Triflic Acid
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Any way to tell if there is bromobenzene in benzene?
I am working on something where I am trying to hydrogenated aryl bromides to get arenes. If I want to take a sample from the reaction flask
periodically, and test if there is still bromide left, how can I do that. Ideally something that would change color on contact with the bromide but
not with the aryl. Is there anything that might work?
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DraconicAcid
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The Beilstein test, I think it's called- dip a clean copper wire into it, and stick it into the flame of a Bunsen burner. Halides will turn the flame
green.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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UC235
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Quote: Originally posted by DraconicAcid  | | The Beilstein test, I think it's called- dip a clean copper wire into it, and stick it into the flame of a Bunsen burner. Halides will turn the flame
green. |
Yes, but if you're removing aryl bromides through hydrogenation, the bromide is going to end up in the reaction mix as HBr or a salt and will still
show up.
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CharlieA
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I would look for a TLC method to follow the course of the reaction.
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ninhydric1
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Benzene BP around 80 deg C, bromobenzene BP around 156 C. Gently evaporate benzene and see if some sort of oil with a "pleasant aromatic order"
(according to Wikipedia) is left over. A few mL of reaction mixture should be sufficient.
The philosophy of one century is the common sense of the next.
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Texium
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TLC against a sample of pure
bromobenzene would probably be the best option. It would be difficult to track the appearance of benzene since it would likely evaporate before you
can visualize the plate.
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Triflic Acid
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On another note, is it possible to track the amount of precipitate that forms, if I put triethylamine in the tolune, I would get precipitate of
triethylammonium bromide.
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Triflic Acid
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Or, even better, I could just track the amount of hydrogen consumed.
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Texium
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Or even better, you could use TLC
(Sorry for being snarky, it’s just that both of the ideas that you mention in your last two posts are not very viable)
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Maurice VD 37
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Instead of TLC, I would use gas chromatography (GC)
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Texium
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Well that's a useless suggestion for 99.9% of the people
on this board. Might as well just suggest he just buy a 20L drum of benzene.
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DraconicAcid
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I actually used to have plans for a dime-store gas chromatograph that used detrgent as a stationary phase, natual gas as a carrier, and a copper wire
in the flame to act as a detector or halogenated organic compounds.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Triflic Acid
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I think that at this point, I'm just going to say to hell with it and run the reaction until I don't see my balloon of hydrogen getting smaller. Its
dirty, but faster than creating a GC system  .
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zed
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Mmmm. Monitor HBr formation. Sometimes hydrogenations of that type, utilize Pyridine to scavenge HBr/HCl.
Reduction can be performed with Mg. I'll check.
Alternate reduction. http://www.orgsyn.org/Content/pdfs/procedures/CV5P0998.pdf
[Edited on 9-2-2021 by zed]
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clearly_not_atara
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Hey, as long as we're talking impractical, how about bombarding the sample with high-energy electrons and trying to measure a characteristic X-ray at
the K-shell energy of bromine? 
But seriously, the flame test (Beilstein?) should work fine if you clean out the HBr first. A bicarbonate solution with agitation should do it. Or, as
zed mentions, reaction with Mg (might destroy bromobenzene, but you want that anyway).
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Triflic Acid
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All of these ideas fall short in one small place, I can't open the reaction flask during the reaction. It'll be filled with hydrogen and Pd/C, and
opening it to take a sample would mean pulling a vacuum on the flask, then allowing the air back in slowly, taking the sample, pulling a vacuum, then
filling back up with hydrogen. @zed, what were you saying about using pyridine to scavenge HBr?
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Texium
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If you use a 2-necked flask, you can cover one neck with a rubber septum. If you make some really thin capillaries (you can do this by heating
disposable glass pipettes with a propane torch until they’re soft and stretching them out) you can insert one through a needle (18-16 gauge) that
you stick into the septum. It’s a great way of obtaining a sample for TLC without opening your flask. Assuming you have a slight positive pressure
of hydrogen in the flask, there’s no way any air will get in through the tiny needle. I’ve used this technique on some very air sensitive
reactions (like stuff containing n-butyllithium) without issue.
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zed
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Ummm. Some Asian Methamphetamine Synthesis, used to convert Ephedrine's alcohol group to a Cl group, via Thionyl Chloride. This was followed by
Hydrogenation, in Pyridine. Stinky stuff. The HCL generated during hydrogenation, was scavenged by the Pyridine, to form Pyridine-HCl.
The exact rational for this approach is lost to me. I'd need a lot of motivation to mess with Thionyl Chloride.
But, anyway. Pyridine can suck up the HBr gas you are generating.
What is your solvent? What kind of an apparatus are you using?
Is it possible to run your hydrogenation in a bi-phasic system?
If a water phase were present, the HBr might be sequestered in it
This would allow you to remove samples via needle, and then titrate them for HBr to determine the progress of the hydrogenation.
I guess what I'm getting at, in my own obtuse way; is that you can monitor the progress of your reaction, by the generation of your side product.
If the composition of your starting material is known, and the amount is known, then the amount of bromine present is known. And, you can monitor the
progress of your reaction, by titrating the HBr generated.
Here high tech is not your friend. A Parr Shaker or Pressure Reactor, might or might not allow sampling.
A stationary flask, powered by a stir-bar, gifted with a rubber septum, is what you need.
DUH! I'm so dumb. Dr. Brown already developed the technique you need. Utilizes Sodium Borohydride solution, in a self regulating system.
Borohydride drips into an acid solution, generating H2..... The H2 is stirred into the hydrogenation flask. When the reaction is done, Hydrogen
pressure ceases falling, and backpressure stops the flow of Borohydride Solution.
The Hydrogenation stops on its own accord, and you get a very good idea, exactly how much hydrogen was
reacted. By the volume of the Borohydride consumed.
Might help you. Maybe someone here has the relevant references?
Mmmmm. Hulk not talk good, sometimes.
[Edited on 11-2-2021 by zed]
[Edited on 12-2-2021 by zed]
[Edited on 12-2-2021 by zed]
[Edited on 12-2-2021 by zed]
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AvBaeyer
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A very complete explanation with diagrams of the Brown apparatus can be found in:
Sandler and Karo, "Organic Functional Group Preparations," Volume 1, 2nd Edition, pages 4ff. [There are 3 volumes in this series and really useful to
have handy on your bookshelf. Used copies can be found at very affordable prices.]
also,
Brown and Brown, J. Am. Chem. Soc., 84, 2829 (1962)
This is an apparatus worth building if you do a fair number of atmospheric pressure hydrogenations. It does require a supply of sodium borohydride
which can be a problem.
AvB
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zed
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Yup! Brown's work was beautiful. Sadly, NaBH4 prices, never really came down a lot. It remains kind of expensive and hard to get. Unless of
course, you buy a large amount.
And, Dr. Brown's elegant and once available apparatus... I have never seen one available for sale, used.
You might have to build your own.
Though if you are patient, and you are working with small reactions, manual addition of Borohydride solution
via syringe, with a balloon to moderate pressure, along with
good sealing and clamping... Would probably be effective.
You are thinking atmospheric pressure is not good enough? Well, with Dr. Brown's catalyst... It's good enough.
Though perhaps a device could now be constructed, that utilizes a hydrogen generator, to produce the required hydrogen. Generators being available
used now, at reasonable prices. (well.... $500.00 or so).
And, H2 consumed might be monitored by water consumed by the hydrogen generator.
I would still utilize Dr. Brown's Catalyst though.
Thank you for the references, I'll check them out.
Yup! https://www.amazon.com/Organic-Functional-Group-Preparations...
Looks like a nice book. Amazon lets you look inside, and see the apparatus.
Enough of Brown's paper to be useful.....
https://pubs.acs.org/doi/abs/10.1021/ja00873a039
OK, I checked around. No really inexpensive volumes right now, at least not in the desirable 2nd edition.
But, I'll keep my eyes peeled. Ebay can be friendly with book prices. Leastwise, here in the USA.
[Edited on 12-2-2021 by zed]
[Edited on 12-2-2021 by zed]
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[Edited on 12-2-2021 by zed]
[Edited on 12-2-2021 by zed]
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AvBaeyer
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Zed,
For book availability, check on ABE Books. It's part of Amazon but far more extensive in used books and easier to search.
Now, for the reduction of aryl halides, check out Organic Syntheses, Coll Vol 5, page 998.
This procedure uses Mg in isopropanol.
AvB
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zed
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Yup! I left a link to that, earlier on. Thanks.
Perhaps conditions are too extreme for the OP's substrate?
Or, his experiment has an alternate objective.
Checked on those books again. I'm familiar with ABE.
The set seems to be more common in Europe.
Oh well, I suppose I can wait a bit.
[Edited on 14-2-2021 by zed]
[Edited on 14-2-2021 by zed]
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zed
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Thanks for the tip on the books. Found a set. Lots of good experimental details.
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