Banjo
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Bromides/bromates from tetrabromoethane
My first post - after much lurking in the shadows, so greetings to all - and many thanks for all the help you have already provided :-)
Some time ago I obtained one liter (weighing almost 3 kg) of Prolabo reagent grade Tetrabromoethane (TBE, Acetylene tetrabromide. Wikipedia), which I have used as my primary source of high purity bromine. By slow and careful distillation of TBE (in a fume hood - these vapors
are deadly) almost pure bromine will come over at about 245C (atmospheric pressure).
The bromine I produce this way is typically bubbled directly from the condenser into water to make HBr, which is safer and easier to store and handle
than bromine itself. The HBr is normally used as is, or used to make bromides and bromates. Needless to say - OTC products containing Br is hard to
come by where I live.
Bromine extraction from TBE is slow, besides both TBE and bromine are pretty toxic substances, and vaporizing them at up to 250C does not make them
any safer. So, I can not help thinking that there must be safer/simpler one pot ways to prepare HBr/bromides/bromates from TBE, which does not require
bromine extraction at elevated temperatures?
Would simply refluxing the TBE with water at atmospheric pressure make HBr, or will the reaction be too slow? How about refluxing TBE with a solution
of KOH to make KBrO3 - or with iron filings and a solution of K2CO3 to make KBr?
TBE is just too nasty for me to start experimenting wildly on my own - so I hope someone here knows, or can think of good ways to do this, preferably
with effective ways to extract and purify the target compounds, which in my case are - HBr, KBr and KBrO3.
I have not found much information on the net as TBE is not widely used as a Bromine source (or at all, really), so any help would be greatly
appreciated :-)
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PHILOU Zrealone
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First of Br2 into water will provide you HBr, but also HOBrO2 (via HOBr).
Refluxing excess strong base and alcoolic solution (methanol, ethanol or glycol) of Br2CH-CHBr2 should provide you the desired bromides.
4 NaOH + Br2CH-CHBr2 --> 4 NaBr + O=CH-CH=O
O=CH-CH=O --NaOH --> HO2C-CO2H + HO2C-CH2OH + HOCH2-CH2OH
Refluxing PbO, water and Br2CH-CHBr2 is said to deliver glyoxal and unsoluble PbBr2.
Same process must be feasible with H2SO4/H2O refluxing because from CHCl=CCl2 and H2SO4, you end up with Cl-CH2-CO2H.
Br2CH-CHBr2 + 4H2O -H(+)-> O=CH-CH=O + 4 HBr
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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Nicodem
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You can react your tetrabromoethane with magnesium or zinc to obtain anhydrous MgBr2 or ZnBr2 correspondingly. With this particular haloalkane the
reaction should be easy. These reactions in general are very exothermic and have an induction period. So there is a danger of a runaway reaction if
you fail to control the reagent addition rate and add too much tetrabromoethane before it kicks in. Ethers are generally the solvents of choice, as
the first part of the reaction is essentially identical to a Grignard preparation. Though toluene might also be suitable for zinc. Do a few
experiments on a 10 mmol scale first.
I think something similar was already discussed, so try UTFSE.
Zinc bromide is a particularly useful reagent as it is also a relatively strong acid.
Heating such halides with alkali hydroxides and other strong bases often produces dihaloacetylenes, dibromoacetylene in this particular case. These
are hazardous and toxic, and though they decompose to other products, their mere presence as intermediates can be problematic.
Quote: Originally posted by PHILOU Zrealone  | Same process must be feasible with H2SO4/H2O refluxing because from CHCl=CCl2 and H2SO4, you end up with Cl-CH2-CO2H.
Br2CH-CHBr2 + 4H2O -H(+)-> O=CH-CH=O + 4 HBr |
That is an electrophilic addition of water on the double bond, which is not applicable for 1,1,2,2-tetrabromoethane.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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PHILOU Zrealone
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"That is an electrophilic addition of water on the double bond, which is not applicable for 1,1,2,2-tetrabromoethane."
Maybe for the first step...but after you stil have to loose two Cl atoms ...and 1,2,2-trichloroethanol is not far from 1,1,2,2-tetrachloroethane 
CHCl=CCl2 + H(+) --> CHCl(+)-CCl2-H
CHCl(+)-CHCl2 + HO(-) --> HOCHCl-CHCl2
I like your idea with Mg and Zn.
Would'nt it make some acetylen what may be a fire and gas explosion hazard?
True that I forgot  the possible side reaction of dibromoacetylen (pyrophoric and
explosive)).
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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Banjo
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| Quote: Originally posted by Nicodem | You can react your tetrabromoethane with magnesium or zinc to obtain anhydrous MgBr2 or ZnBr2 correspondingly.
Heating such halides with alkali hydroxides and other strong bases often produces dihaloacetylenes, dibromoacetylene in this particular case.
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Thanks Nicodem - anhydrous MgBr2 and ZnBr2 - that's very convenient. But even more thanks for the dibromoacetylene info. I UTFSE to find out more, and
from the documents I could access, I get the feeling that dibromoacetylene is not formed unless an alcohol is present in the reaction. All
the refs I found require an alcohol for this monster to appear, while only some refs require a strong base (too). Alcohol and zinc dust alone
apparently converts tetrabromoethane into dibromoacetylene - at least according to one TBE purification procedure:
Treatise on general and industrial organic chemistry - vol 1, page 22:
"...It is obtained pure by treatment with alcohol and zinc dust, which converts it into dibromoacetylene; the latter is then purified by distillation
and transformed into the tetrabromo-compound by means of bromine".
And, just like you said - tetrabromoethane, zinc and an ether yields pure anhydrous ZnBr2 - I just dried and bottled ~2g of white crystals this
morning, after a 12 hour reflux in THF. I am glad I did not use any alcohol in this reaction - apparently there may be some truth to the opinion that
misuse of alcohol can get you hurt (or set your garage on fire) :-)
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unionised
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Mixtures of halogenated hydrocarbons and reactive metals like Zn or Mg can explode.
You wan to be very careful if you do this.
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Nicodem
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| Quote: Originally posted by Banjo | | I UTFSE to find out more, and from the documents I could access, I get the feeling that dibromoacetylene is not formed unless an alcohol is
present in the reaction. All the refs I found require an alcohol for this monster to appear, while only some refs require a strong base (too).
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The alcohol only has the role of solvent for both reactants. It has no mysterious role. Dihaloacetylenes form by E2 elimination and obviously this is
not going to happen at a useful rate unless a proper solvent is used and for obvious reasons alcohols are the most proper solvents for this reaction.
| Quote: | | Alcohol and zinc dust alone apparently converts tetrabromoethane into dibromoacetylene - at least according to one TBE purification procedure:
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That is an obvious error. Treating tetrabromoethane with zinc in ethanol would have to give 1,2-dibromoethene (dibromoethylene), as
it does with other similar vicinal dibromides. Dibromoacetylene is not something you would use as a purification intermediate and most certainly not
something anyone would try to distil. Furthermore, tetrabromoethane can not be formed from dibromoacetylene by its reaction with Br2 as claimed in
that source (where would the two hydrogen atoms come from?).
| Quote: | | And, just like you said - tetrabromoethane, zinc and an ether yields pure anhydrous ZnBr2 - I just dried and bottled ~2g of white crystals this
morning, after a 12 hour reflux in THF. |
Well, good for you, but be a nice guy and provide the experimental! Other members might benefit from it.
Edit: Wrote 1,2-dibromoethane instead -ethene. Some err by unsaturation, I err by saturation.
Thanks for the info bellow.
[Edited on 25/6/2011 by Nicodem]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Banjo
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Thank you for taking the time to set me straight, and to name the mechanisms involved, Nicodem
Now I have something to read up on before experimenting further. Being a rank amateur, and not really having the background to know what to look for,
I am easily mislead to draw false conclusions from false claims, like the purification procedure referenced.
I did not have the time to include details on the preparation this morning - but here it is:
Preparation of anhydrous ZnBr2 was taken from an article by Moncef Bellassoued, Jerome Grugier, Nathalie Lensen on "Direct synthesis of
unsaturated [beta]-amino acids". Journal of Organometallic Chemistry, Volume 662, Issues 1-2, 15 November 2002, Pages 172-177
| Code: | Anhydrous ZnBr2 was prepared by heating ground zinc (3.6 g, 55 mmol) and 1,2-dibromoethane (9.5 g, 50 mmol) in tetrahydrofuran (THF, 100 ml) for 16 h at a t reflux. The resulting solution is allowed to cool slowly to RT. The remaining excess zinc was removed by filtration. Complete evaporation of solvent under high vacuum gave pure ZnBr2 as a white powder. |
I scaled this down by a factor of 5, and used 5mmol 1,1,2,2-tetrabromoethane instead of 10 mmol 1,2-dibromoethane, as the former has twice the number
of Br atoms available. The zinc used was taken from the case of a zinc-carbon battery, and cut into very small pieces. The reaction vessel used was a
boiling tube fitted with a home made "reflux condenser" (a 16mm test tube with the bottom cut off, and with a folded glass pipette hung down the
middle, through which cold water was circulated). The reaction was heated to reflux (about 70C) in a fume hood for 12h. Excess zinc was easily
filtered out and the filter was washed with 20ml THF. The THF was evaporated by heating to about 130C under a heating lamp for 15 minutes. The yield
was 1.8g in the form of translucent white crystals.
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