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?DraconicAcid - 3-2-2021 at 19:38
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.UC235 - 3-2-2021 at 20:14
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.CharlieA - 4-2-2021 at 14:47
I would look for a TLC method to follow the course of the reaction.ninhydric1 - 4-2-2021 at 22:10
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.Texium - 5-2-2021 at 09:16
I would look for a TLC method to follow the course of the reaction.
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.Triflic Acid - 5-2-2021 at 10:53
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.Triflic Acid - 5-2-2021 at 11:05
Or, even better, I could just track the amount of hydrogen consumed.Texium - 5-2-2021 at 19:11
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)Maurice VD 37 - 6-2-2021 at 05:22
Instead of TLC, I would use gas chromatography (GC)Texium - 6-2-2021 at 16:14
Instead of TLC, I would use gas chromatography (GC)
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.DraconicAcid - 6-2-2021 at 17:48
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.Triflic Acid - 6-2-2021 at 22:29
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 .zed - 8-2-2021 at 23:14
Mmmm. Monitor HBr formation. Sometimes hydrogenations of that type, utilize Pyridine to scavenge HBr/HCl.
[Edited on 9-2-2021 by zed]clearly_not_atara - 9-2-2021 at 07:19
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).Triflic Acid - 9-2-2021 at 22:17
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?Texium - 10-2-2021 at 08:10
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.zed - 11-2-2021 at 04:56
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]AvBaeyer - 11-2-2021 at 19:14
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.
AvBzed - 12-2-2021 at 04:29
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.
.