Stibnut - 21-7-2016 at 16:24
I've started playing around with bromine. I've made weak bromine water solutions but I wanted something stronger, to see if I could make an isolatable
amount of 1,2-bromocyclohexane from a incoming bottle of 50 mL cyclohexene that is on its way. So I decided to really overdo it on acid and H2O2.
In a 100 mL media bottle with a blue cap, I dissolved 20 g of NaBr into 30 mL water, then added 20 mL of H3PO4 to acidify it. Then I added 15 mL of
17.5% H2O2. Sure enough, bromine started forming quite rapidly, and I closed the bottle. However, bromine vapor started leaking through the cap
slowly, so I took everything outside with some sodium thiosulfate at the ready.
It fumed fairly heavily when I opened the bottle, while a few beads of bromine were visible at the bottom of the deep red solution. I added 20 mL of
chloroform to serve as a dense organic solvent for the bromine, which caused it to fizz up and overflow with bromine solution for a bit, then settle
back down into roughly what I wanted: much less bromine dissolved in the water layer, and a very concentrated bromine solution at the bottom. After
closing it, fewer fumes leak out of the bottle now.
So the question is: how best do I contain this for storage? I'm going to be gone for a few days before I can come back and do the cyclohexene
experiment, and I don't want any surprises. What I'm doing right now is that I have the bromine bottle inside a cheap, wide-mouth 1-gallon glass jar
from Walmart. I've poured a solution containing about 60 g of sodium thiosulfate into the glass jars so that it forms a ~1 cm deep puddle at the base
of the bromine bottle, with the idea that since bromine is denser than air, any fumes will sink and hit the thiosulfate before they hit the lid of the
jar. The lid of the bottle is still tightly closed, and the large jar's lid is closed as well.
Is this a reasonably safe way to store my concentrated bromine solution? And would it be better if I stored it at a lower temperature like in the
refrigerator, or is room temperature okay? It seems like bromine eats through pretty much everything given enough time, and I want to make sure this
is will be safe at least in the medium term.
subsecret - 21-7-2016 at 20:33
Bromine solutions may be convenient in some cases, but I really prefer storing bromine by itself. It's easier to measure how much bromine you need,
and you can just store it under sulfuric acid. In your case, I'd recommend storing the solution in a fridge (freezer if possible), and even under
sulfuric acid on top of that.
It sounds to me like you've chosen a reasonable way to store your bromine, but I'd recommend having some way for the large jar to vent just a little.
Maybe a 1mm hole with a cotton ball taped over it? Just keep it as cold as you can. Of course, with water in the container, you don't want to crack it
by freezing.
Praxichys - 22-7-2016 at 06:11
For what it's worth, I isolate the pure bromine and keep it in a "boston round" type glass bottle in the back of the freezer. It freezes solid and has
negligible vapor pressure at this temperature, and an ordinary plastic cap on the bottle has kept it contained for years. Since it melts at -7C, a
short dip in a warm water bath easily melts enough so that some can be taken up by pipette or poured out for use.
Fleaker - 22-7-2016 at 08:02
Make sure it is plastic wrapped bottle when storing. I had 2 L bromine stored that way with that weird orange bakelite cap with PTFE insert. Wrapped
the outside in parafilm and it was fine for years.
Stibnut - 22-7-2016 at 09:30
Thanks for the answers. From what I can tell it sounds like I'll be fine just sticking it in the fridge, at least as a short-term solution until I get
back. I added some more thiosulfate to the jar to bring it up to ~90 g, for good measure. It does appear the media bottle lid I'm using is not
PTFE-lined - I think it's just PP, hence the leakage. But even if the lid were to fail completely, there's more than enough thiosulfate to reduce all
the bromine I have back to bromide. When I opened the large jar this morning, there was no discernible bromine smell unless I picked up the media
bottle and smelled near the cap. Otherwise the dominant smells were just sulfur and some chloroform that I had spilled in as I added it to the bromine
solution.
I figured it would be easier and a bit safer to work with bromine if it is in a solution with chloroform than just by itself, but obviously I'm
sacrificing the ability to know how much bromine I am adding (besides knowing it can't be more than the Br- in the original solution). It might be
better to make the pure stuff and keep it under acid or in an all-PTFE bottle, in the freezer to keep it solid.
How do you guys get your pure bromine? Do you just distill it out of a bromine solution produced by bromide+acid+oxidizer, or is there some other way?
And how dangerous is bromine distillation performed outdoors? Also, can straight bromine just be added to straight cyclohexene or anything else with a
C=C double or triple bond to produce the brominated compounds, or is there more work involved? And finally, if I took some calcium carbide and dripped
water onto it to make an acetylene generator, and bubbled the acetylene through either straight bromine or a chloroform-bromine solution, would it
make tetrabromoethane or would there be side products too?
I'm ordering a few bottles with PTFE-lined lids, and also a 100 mL bottle made entirely of PTFE (that one bottle set me back $30, but probably worth
it if I'm going to work with this stuff). I assume that would work well, especially if I plastic wrapped it as well.
Praxichys - 22-7-2016 at 10:57
I made my bromine by oxidizing HBr. I added H2SO4 to a saturated solution of KBr which made a concentrated solution containing HBr. I then slowly
dripped 29% H2O2 into it to oxidize the HBr to Br2, which was continuously removed by distillation, first through a reflux condenser to minimize the
escape of water from the pot, through the still head, and down a graham condenser pumped with ice water. The bromine condensed into a flask which was
in an ice/salt bath to prevent Br2 evaporation.
After the bromine was collected, it was washed in a sep funnel with conc. H2SO4 to remove water, then separated into its own clean, dry glass storage
bottle.
I did this in my homemade fume hood, but there was almost no escape of Br2. I connected a vinyl tube to the vacuum port of the distillation setup and
ran it to the back of the hood so any excess vapors would vent safely outside. You should easily be able to do this outside if you are careful about
wind direction. Keep in mind that it only takes a little bit of bromine to stink up a lot of area, so be careful with neighbors, etc.
The main exposure route is not when you run the reaction but when you disassemble the apparatus when you are done. A spray bottle full of cleaning
ammonia is very helpful. Misting it liberally into the air and into the setup will cause large white clouds of solid ammonium bromide to form, but
will effectively neutralize the bromine into an odorless compound which is very soluble in water and easy to clean up.
Yes, adding bromine slowly and carefully to alkenes will cause them to undergo vicinal dibromination spontaneously. A common test for unsaturated
hydrocarbons is to shake them with bromine water. If the bromine water loses its orange color (because the alkene has been brominated, removing the
bromine from the water) the test is positive.
A word of caution though - this reaction is quite exothermic. You may need a solvent to prevent localized overheating and potentially a fire. You can
dilute bromine first with an alkane like cyclohexane or petroleum ether, or a chlorinated or brominated solvent like chloroform. Make the bromine
solution and add the alkene dropwise with stirring and temperature control. The dibromocyclohexane should be easy to separate if you choose a solvent
that boils at a low temperature.
If you choose to use an alkane for your solvent, watch that the reaction is not exposed to much UV light or they will be free-radical brominated.
You can make tetrabromoethane from acetylene. However, note that the rate of reaction slows with each bromination because of steric effects.
Converting acetylene to 1,2-dibromoethene will be the fastest, but it might take some time to fully brominate. I would use the same procedure as
outlined above - dissolve the bromine into a suitable solvent, then bubble the acetylene through.
You may want to wash your acetylene first as calcium phosphide is often an impurity in commercial calcium carbide. It reacts with water to form
phosphine and diphosphine which is very poisonous. It may react with bromine to form some other toxic compounds, so be careful to avoid the exit
vapors of this reaction. I'm not well-versed in the chemisry of phosphine, so I'm speculating here. Maybe someone else can shed some light on this?
You should probably run your acetylene through a drying tube first as well, as traces of water may become acidic with impurities in the bromine and
form HBr, which would give you side reactions of primarily bromoethene, and 1,2-dibromoethane, and several other less favorable combinations.
Hope that helps!
Fleaker - 22-7-2016 at 11:13
Best way I ever made bromine was from sodium bromide, sodium bromate and sulfuric acid. Yield is virtually quantitative.
byko3y - 22-7-2016 at 16:33
Bromine and iodine catalyticaly decompose hydrogen peroxide.
I prefer to store the bromine in a glass stoppered PTFE sealed bottle, which is placed in a dessicator. Small amount of bromine vapors in dessicator
are absorbed by your favorite bromine absorbent. I can definitely see some low concentration bromine vapors in the dessicator otherwise.
AJKOER - 25-7-2016 at 04:07
I would recommend using a lot of force (or just heat the metal lid) in making the jar difficult to open along with a warning label if the fridge is
accessible to anyone ignorant of the danger or a minor child. You can apply heat to the metal lip to re-open the jar.
The best jar would be one less likely to break if dropped.
MrHomeScientist - 25-7-2016 at 06:19
One method I really enjoyed is the electrolytic method - I have a video of it here: https://www.youtube.com/watch?v=NKjyM2AkZSY
The advantage of this is that you don't have to do distillation to get a reasonable amount of Br, so no gassing yourself when disassembling the
apparatus. The disadvantage is that it takes 26 hours to electrolyze! I got this method from woelen's site, so credit to him for the idea.
NEMO-Chemistry - 25-7-2016 at 07:27
What about small glass ampoules? 1=2ml size? at least they will be sealed.
Nucleophile - 18-8-2016 at 01:03
I recommend using glass ampoules as they will contain the bromine safely for a long time. However they do break so if you make them, store them in jar
of some reducing agent. If you want to use the bromine in smaller amounts and use it often I recommend using PTFE coated glass stopper reagent
bottles. I am not really sure about PTFE bottles for bromine, because I think the PTFE is porous IIRC?
My way of producing bromide is the method involving HCl and TCCA, which produce enogh cl2 in situ to react with the KBr. I get 92-95 percent yield
based on the potassium bromide.
The best way of disassembling a apparatus containing bromide is to submerge it (when it's cold) into bucket with some reducing agent (I use
metabisulfite). When the brown colour disapears, it's safe.
laserlisa - 18-8-2016 at 04:08
I found an amber glass bottle full of bromine in a chemical storage room once, it was probably at least 40 years old and the glass in the bottom of
the flask was soft like rubber, if I wanted to I could have easily poked a hole in the bottle with my finger.
Very interesting, I guess not even glass is safe if you are planning to store it for decades.
Metacelsus - 18-8-2016 at 05:03
Odd. Are you sure the bottle was glass, and not some kind of plastic?
woelen - 18-8-2016 at 05:29
I also think it is not glass. Maybe glass, covered with some plastic?
I have an ampoule of bromine, of fairly thin colorless glass and it is from the 1970's according to its label. The ampoule still is perfect and clean
and no bromine has leaked out over all these years. I like the (big) ampoule with its nice label and I keep it around as a weird curiosity. My bromine
needs are fulfilled by making it from NaBr and electrolysis and acidifying the resulting solution.
laserlisa - 18-8-2016 at 23:48
Yes it was a glass bottle for sure, it was stil in the original container from some chemical supplier.
In the same cabinet there were plenty of various acid chlorides and items such as POCl3 and SOCl2. But I dont think these would affect the glass would
they? Strange indeed.