Sauron
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Carbon Tetrabromide
Someone recently posted a very interesting article on the preparation and use of CBr4.
This solid is made by the haloform reaction of acetone (or MEK or ethanol, etc.) with sodium hypobromite, which is usually prepared in situ from NaOH
and Br2
The reaction proceeds through the bromoform stage to the solid CBr4 in good yield. For example 200 g Br2 gets you 150 g CBr4.
While CBr4 is commercially available it is very expensive, on the order of half a dollar a gram. So that 150 g CBr4 is worth about $225.
Meanwhile 500 ml Br2 is about $75 so that's only about 5 cents a gram.
CBr4 is a useful brominating reagent.
I am reposting the article here for benefit of those without References access
[Edited on 17-7-2007 by Sauron]
Attachment: cbr4.pdf (263kB)
This file has been downloaded 1252 times
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chemrox
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Sauron, where are you getting these great prices? My supplier was going to charge me about $75-80 for a half liter of Br2 but the shipping was going
to run $137! I found it locally from a company that buys in bulk and packages it down. 500 ml. is costing $187 .. only slightly better. Que
lastima!
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Fleaker
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@ Both of you: still too high a price.
I can get a liter of Br2 from Acros for 115 with shipping.
That's 3.1 kg of Bromine thus it's a little under $0.04/g
Here's the link guys:
http://www.acros.com/DesktopModules/Acros_Search_Results/Acr...
BROMINE 99%+, extra pure 1L 86.3 USD CATALOG NO. 402840010
You're welcome :-P
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Sauron
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The supplier I use here is the agent for both Acros and Carlo Erba. They stock Br2 locally and it is the Carlo Erba product. I am only going by memory
re price. But typically I end up paying about 1.5 to 2X the ex works price by the time transport, duty and 7% VAT have been calculated in.
The point was that even at 10 US cents a gram it is still 1/5 the ex works price for CBr4 and thus as much as 10X cheaper than the landed cost.
CBr4 is a rather specialized reagent, only cost effective compared to other methods in a few cases. But it is good to have it in the arsenal.
I am not at all sanguine about its stability. I would expect it to be no more stable than bromoform, and perhaps a little more stable than CI4 and
iodoform but that's not saying much. So if it is needed, make it fresh and use it fast.
There's an Org.Syn. procedure that employs it, in concert with triphenylphosphine for making alkyl bromides from sensitive alcohols.
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garage chemist
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BromicAcid may have unknowingly prepared it before, he got a crystalline substance from NaOCl, NaBr and acetone:
https://sciencemadness.org/talk/viewthread.php?goto=lastpost...
He thought it may have been bromobutanol, but it could just as well have been carbon tetrabromide.
You have to know, when an alkali bromide is added to NaOCl solution, there will be an instant redox reaction forming NaOBr solution. This is a very
good method ro prepare NaOBr solution without using bromine.
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Sauron
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Perfectly true. Purists however make their own NaOCl soln fresh from NaOH and Cl2 rather than relying on off-shelf bleach or pool-treatment
hypochlorite solns or on the tedious grinding of bleaching powder and water.
I am blessed with a ready supply of Br2 in reasonable qty (a Kg or two a month) so at present I can ignore the fallback positions. Frankly I have no
immediate need to CBr4. It is very good for making benzyl bromide but so is TCT/DMF/KBr and benzyl alcohol which is quantitative compared to CBr4's
76%. Benzaldibromide might be interesting but, the prep of the benzotribromide is a very disappointing 21-26%.
That Org.Syn. exploitation of CBr4 and PPh3 for halogenation of sensitive alcohols requires a tedious drying of the reagents in a drying pistol, and
therefore means buying a specilized piece of costly glassware with inherent limits on scale of operation.
Another Org.Syn.Prep using CBr4 to dibrominate aldehydes as first step in a long sequence to prepare 1,3-dienes requires both a cylinder of ethylene
and a complex ruthenium catalyst in final step. It's lovely chemistry but one I don't feel very motivated to add to my to-replicate list.
So I might make a little CBr4, but it seems like I won't be after Kg batches anytime soon.
I just like to have as many options as possible, is all.
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hashashan
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| Quote: | Originally posted by garage chemist
You have to know, when an alkali bromide is added to NaOCl solution, there will be an instant redox reaction forming NaOBr solution. This is a very
good method ro prepare NaOBr solution without using bromine. |
That is intersting. I should try this method.
It does make me wonder why the hell i had to produce 40ml of bromine and stink the whole house, yard, neiborhood, burning nose, lungs, eyes .....
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Sauron
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A few hints
COLD bromine has very little vapor pressure. You can watch the dark red fumes disappear as you chill the liquid in bottom of the flask.
Ordinary ice-water or brine and ice is cold enough.
Then you don't stink up the environs.
A fume hood and scrubber are very good ideas if you are going to work with Bromine.
Transferring Br2 by syringle through a septum is also an effective way to avoid releasing a lot of those vapors. A 100 ml glass syringe such as
Hamilton and stainless steel or PETF needles work very well.
Combining the chilling and the syringle transfer/septum regimen, even better.
Bromine is not hard to handle, you just need to respect it.
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hashashan
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Well fumehood is a problem when you dont have a proper lab.
And the chilling, yeah it sounds like a good idea 
About the NaOBr, will this work also : NaOCl + KBr ? and how will the process sustain ? i believe that NaOBr is soulble, just like NaCl and KCl.
How can one extract the NaOBr? or should i just add the acetone to the NaOCl + KBr solution and the CBr will be just falling out?
Oh and another thing, will the sitty NaOCl that is sold for cleaning work, or is it just too contaminated with crap?
[Edited on 19-7-2007 by hashashan]
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garage chemist
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As a general rule, you cant isolate alkali hypochlorites nor free them from other salts in their solution.
For example, any grade of NaOCl solution you can buy (no matter if for cleaning or high-purity for the lab) always contains an equal amount of NaCl in
it, due to the equation of its formation:
2 NaOH + Cl2 ---> NaOCl + NaCl + H2O
The NaCl cant be removed, so NaOCl solution is always really NaOCl + NaCl solution.
Same with the NaOBr, you have no chance of removing the NaCl or KCl byproduct. It is always left in there, it doesnt interfere with any of the uses.
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hashashan
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And it doesnt harm the haloform reaaction? nor the reaction to form CBr4?
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Sauron
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Well, obviously not, because if it did, there would be no haloform reaction, yet certainly and inarguably there is such a reaction.
QED.
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Blind Angel
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Well nice to find a use from my recently donated 150ml of Bromine, but if I may ask, is it easy to order from Acros or you guy just lucky?
/}/_//|//) /-\\/|//¬/=/_
My PGP Key Fingerprint: D4EA A609 55E4 7ADD 8529 359D D6E2 33F6 4C76 78ED |
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Sauron
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I don't know who the Acros agent is in Canada. I deal with their agent in Thailand where I am. I believe their US agent is Fisher and that may or may
not also be the case in Canada. Their website www.acros.be will tell the tale.
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hashashan
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tried to make the following :
bought bleach (NaOCl) dissolved some KBr in it.. it gained some yellow color.
added some NaOH and then acetone.... and what do you think happned? nothing
no participate
what might be wrong here?
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woelen
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I had similar problems with household bleach and acetone. Household bleach simply is too dilute. It just is 4% 'active chlorine', which is weak. In
the past, one could buy 12% 'active chlorine' bleach, but that is no longer available. The dilute stuff certainly makes chloroform (or bromoform,
when excess KBr is added), but the amount of haloform is so low, that it remains dissolved.
If you want to make bromoform (or chloroform), then use more concentrated bleach. You can make that by using swimming pool chlorine tablets (e.g.
TCCA, Na-DCCA, or calcium hypochlorite) and hydrochloric acid for making Cl2. That Cl2 must be bubbled through a fairly concentrated solution of NaOH
(and if you want bromoform + KBr). The solution must be kept cold, otherwise the hypochlorite disproportionates. Hypobromite is even less stable. It
VERY easily disproportionates to bromate and bromide. It all is quite cumbersome and requires a lot of time and preparation. Making chloroform and
bromoform and isolating it is not as easy in practice as it is on paper.
[Edited on 23-7-07 by woelen]
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vulture
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I've seen some pool suppliers stock pretty concentrated Ca(OCl)2 solutions. That might be a better bet in this case, although it is somewhat
expensive.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Sandmeyer
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| Quote: | Originally posted by vulture
I've seen some pool suppliers stock pretty concentrated Ca(OCl)2 solutions. That might be a better bet in this case, although it is somewhat
expensive. |
You can buy pure Ca(OCl)2 over the counter (pool stores)...
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hashashan
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its ok i can just buy 12% bleach .. in a chem store of course ... but will a 12% work or should i have one more concentrated .. i dont want to spend
my money on nothing
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panziandi
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i have a prep of carbon tetrabromide. it is sourced from Experimental chemistry a laboratory manual, Rendle et al. i have used this prep before and
have had no real problems my yields were never better than 50%. i have condensed the method down but it should be easy enough to follow.
dilute 100ml of 2M NaOH to 300ml with distilled water. add 2.5ml of bromine and shake until the colour disappears. in a seperate flask dilute 5ml of
acetone to 50ml with distilled water, and add 9ml of this solution (1/80 mole acetone), with shaking to the hypobromite solution. stopper and shake
vigorously. initially bromoform droplets separate, on continued shaking crystals of carbon tetrabromide deposite. leave to stand for a while then
filter at the pump. return the filtrate to the reaction flask and add a second lot of bromine and acetone solution and repeat the process. add the
precipitate to the first and wash with distilled water and dry over sulphuric acid in a desiccator.
CBr4 m.p. 94*C
yields usually 50%
hope this is useful! andi
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BromicAcid
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Garage Chemist, I actually have some pictures of my attempt at bromoform that may well have been carbon tetrabromide, check them out.
It was darn cold outside, in the foreground you can see a solution of sodium hypochlorite that I had, but further back you can see what happened when
I added a stoichiometric amount of sodium bromide to the solution, it turned darker, making the hypobromite.
Now, here is the interesting one, what you actually see are three different layers. The top is an aqueous layer that is the byproduct of the haloform
reactions. The second layer is chloroform, I made a batch of chloroform from sodium hypochlorite along side my attempt at bromoform to compare the
reactions. The third and final layer, the granular bottom layer was the solid material that I got from my reaction of acetone with excess hypobromite
(not sure how much excess) but it may well be the tetrabromide, I got the solid and poured it in with the chloroform and it immediately sunk to the
bottom, it took a while but it dissolved, apparently without melting. Nothing conclusive here but I like to get a chance to post pictures when I can
since I don't experiment any more. 
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