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Author: Subject: 1,1,2,2-tetrabromoethane Synthesis & Impurities
Polesch
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[*] posted on 24-6-2013 at 02:37
1,1,2,2-tetrabromoethane Synthesis & Impurities


I've successfully made tetrabromoethane by passing acetylene gas directly into dry bromine, but the color of the solution never turned colorless, but something light-yellowish:



I don't know how to explain that discoloration, I could only think of unreacted bromine (very dilute?). Tried adding some metabisulfite, but not change. I also don't have the capabilities of distilling it, so I need some ideas on what this is, and how I could obtain the colorless tetrabromoethane.

I have a quick video of my procedure below, just so you can see the colors of the solution as the reaction proceeds: http://www.youtube.com/watch?v=VU3DgEF4haM
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[*] posted on 24-6-2013 at 03:51


Finnally someone doing real chemistry with acetylene! I have been waiting to see this reaction for a long time.

You seams to have all the glassware needed for distillation, no? I would react it with Na2CO3, and then a dessicant if I where not distill it. I strongly reccomend you distilling your product.




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Polesch
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[*] posted on 24-6-2013 at 04:21


I became interested with acetylene when I learned about the metal acetylides, Cu/Ag acetylides are rather fascinating. But I'm more or less done with them now, but still have a bunch of calcium carbide left, so if you know any interesting reactions I would be happy to try them out.

I do have a complete distillation setup, but tetrabromoethane has a very high boiling point of 244 C, and I do not have a proper vacuum. I did attempt a distillation earlier, but the vapors would either recondense immediately (not make it up the stillhead) or it would decompose from the vigorous boiling I had to achieve (bunsen burner). I suspect the metabisulfite caused some debromination and formation of dibromoethane at the high temperature also.
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[*] posted on 24-6-2013 at 04:47


Did you clean the acetylene from the calcium carbidebefore brominating it? Acetylene from calcium bromide contains some PH3, H2S and traces of organic sulphur compounds. These impurities give it the typical bad smell of acetylene from calcium carbide. PH3 leads to formation of PBr3 and/or PBr5 (yellow) with bromine and H2S leads to formation of S2Br2 (red). These impurities may lead to yellow coloration of the final product, especially when strongly diluted.



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[*] posted on 24-6-2013 at 04:57


I did not clean the acetylene in any way, and the gas produced did have a distinct smell (of old wet clothes or something, musky?), so you might be right on the impurities you list. Any idea how I would separate them safely? Those impurities should hydrolyze in water, while tetrabromoethane should be insoluble in water, would it work by simply shaking the solution with water in a sep funnel?
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[*] posted on 24-6-2013 at 06:36


Acetylene gas can be scrubbed in an alkaline scrubber to remove PH3 and H2S.
That is if you have enough reagents to perform another synthesis.
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[*] posted on 24-6-2013 at 07:03


I agree with <strong>woelen</strong>. <strong>Polesch</strong>, it couldn't hurt to try a simple wash with distilled water, as you have suggested. Finally, I noticed that your bromine was sucked back into the flexible tubing you had leading from the gas generator to the reaction flask. It is also possible that the bromine reacted with and/or picked up a contaminant from the tubing.



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[*] posted on 24-6-2013 at 07:50


I don't have that much bromine left, and I plan to use the rest on a methylamine synthesis. If I fail to purify it I will probably leave the project on hold for a month or so as I get reagents for making more bromine. I will try a water wash immediately, will report back!

High-res picture of the mixture after shaking with distilled water, the color went from reddish to brown: https://dl.dropboxusercontent.com/u/15526481/Permanent/WP_20... ... Didn't do much. And that foamy layer with the bubbles, kinda look like some kind of synthetic polymer/fabric. That soup is now tetrabromoethane, possibly dibromoethane, sodium metabisulfite, most likely phosphorus bromides and H2S/PH3 impurities. No idea what's going on anymore.

<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: merged sequential posts]

[Edited on 7/8/13 by bfesser]
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[*] posted on 24-6-2013 at 09:31


If you can't distil it, how about going the other way. Freeze it and pour off any liquid impurities.
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[*] posted on 24-6-2013 at 09:49


Or if the impurities aren't enough to pour off, you may be able to recrystallize it at low temperature. The Merck index says it's miscible with alcohol, chloroform, ether, aniline, and glacial acetic acid. It might be too soluble in those to recrystallize from them (unless you can cool it down to a ridiculously low temp like -80 oC), but I'd try a small amount of your stuff, mixed with a small quantity of alcohol, and placed in the freezer. See if it crystallizes.

<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: removed unnecessary quoting]

[Edited on 7/8/13 by bfesser]




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[*] posted on 24-6-2013 at 11:42


Not too ridiculous, a dry ice and acetone bath gets you to -78C (with dry ice being available from some grocery stores).

<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: removed unnecessary quoting]

[Edited on 7/8/13 by bfesser]




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[*] posted on 24-6-2013 at 14:22


The C-H bond in acetylene is quite reactive also. (think about the double displacement reaction that happens when acetylene is passed into a solution of silver nitrate)

I would not be surprised if there were some aromatic impurities forming in your reaction, which would impart the yellow color. Once a Br-CΞCH forms, the proceeding side reactions can be quite complicated.

Quote: Originally posted by Polesch  
Tried adding some metabisulfite, but not change.

Not really sure, but metabisulfite may not necessarily get rid of the yellow color. Woelen has done some experimentation of iodine with sulfites, having found that a yellow-colored complex exists. And furthermore, the metabisulfite could simply be reacting with your brominated product.

Do not forget that bromoalkanes can substitute off much more readily than chloroalakanes. In fact, I think they are incompatible with water.

[Edited on 24-6-2013 by AndersHoveland]
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