Sciencemadness Discussion Board - Free Bromine from Hydrobromic acid synth

Free Bromine from Hydrobromic acid synth

Cabalaba - 10-6-2016 at 15:38

I think the temp must have raised above 70degrees and Ive noticed that the solution looks like dilute bromine.
I did a slow drip of 3M H2SO4 in to 600mL in to a solution of 3M NaBr, took over 4hrs, with 1 drip at most every 3s.
Is it possible that this is free Br2 or contaminants from either the NaBr or the H2SO4?
If so, any ideas what I should do?

Sorry if this is a dumb question, Im a neuroscientist who is slowly teaching themselves chemistry,

Cabalaba - 11-6-2016 at 03:08

Thanks for all the help or references.

DJF90 - 11-6-2016 at 04:28

I would suggest passing a little sulfur dioxide through the solution, or adding a pinch of (red) phosphorous. Both will reduce any free bromine to HBr in an aqueous environment, with non-volatile byproducts enabling purification by distillation of the (azeotropic) hydrobromic acid.

Cabalaba - 11-6-2016 at 05:06

Strange though, My first batch of HBr was clear, with a b.p. of 121-122C. The only difference was A) poss temp rise and B) I used LiBr, but thats not the case here.

@DGF Thanks, no red p around (try not to keep certain chems around) but Ill see what I can do with SO2.

Im honestly dismayed by the non-responses, you would think more people would encourage safe hobbyist chemistry.

It probably why I dont enage forums, either the topic goes off course or the original posted is subject to sarcasm or direct attacks.
Ive seen it get better around here (much better) but I guess when one thinks their anonymous...

[Edited on 11-6-2016 by Cabalaba]

[Edited on 11-6-2016 by Cabalaba]

j_sum1 - 11-6-2016 at 05:38

Quote: Originally posted by Cabalaba

Im honestly dismayed by the non-responses, you would think more people would encourage safe hobbyist chemistry

Well, you are dealing with a comparatively low-traffic day round here. It does vary.
Additionally, I don't think your question was that clear. "If so, what should I do?" Well the "if so" could have been referring to any one of three things in the previous sentence -- free Br2 or contaminants from one of two possible sources. As well as this, it wasn't very clear to me what your objective was. Were you after Br2 liquid or Br2 aqueous or something else?

I don't think anyone was being rude. I bet there were a number of people like me who read your post and decided not to take the time to figure out your meaning. The clearer you are the better your chances of getting an answer.

Welcome to SM. Stick around. If you are interested in increasing your skill set in chemistry, I don't think you could ask for a better place.
J.

Cabalaba - 11-6-2016 at 13:10

Fair enough, I thought the title said my intention but many details were left out (how long it was above 70F), etc). Also never considered a 'slow day' as my slow days are during the week when Im trying to patch.

Anyway, I aimed to make a liter to 1.5L of HR through several smaller batches. My last HBr ssyth wasnt perfectly clear but my sense is the temp rosse and free Br2 formed. I generated some SO2, though I suspect not enough.
Of the 1500mL raw material (NaBr and SO4), about 100mL came over between 121-122C.
Im running the distill again, see if I get anymore.

Could find a use for free Br2. If I could use it, so its not all a loss. Ill have to distill off the water, filter the sodium bisulfate (reuse later), My worriers are the contaminants from the SO4

Sorry if I came off as a ass, Ive been following this forum since I want a congeinal forum where drug synth is avoided (as much as possible).

Dont worry I will stick around but wont bother until Ive exhausted all my resources (but patents tend to be out of my realm, it'd be like you reading one of my papers).

Thanks for your patience and understanding.

Cabalaba - 11-6-2016 at 13:25

Quote: Originally posted by Cabalaba

Anyway, I aimed to make a liter to 1.5L of HR through several smaller batches. My last HBr syth wasnt perfectly clear but my sense is the temp rose and free Br2 formed.

After the sample runs at 122 for a while, it jumps beyond 127, at which point I turn it off, maybe this will help.

Cabalaba - 12-6-2016 at 07:44

Quote: Originally posted by DJF90

I would suggest passing a little sulfur dioxide through the solution, or adding a pinch of (red) phosphorous. Both will reduce any free bromine to HBr in an aqueous environment, with non-volatile byproducts enabling purification by distillation of the (azeotropic) hydrobromic acid.

As you said, I rigged a heater with a 100mL flask (flushed with 20% O2) with 4g sulfur. Before I did this my HBr yield was pathetic (<10%) but after heating again, Ive spent 8 more hrs collecting dilute HBr between 120-124C, and it still coming. Now I got 0.5L and counting. I even had to double the size of the receiving flask.

Thanks DJF90

bluamine - 13-6-2016 at 06:54

Quote: Originally posted by Cabalaba

I think the temp must have raised above 70degrees and Ive noticed that the solution looks like dilute bromine.
I did a slow drip of 3M H2SO4 in to 600mL in to a solution of 3M NaBr, took over 4hrs, with 1 drip at most every 3s.
Is it possible that this is free Br2 or contaminants from either the NaBr or the H2SO4?
If so, any ideas what I should do?

Sorry if this is a dumb question, Im a neuroscientist who is slowly teaching themselves chemistry,

WOW! you have acces to hydrobromic acid!!

UC235 - 13-6-2016 at 18:20

Have you seen the attached paper? It's worked well for me repeatedly when adjusted for NaBr and seems to produce colorless distillate despite how the pot looks. I do find that even if I discard any forerun and slowly slowly distill at 122C, the product manages to be more dilute than azeotropic. An excruciatingly slow fractionation of the water out of the initial distillate through a large vigreux produces azeotropic HBr containing a trace of Br2 and without much loss but I usually just check for approximate concentration via density and use it as-is.

Attachment: HBr.pdf (1.2MB)
This file has been downloaded 378 times

[Edited on 14-6-2016 by UC235]

ave369 - 14-6-2016 at 23:58

You can separate free bromine from this solution. Bromine solubility greatly increases when Br<sup>-</sup> is around. Because your mixture contains HBr, bromine does not separate from the solution and keeps being dissolved in it.

One way to separate it is fractional distillation. The boiling point of bromine is around 50-60 Celsius, while azeotropic hydrobromic acid has it much higher. You can distill all bromine from the solution at first, and all hydrobromic acid second.

The other way is oxidizing all HBr to Br2. If you get rid of that pesky Br<sup>-</sup>, bromine will separate from the solution and you will be able to get it via a separatory funnel. This way you'll get a lot of bromine but lose all the acid. Be warned: while you oxidize HBr, the concentration of the bromine solution will rise. And, even before the point when bromine starts to separate, you will see a lot of bromine vapor emerging. Do it in a well ventilated area or outside.

[Edited on 15-6-2016 by ave369]

woelen - 15-6-2016 at 00:51

Even commercial samples of HBr are yellowish brown. I never have seen colorless solutions of HBr.
I myself did the synthesis of HBr from KBr and concentrated H2SO4. I dissolved as much of KBr in water as possible, keeping track of the weight added. To this, I added a calculated amount of concentrated H2SO4 (one mole of H2SO4 for 2 moles of KBr with a very slight excess of H2SO4 to be sure that all bromide is converted), slowly, while stirring. This leads to formation of some free Br2, but only marginally. This is expected, because for any free Br2 formed, there also is formation of SO2 and on dilution with the water, they react again to H2SO4 and Br(-) ions. I allowed the liquid to become quite hot. This allows all material to remain dissolved.
On cooling down, a lot of crystalline material settled at the bottom. This is K2SO4. I allowed the liquid to cool down to room temperature and then I put it in a fridge, allowing it to cool down further to a few degrees Celcius. I decanted the liquid (which was yellowish/brown) from the crystal mass and distilled this liquid. It yielded me appr. 40% HBr with a yellowish/brown color, but with hardly any oxidizing properties. Adding SO2 to this liquid does not make it colorless. This still is a riddle to me, apparently the brownish/yellow color is due to some impurity, other than Br2 (its color also is somewhat different, not the bright orange/brown of Br2, but a more brown/yellow color).

Hopefully this information helps you make HBr at decent concentration and of decent purity. You even could consider skipping the distillation step, the amount of K2SO4 in the liquid is quite low and for many experiments it does not interfere.

Cabalaba - 15-6-2016 at 05:35

Thanks for the info. Actually my first guess was to do a fractional, but I wanted to maximize yield. Trying to nominate some alkyl groups.

From now on I'm doing all that in 1 pot.
I'm very happy I decided to ask for help rather than sitting on the sidelines as I have for years.

[Edited on 15-6-2016 by Cabalaba]

Cabalaba - 17-6-2016 at 05:01

Quote: Originally posted by Cabalaba

Trying to nominate some alkyl groups.

[Edited on 15-6-2016 by Cabalaba]

To be more clear, I wanted more freedom to halogenate alkyl groups as need be and not be tied down to one.

Cabalaba - 17-6-2016 at 05:23

@woelen
I usually run one distillation because I do sell of HBr as well.
But I like the the idea of skipping the step when Im using it on an alcohol.

And @bluamine, thats just half the prep, I dont want to buy a 5L vessel or mantle (too much attention as is). I aim to recover at least 2L of HBr.

I want to Brominate ethanol, n- and iso-propanol, maybe n-, iso, tert-butanol, (and sec- butanol if I find it) and n-pentanol. And possibly a 1, 2.5, or 5M methyl bromide aqueous solution, just for fun.

I expect to focus on propyl, because once you get to longer alkyls, you can get mixtures of n-propyl bromide and iso-propyl bromide. I dont need that headache when I really need to work on getting a paper from my postdoc. Should have gone with chem, not neuro.

Id like to specialize in this subfield, these things are so ubiquitous, they're used in so many protocols.

Enough rambling. Thanks guys, I wont say it a lot, but your a good group. Have a nice weekend.