MrHomeScientist
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"Large" Scale Bromoform Synthesis
While perusing topaz mining sites for an upcoming rockhounding adventure, I read a very interesting tidbit. The site claims that topaz can be
distinguished from quartz by the use of bromoform - topaz will sink in it but quartz will float. That sounds like an interesting
experiment to try, so I'd like to make some bromoform. The videos I've seen end up making tiny amounts, though, and to see this effect I'd need
several tens of milliliters. So I need to scale up. Disclaimer: I've never done any haloform reaction before, so this is new ground for me.
From the videos I've seen it seems the standard 'recipe' is:
1) Dissolve ~8g NaBr into 100mL distilled water.
2) Add 4mL of acetone to the solution.
3) Add 150mL household bleach. With stirring, the bromoform is quickly produced and is allowed to settle out as a dense liquid.
The bleach oxidizes the bromide to bromate, and then the overall reaction (I believe) would be:
CH<sub>3</sub>-CO-CH<sub>3</sub> + 3NaBrO → CH<sub>3</sub>COONa + CHBr<sub>3</sub> + 2NaOH
Is this suitable for scaling up to larger quantities? I seem to remember making chloroform requires careful temperature control to prevent runaway;
would that be required here?
I also have elemental bromine available if that affords an easier path. Appreciate any advice from haloform pros!
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Cryolite.
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Temperature control is definitely required with haloforms: the standard haloform reaction (using bleach to make chloroform) usually requires putting
the bottle of bleach in the freezer for a few hours to cool, and by the end of the reaction the bottle reaches 30 C or so anyways. However, I believe
this is needed primarily to prevent the unstable haloform product from evaporating away or reacting with the hydroxide formed in the reaction, rather
than any issue with quality of product collected.
To produce bromoform, you will need to prepare some hypobromite solution. This is slightly complicated by the fact that hypobromite (like
hypochlorite) disproportionates into bromate and bromide, which will hurt your yield. My advice would be to take a slight excess of sodium hydroxide,
dissolve in some water, and cool in the freezer to -5 C. Once this is reached, add the correct molar amount of bromine SLOWLY with strong stirring,
ensuring the temperature remains below 5 C. This should now be a solution of sodium hypobromite. Once this is obtained, due to the concentration
adding all of the methyl ketone at once will induce a thermal runaway. To prevent this, I would place the hypobromite soluton in an ice bath and
slowly drip in acetone, ensuring the temperature stays below 20 C to prevent evaporation of the product. Once the ketone has been added, allow the
product to settle for an hour or so and decant (do a DCM extraction of the aqueous phase if you want to get as much out as possible). Wash with water,
distill for purity, and you should have pure bromoform.
For a literature example of such a reaction, see https://www.google.com/patents/CN1417190A?cl=en .
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byko3y
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Above approximately 40 C hypochlorite converts into chlorate, and the latter is either inert or burns the shit out of organic matter.
I see not problem with avaporation, because acetone is the main compound to evaporate.
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woelen
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Using bromine for making bromoform indeed is possible, but for a home chemist, the use of simple NaBr and bleach is much more interesting. Bromine is
quite valuable, while NaBr hardly is.
I would use a slight excess amount of NaBr, added to bleach and dissolved in it. For each mole of hypochlorite, add a little more than a mole of NaBr.
This gives a hypobromite solution, with besides the hypobromite a lot of chloride and some bromide.
To this solution, SLOWLY, while stirring drip acetone. It is best to keep the solution cool. Putting the beaker in a larger beaker or bucket, filled
with crushed ice is a good idea.
Do not add excess acetone. Excess acetone will react with bromoform.
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AJKOER
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For large scale, first test out as an alternative strategy a halide exchange reaction assuming you can acquire large amounts of CHCl3 and the
appropriate solvent (THF?):
CHCl3 + 3 NaBr = CHBr3 + 3 NaCl
A general reference on like reactions: https://books.google.com/books?id=JlfuCAAAQBAJ&pg=PA120&...
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MrHomeScientist
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Thanks for the replies so far. I like your suggestions, woelen.
My girlfriend is somewhat concerned over toxicity, knowing already a bit about chloroform. How does bromoform compare? Looking at the SDS it doesn't
seem that scary, and having such a dense vapor means it should flow away from nearby noses. Obviously safety precautions will be taken when
synthesizing it; gloves, goggles, and working outside for starters.
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byko3y
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There are much toxic substances than bromoform, like phosgene and HCN, but I would start making bromoform without some good protective measures only
when my life depends on it.
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Velzee
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Quote: Originally posted by MrHomeScientist  | Thanks for the replies so far. I like your suggestions, woelen.
My girlfriend is somewhat concerned over toxicity, knowing already a bit about chloroform. How does bromoform compare? Looking at the SDS it doesn't
seem that scary, and having such a dense vapor means it should flow away from nearby noses. Obviously safety precautions will be taken when
synthesizing it; gloves, goggles, and working outside for starters. |
A quick look at the Wiki suggests you are correct, sir:
| Quote: |
The substance may be hazardous to the environment, and special attention should be given to aquatic organisms. Its volatility and environmental
persistence makes bromoform's release, either as liquid or vapor, strongly inadvisable. Bromoform can be absorbed into the body by inhalation and
through the skin. The substance is irritating to the respiratory tract, the eyes, and the skin, and may cause effects on the central nervous system
and liver, resulting in impaired functions. It is soluble in about 800 parts water and is miscible with alcohol, benzene, chloroform, ether, petroleum
ether, acetone, and oils. Its LD50 is 7.2 mmol/kg in mice, or 1.8g/kg. Bromoform is a confirmed animal carcinogen; (ACGIH 2004). Carcinogen category:
3B; (DFG 2004).
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Check out the ScienceMadness Wiki: http://www.sciencemadness.org/smwiki/index.php/Main_Page
"All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."
—Arthur Schopenhauer
"¡Vivá Cristo Rey!"
—Saint José Sánchez del Río |
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woelen
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Please read this thread. This also applies to bromoform (and in fact, for nearly all chemicals):
http://www.sciencemadness.org/talk/viewthread.php?tid=68234
So, feel free to try to make some, use common sense, try to avoid excessive inhalation, but if you accidently get a whiff of it, don't worry.
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byko3y
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Formal Toxicity Summary for BROMOFORM
Inhalation of 24 ppm for 2 month leads to toxic effect. Inhalation/absorption of 1 g in a day should be avoided. A human inhales approx 0.5 m3/h of
air, odor threshold for bromoform is 1.3 ppm.
Without good ventilation and without gloves the 1 g absorbed can be easily reached, not leading to death but to headaches, change in a mood, change in
a blood content. Just having a good ventilation and gloves can be called good protective measures for this case.
Of course I have an experience of inhalation such a natsy stuff as phosphine, benzene in high cocentrations, tiethylamine, bromine, chlorine. I'm
still alive, but the impact on my health is perceptible.
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Cabalaba
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When I used a haloform reaction for chloroform, I had to distill and dry the chloroform.
When I attempted to distill Bromoform at about 150C, the reaction must have runaway, lost it all to black gunk. Any tricks to cleaning homemade
bromoform?
I dont really need it but wouldn't mind tinkering with it again.
[Edited on 19-8-2016 by Cabalaba]
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byko3y
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Throw a kilo of NaCl in the mixture, separate, distill. Or simply neutralize with thiosulfate and then distill of.
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Cabalaba
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Thanks, weekend project!
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Justin Blaise
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There is a prep for bromoform in Vogel's, I believe. I think they use Br2. If you use bleach, you may get a mixture of chloroform and bromoform.
Luckily, the two have boiling points almost 100 degrees apart, so distillation should be easy.
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nezza
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I have tried using bleach and the product has an intermediate desnity between bromoform & chloroform so I suspect it is a mixture of the two. I am
going to try hydrolysing some to check that it contains enough Bromoform to be worth fractional distillation.
If you're not part of the solution, you're part of the precipitate.
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battoussai114
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I wonder if it would work to crystallize the bromoform out... it's supposed to have a favorable melting point at around 10ºC
Batoussai.
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Stibnut
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I'm doing the same reaction today, using 3.5 L of household bleach with 8.25% NaClO (currently chilling in the freezer) and 527 g of NaBr, which is in
excess.
Here's my question. Is it possible to end up with CHClBr2 and CHCl2Br as well as bromoform and chloroform? If not, I'm just going to distill off the
chloroform in a hot water bath, dry the bromoform with NaCl and then MgSO4 or CaCl2, and call it good enough after measuring the density. But if
CHClBr2 and CHCl2Br can show up, then I might have to use a higher temperature. Come to think of it, it actually might be interesting to isolate those
as well if they're there.
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battoussai114
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| Quote: Originally posted by Stibnut | I'm doing the same reaction today, using 3.5 L of household bleach with 8.25% NaClO (currently chilling in the freezer) and 527 g of NaBr, which is in
excess.
Here's my question. Is it possible to end up with CHClBr2 and CHCl2Br as well as bromoform and chloroform? If not, I'm just going to distill off the
chloroform in a hot water bath, dry the bromoform with NaCl and then MgSO4 or CaCl2, and call it good enough after measuring the density. But if
CHClBr2 and CHCl2Br can show up, then I might have to use a higher temperature. Come to think of it, it actually might be interesting to isolate those
as well if they're there. |
Assuming you use excess NaBr, it should consume all the NaClO in the first step. So chlorine would only find it's way in by halide exchange. Which
again wouldn't be favored if there's excess Br- in the solution. Guess you should expect to see, at most, trace amounts of Cl/Br mixed halomethanes.
Batoussai.
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MrHomeScientist
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Stibnut, I'm interested to hear how your synth works out. I won't be going quite that large scale, but if you could post a write-up of the whole
process that would be great to see.
I went to the topaz mine this weekend and it was great fun. I don't think I found any topaz, but there are a few little rocks I think are still worth
testing. And if all else fails, I bought a piece from a local shop so the show must go on! 
Once work settles down in the next few weeks, I plan on moving forward with this experiment.
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Stibnut
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Sorry, forgot to follow up - RL got in the way.
The reaction was successful at producing bromoform, but at a yield of only 33.3%. I'm not sure what went wrong - the temperature never got above 40 C,
and I even used ~150 mL of DCM in an attempt to get the dissolved fraction of it by liquid-liquid extraction after pipetting up the bromoform that
fell out of solution. I distilled off the DCM and any chloroform using a boiling water bath, dried the product over MgSO4, and then dried further with
a copious amount of molecular sieves. The product has a density of only 2.72 g/mL as measured using a 5 mL volumetric flask, compared to 2.89 in
theory, making me suspect that something else with a high bp is in there too.
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woelen
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Most likely you do have some CHBr2Cl (and possibly CHBrCl2) in your end product. These almost certainly will lead to a somewhat lower density, while
at the same time their boiling points may be so close to that of CHBr3 that they are not easily separated by distillation.
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clearly_not_atara
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Would NaBR/H2O2 be more selective and produce less chlorodibromomethane and other chlorinated side products? Or is that a problem with acetone, even
at high pH?
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MrHomeScientist
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Avoiding chlorine atoms in your reactants will eliminate the possibility of chlorinated products. Bromine, sodium hydroxide, and acetone can be used
instead, which should result in a purer product:
| Quote: | A 1000 ml three-necked flask is fitted with a mechanical stirrer, a dropping funnel with stem reaching to almost the bottom of the flask, and another
separatory funnel but with a short stem. The flask is immersed in water maintained at 50°. To the flask, 30 ml of acetone and 150 ml of 20% sodium
carbonate solution are placed. 75 ml of bromine in the long-stemmed funnel and 800 ml of 10% sodium hydroxide solution in the other funnel are placed.
The bromine is slowly added dropwise while stirring into the alkaline mixture. Bromoform starts to separate out and as soon as the bromine is no
longer decolorized, the sodium hydroxide solution is slowly added from the second separatory funnel at such a rate that the mixture in the flask does
not become strongly alkaline. The correct rate of addition of the sodium hydroxide solution is attained when, on stopping the addition, the liquid
immediately assumes the red color of bromine. During the addition, the temperature should be maintained at 45-50° C throughout the experiment. When
all the bromine has been introduced, the addition of the sodium hydroxide is stopped. The heavy layer of bromoform is separated, washed with water,
and dried with anhydrous calcium chloride. The crude bromoform is purified by distillation, preferably in a current of inert gas, and the fraction
boiling at 148-149.5° C is collected yielding 68 grams of pure bromoform (m.p. 7-8° C). Pure bromoform is stabilized with of 4% of its weight of
ethyl alcohol or of a small quantity of diphenylamine.
A text book of practical organic chemistry, by A. I. Vogel, 299, 1974 |
Source: http://www.prepchem.com/synthesis-of-bromoform/
(copied before the annoying popup blocked the text. Take that, prepchem!)
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Maroboduus
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| Quote: Originally posted by Stibnut | Sorry, forgot to follow up - RL got in the way.
The reaction was successful at producing bromoform, but at a yield of only 33.3%. I'm not sure what went wrong - the temperature never got above 40 C,
and I even used ~150 mL of DCM in an attempt to get the dissolved fraction of it by liquid-liquid extraction after pipetting up the bromoform that
fell out of solution. I distilled off the DCM and any chloroform using a boiling water bath |
Is it possible the DCM did an SN2 reaction with the bromoform and displaced some of the Bromine? Or what about the possibility that some of the
bromoform got hydrolyzed somehow?
Oops. Not much of a driving force behind that reaction. But there's still the question of hydrolysis...
[Edited on 20-9-2016 by Maroboduus]
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