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I'm trying to understand the fate of bromide anions during and after they undergo anodic oxidation. I have a solution of 94% DMF/6% water (by volume) with 0.1 M tetrabutylammonium bromide (TBAB) as the supporting electrolyte. I've used this electrolyte to reduce a cathode at -2.5 V or less v. the Ag/AgCl reference electrode. During the reaction, the solution near the platinum counter electrode (the anode) turned yellow. I would assume this is due to the reaction: $$\ce{2Br–(aq)-> Br2(l) + 2e– }$$

The next day, I noticed that the yellow liquid in the solution had completely disappeared. I'm assuming that the bromine had dissolved into the solution into $\ce{Br-}$ ions again. However, I'm not 100% sure of the chemical reaction associated with this. Also, would there be any side products associated with the dissolution? Or, do we end up with the species we initially started with in terms of bromine?

I ask because I would like to be able to reuse the electrolyte multiple times and so want to figure out exactly how the electrolyte changes after the reaction. At the cathode, there will be reactions such as the hydrogen evolution reaction which will make the solution more basic. However, I'm not sure what happens with the bromine.

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A possibilty is that elementary bromine formed -- causing the the yellow color -- then solubilized in water yielding bromine water.

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(source)

It does not dissolve well (2.8% ref -- but do not forget the high density (about 3.10 g/mL) of liquid bromine here). It then may disproportionate into then colorless hypobromit and bromide

$\ce{Br2 + H2O -> Br- + BrO- + 2H+}$ (ref.)

especially under light, forming an acidic solution. The hypobromit may act as an oxidizer.

To draw an analogy, chlorine gas does dissolve into water, too (ref, ref).

Proceed with caution since bromine is corrosive, though.

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  • $\begingroup$ Thanks for this answer. I think you're probably right. I do wonder if there is some reaction with the DMF, though. I recently learned that bromine and DMF violently exothermically react (in situations without water, at least). I couldn't figure out what reaction this is exactly (would be good to know). However, I'm thinking that this is not a good system if there is that safety hazard. $\endgroup$ – Electrolyzer Feb 17 at 3:49
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    $\begingroup$ [Link to info about bromine and DMF] (webwiser.nlm.nih.gov/…) $\endgroup$ – Electrolyzer Feb 17 at 3:50
  • $\begingroup$ @Electrolyzer Interesting OSHA-related data base! $\endgroup$ – Buttonwood Feb 17 at 10:38
  • $\begingroup$ @Electrolyzer I speculate a brief reduction of a tiny amount of elementary bromine at a very low concentration, e.g. in a sweep of cyclovoltammetry (e.g. DOI: 10.1021/acs.jchemed.7b00361, OA) is lesser dangerous than extended presence of concentrated (neat) bromine in DMF in an excited state (like heat, sun light). Otherwise, NBS-DMF would not be listed as a mild monobromination reagent at room temperature in Leo Paquette's encyclopedia EROS (cf. DOI: 10.1021/jo00393a066, or scheme 1 of DOI: 10.1021/ja00040a063 in the preview). CV indeed uses NMe4BF4 instead to limit redox on your sample. $\endgroup$ – Buttonwood Feb 17 at 11:12
  • $\begingroup$ Good point. I imagine in very small amounts, it would be safe as suggested by the references. In my case, I will be running at a high current for possibly hours, so can't have amides+bromine building up! It appears that acetonitrile also works for my reaction and is compatible with bromine, so I'll go with that. I think the solution to reusing the electrolyte is just to use an anion that oxidizes to something that can be removed easily (like a solid or gas). I wonder if chlorine gas from Cl- ions would dissolve into acetonitrile... $\endgroup$ – Electrolyzer Feb 17 at 15:53

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