Sciencemadness Discussion Board

Bromine monochloride

Pumukli - 12-7-2015 at 12:42

I was wondering wether it would be possible to make BrCl from TCCA and a bromide salt plus acid in a similar fashion as chlorine can be generated from TCCA and HCl soln. ?
Is it a walkable path? Has anyone tried it yet? Drawbacks?

Gooferking Science - 12-7-2015 at 14:56

I can honestly say I'm a bromine expert. I have worked with it many times, so I feel I can reliably answer you. Using the chemicals you described would make trace amounts of BrCl, but it would basically just make elemental bromine. In fact, the mixture of those chemicals is a viable synthesis of bromine. You would just need to distill it. That being said, I would be interested in making BrCl. I will see if I can find a good procedure for making it.

EDIT:
I can't find a good way to make BrCl, and from my research it appears to be an unstable molecule. It doesn't have many uses either...

[Edited on 12-7-2015 by Gooferking Science]

Volanschemia - 12-7-2015 at 15:11

It's actually a very good method of making elemental Bromine. In fact, I performed the isolation myself two days ago. BrCl is formed, but in very small amounts when the HCl is added dropwise by use of an addition funnel. It may be possible to recover any BrCl formed, by simple distillation at very low temperatures as BrCl boils at 3C.

woelen - 13-7-2015 at 01:51

You make BrCl as long as there is no excess of bromide. Any excess of bromide will lead to formation of bromine and chloride ions.

CuReUS - 13-7-2015 at 03:36

can't you just make it by passing chlorine through bromine.I thought inter halogen compounds could be made just by mixing both the halogens.

Gooferking Science - 13-7-2015 at 03:55

Quote: Originally posted by CuReUS  
can't you just make it by passing chlorine through bromine.I thought inter halogen compounds could be made just by mixing both the halogens.


That's what i thought too. Couldn't you just fill a flask with bromine vapor and pass chlorine through it? Or would the bromine stay as Br2 rather than bonding with the chlorine?

Pumukli - 13-7-2015 at 09:30

Thanks for the input(s). :-)

Something about the background of the original question: I decided that I'd need a particular Br-Cl-alkane. The easiest route to this compound seemed to be the addition of BrCl to a precursor alkene. (Being a symmetrical one the appearance of isomers is fortunately not a problem, all the reaction products are the same.)
I love OTC synths and as I already have TCCA and NaBr on the shelves I thought it might work.
Exactly as Woelen wrote: I thought of balancing the ammounts and avoid Br- excess. The forming BrCl gas would be swiftly consumed in the addition reaction, so its short shelf-life does not count either.

So Woelen, if I used a slight excess of TCCA (just to be on the safe side), thoroughly mixed the solid, dry reactants (ball mill maybe?) and then slowly introduced some dilute sulfuric acid (to get a stirrable slurry and not a solid crystall mass) then (maybe) introduced some overpressure from an aquarium air pump to continuously expell the forming BrCl and bubbled it into the solution of the alkene (inert solvent) then may I get what I wanted? ;-)


woelen - 13-7-2015 at 09:40

It will be hard to obtain precisely BrCl in this way. I can imagine that not all Cl2 reacts with the bromide and that you get BrCl, mixed with Cl2.

Also keep in mind that your method gives humid BrCl. Whether that is a problem or not depends on your reaction. Probably it will do no harm for the reaction itself, but workup/purification may become harder in the presence of water.

Another way of making BrCl of more precise composition is to put Cl2 in a bottle of known volume. You then can compute how much Cl2 you have and then you can compute how much Br2 you need to react with the Cl2. E.g. if you use 1 liter of Cl2 at room temperature, you have appr. 0.041 mole of Cl2. This method also is not without complication. You have to add the Br2 to the Cl2 with at little as possible loss of gas and you have to keep in mind that the pressure increases (BrCl is a gas at room temperature) when the Br2 reacts with the Cl2.

Pumukli - 13-7-2015 at 10:47

Umm, I was expecting something like this. I mean the not quantitative conversion.
Anyway, I think the reaction products that would arise from the addition could be separated by distillation. There is about 45 Celsius difference in b.p. of dichloro-alkane and dibromo-alkane so bromo-chloro-alkane should be somewhere halfway in between. :-)
Thanks!

annaandherdad - 13-7-2015 at 11:23

woelen is too modest to tell you that he has a photo of this process. It's on a very interesting thread on this forum,

http://www.sciencemadness.org/talk/viewthread.php?tid=9690

Just google on "sciencemadness bromine chloride"

Metacelsus - 13-7-2015 at 19:19

Could you try forming a halohydrin (chlorohydrin) instead, and then reacting with hydrobromic acid to form the desired compound?

[Edited on 14-7-2015 by Cheddite Cheese]

Pumukli - 14-7-2015 at 10:42

Ahh, a halohydrine!
It may work, actually I overlooked this route because I was so excited by the chance to generate/use BrCl. :-)

I will explore this more thoroughly because it may work. Thanks for the tip!

CuReUS - 15-7-2015 at 03:06

there might be some problems with the halohydrin route

1.OH- is a good nucleophile.Br- ion won't be able to replace it unless the conc of HBr is high(70%).normal HBr is 48%.azeotrophe

2.you would have to use dry HBr gas,as the water present in HBr will give OH- ,that might replace the Cl- of the halohydrin to give a diol,which under acidic conditions(due to the HBr)might lead to a pinacol rearrangement.

you could try using HBr dissolved in GAA.
BTW,this thread might be useful
http://www.sciencemadness.org/talk/viewthread.php?tid=1091

Nicodem - 15-7-2015 at 04:13

CuReUS, instead of resorting to groundless arguments, try referring to the literature instead. People have been using 48% HBr(aq) for over a century to prepare alkyl bromides from the corresponding alcohols. It does not even need to be that concentrated, as long as you give it some more reaction time and temperature. Actually, it does not even need to be HBr, as one can use aq. H2SO4 and a bromide salt. See the numerous examples posted on this forum.

BrCl can be prepared from TCCA and bromine, but the literature example is a bit of no go for the average chemist. It requires heating up bromine with TCCA in a closed vessel! See the attached article Monatshafte für Chemie, 103, 1150-1157 (1972). I might guess that this could be performed at normal conditions by using a suitable solvent like dichloromethane (which does not dissolve much TCCA) or acetonitrile (not completely inert to TCCA, but at least it dissolves it well).

Aqueous BrCl is easily prepared from KBrO3, KBr and HCl(aq) by using the appropriate stoichiometry of bromide/bromate (such a solution will contain also excess HCl and KCl). See Chem. Abstr. 54: 67142 cited here:
Quote:
Preparation and testing of BrCl solution
Schulek, Elemer; Burger, Kalman

A 0.1N BrCl soln. was prepd. from stoichiometric amts. of KBrO3 and KBr in the presence of excess HCl soln. After 3 months storage in the dark the BrCl content was only 3-​5​% lower. The BrO3-​, Br2, and Cl2 contents of the BrCl soln. were detd.

Magyar Tudomanyos Akad. Kem. Tudomanyok Osztalyanak Kolzemenyei, 12, 9-14 (1959)



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CuReUS - 15-7-2015 at 04:36

Quote: Originally posted by Nicodem  
People have been using 48% HBr(aq) for over a century to prepare alkyl bromides from the corresponding alcohols.

would you be kind enough to provide a reference ?
Quote:
Actually, it does not even need to be HBr, as one can use aq. H2SO4 and a bromide salt. See the numerous examples posted on this forum.

even I know that H2SO4 and a bromide salt will work.The trick there is that H2SO4 will dehydrate any alcohol that might form so that Br- has another chance to add to the alkene.

Nicodem - 15-7-2015 at 10:22

Quote: Originally posted by CuReUS  
Quote: Originally posted by Nicodem  
People have been using 48% HBr(aq) for over a century to prepare alkyl bromides from the corresponding alcohols.

would you be kind enough to provide a reference ?


There are more than 2000 references for the nucleophilic bromination of alcohols with aqueous HBr, so I hope you don't expect me to select one out of them all. It is pretty much a method of choice for some chemists, particularly if the substrate is soluble in that acid. Even I used it on a couple of substrates in my work (and I rarely need to brominate alcohols). If the substrate has poor solubility, acetic acid can be used as a cosolvent,* though it does work, albeit more slowly, also under biphasic conditions (see about 3-phenylpropanol). Alternatively, on liquid alcohols, gaseous HBr can be used directly.

* Like in the Org. Synth. example of monobromopentaerythritol, though there the acetic acid may have another role (selectivity issue). As noted in the Discussion part, the reaction works also without acetic acid.
Quote:
even I know that H2SO4 and a bromide salt will work.The trick there is that H2SO4 will dehydrate any alcohol that might form so that Br- has another chance to add to the alkene.


Unless you provide a reference demonstrating this is supposed to be the prevalent pathway on simple aliphatic alcohols, I will consider that as another groundless argument. If what you said was the case, then such a reaction would not work as well on primary alcohols such as ethanol at the conditions it does, and it would not work at all on methanol or other alcohols lacking a beta-hydrogen (unless trough a different mechanism). It works excellently on both as testified by posts on this forum and the literature examples. Besides, if that was the case, the reaction on iso-butanol would have to give tert-butyl bromide as the sole product. Instead the main product is isobutyl bromide (DOI: 10.1080/00304948809355804). Furthermore, the sulfuric acid with bromide method works best in the presence of some water (see discussions and references posted on this forum), which goes against any elimination promoting role of sulfuric acid. Not to mention, that on most substrates the addition of HBr on alkenes does not necessarily go faster than the SN2 of HBr on alcohols, so what point is there to even invoke an elimination-addition mechanism in the first place? It is reasonable for tertiary alcohols, but certainly not for primary alcohols.

Paradoxically, at least for iso-butanol, the only evidence of a presumably elimination-addition pathway that I could find, was the reaction not in the presence of sulfuric acid, but rather with HBr gas (WO2010104742). Apparently, under non-optimal conditions, up to 10% tert-butyl bromide forms. Since SN1 is not possible on simple primary alcohols like iso-butanol, the most likely explanation for the formation of tert-butyl bromide is the elimination-addition pathway.

CuReUS - 16-7-2015 at 04:36

I am sorry,I should have double checked.After your post yesterday,I immediately checked Vogel,and there was a procedure for converting secondary OH to Br using constant boiling HBr(48%) at reflux without H2SO4.
H2SO4 was used to convert a primary OH to Br.

also I found that they use sulphuric acid to make the OH a better leaving group,not a dehydrating agent,as I thought.So you are right
http://www.orgsyn.org/demo.aspx?prep=CV1P0025

Pumukli - 16-7-2015 at 12:10

Nicodem, thanks for the tip!
The aqueous solution of BrCl seems interesting. The HCl excess may not cause harm because even if it would add to the olefine bond the resulting monochloro-alkan had way lower boiling point than the required bromo-chloro variety.

As a sidenote: I always enjoy your posts, especially those where you cite something. It makes me wonder HOW do you find those articles!? I'm sitting in the middle of nowhere in the rural countryside (where fortunately high-speed internet is available), far from university libraries and wondering whether I could have find that aqueous BrCl synth if I knew where and how to look for or you have something more than a naked internet connection?

chornedsnorkack - 19-7-2015 at 00:35

How easy is it to purify liquid BrCl?
The boiling points are
Cl2 - -34 degrees
BrCl - +5 degrees (39 degrees higher)
Br2 - +58 degrees (53 degrees higher).
Can fractional distillation give pure liquid BrCl? Or is it unstable and decompose as the other fractions are drawn off:
2BrCl<->Cl2+Br2