. Has anyone done this before?

My idea is to precipitate the bromide from the Grignard water layer using alcohol and then use that instead of KBr to produce alkyl bromides.

Dr.Bob - 27-12-2019 at 14:31

I doubt that you can collect the salts in any useful manner, KBr is dirt cheap compared to most of the organics used.

I was first thinking that you meant the alkyl or aryl bromide, but usually if is mostly used up, the main impurities that I find are homocoupled (dimerized) alkyl or acyl group (biphenyls mostly), hydrolysis of the Grignard by water (alkyl hydrocarbon or benzene), and the reaction with CO2 to form the alkyl acid if you don't keep out all traces of air. These are found in most Grignards to some degree, thus in most cases you use an excess of the bromide (1.2-2.0 equivalents) if it is not the expensive partner in the reaction. Most people also use a large excess of Mg, as that is so light and cheap, and then either ignore the remaining metal or decant or cannulate the Grignard to a fresh flask. But in the workup, you usually quench the reaction with acid to break up the Mg salts and they all dissolve away, and you extract the product with an organic wash.

Assured Fish - 27-12-2019 at 14:33

This is a good idea.
Use isopropanol as your alcohol as im pretty sure it would be the cheapest least polar alcohol you could aquire.

Unfortunetely because you ended up Mg(OH)Br you are gonna waste half your acid to prepare HBr, sadly i have no idea how you could reconsile this beyond just using a very cheap source of sulfuric acid.

clearly_not_atara - 27-12-2019 at 17:07

EtOH won’t precipitate MgBr2. Or alkali bromides. You might get an MgBr2/MgO Sorel cement which could be annoying. I guess you might get phase separation but crystallization seems unlikely as MgBr2 is deliquescent.

You could oxidize to bromate (H2O2 iirc) and precipitate by adding Ba2+, then roast the Ba(BrO3)2 precipitate so it loses oxygen leaving BaBr2. Extraction of bromide directly is hard.

[Edited on 28-12-2019 by clearly_not_atara]

Tellurium - 28-12-2019 at 05:30

Maybe just acidify, oxidize and extract using DCM if you want to recycle the Br- Ion. But for myself I'm just recycling iodine and iodides, because they are quite more expensive. I don't know on which scale you are doing your grignard reactions, but for example with 30g of EtBr you would have a maximum off around 22g of bromide(assuming you would have a 100% conversion). And as bromide is quite cheap(I pay around 20€ including shipping for a Kilo of NaBr) bromide from the grignard is only like 40-50 cents. I think working this up would require chemicals at least worth the same.

draculic acid69 - 29-12-2019 at 06:35

Yes I see the above posters point but if done frequently or on a large enough scale it can make sense to recycle or if bromides are hard to get or precious and rare to you then it's another reason to recycle them.and if you evaporated the water layer down acidified with HCL would it convert mgOHbr -->mgclbr which if gassed with cl2 would yeild mgcl2+br2 which can be distilled out or take the mgOHbr solution boil down to saturation and use h2so4 and distill out the hbr if that oxygen doesn't get in the way somehow.just what I'd try if I was going to do it.

draculic acid69 - 29-12-2019 at 06:37

Quote: Originally posted by michalJenco  
Hi fellow chemist,

I am currently on a Grignard reaction spree and I wonder if it possible to recycle the Mg(OH)Br for preparing further alkyl bromides. Has anyone done this before?

My idea is to precipitate the bromide from the Grignard water layer using alcohol and then use that instead of KBr to produce alkyl bromides.


Probably best to boil down or leave the water to evaporate instead of trying to precipitate with alcohol