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morsagh
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glyoxal synthesis
Hi there, i am going to synthetise glycouril by reaction of urea, glyoxal, catalysed by P4O10 in aqeous solution (ref.: http://www.tandfonline.com/doi/full/10.1080/17518253.2012.71...) , and then polymerize to (6)cucurbituril with formaldehyde in conc. H2SO4. The
only problem is i do not have any glyoxal so i want to prepare it by reaction of ethanol with nitric acid (maybe copper catalyst)... but i can´t find
any reference to this method, does it really work, what are the yelds?
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Boffis
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Have you looked the couple of existing threads on glyoxal synthesis? Please use the search engine, you might find your answer.
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morsagh
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I found that thread but nothing usable. I would like to try HNO3 ethanol method but there aren´t written yelds etc..
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WGTR
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This has been a frustration of mine recently (in a fun sort of way). Making glyoxal isn't too difficult, I think. It's the workup and
characterization that's difficult. Glyoxal polymerizes in traces of water, and isn't volatile up to its decomposition point unless first being
dehydrated with phosphorus pentoxide. As long as glyoxal is kept in the gas phase, without being allowed to re-condense, it doesn't re-polymerize
(I'll have to look for the patent that contains that information, I haven't tried that personally). It's slightly sensitive to "long" boiling in
aqueous solution, so evaporation during work-up is usually done under reduced pressure, slightly above room temperature. Alcohol oxidation with
nitric acid produces a mix of products, including glyoxal, glycolic acid, glyoxylic acid, oxalic acid, and acetic acid. Out of those, both glyoxal
and glyoxylic acid are aldehydes, and give positive reactions to Tollen's reagent.
To answer your question about references, you can check on Google Books for "Experimental Chemistry for Junior Students, part 4", by J. Emerson
Reynolds, published in 1887, pages 156-159. It explains the experimental procedure, as well as how to separate the products from one another.
Keep in mind that air conditioning didn't exist when the book was published, so the temperature of the room was merely recorded for reference. It
might be possible to do the reaction faster, so long as the temperature can be kept low. A microchannel reactor design might be beneficial in this
case. Also, the suggested calcium carbonate won't cause a Cannizzaro reaction with glyoxal, although calcium hydroxide will. Don't neutralize the
reaction mix with something like sodium carbonate, because all of your glyoxal will end up as glycolate. Use ethanol to separate out the calcium
salts, don't substitute with methanol or acetone. I tried acetone, and ended up with some strange, unidentified goop. The Tollen's test on that
resulting mess was negative.
Getting rid of the residual nitric acid is a challenge, as it seems to stop reacting below a certain concentration. Evaporation merely concentrates
it up to its constant boiling point, around 68% concentration. It's common to see what looks like a finished reaction, upon evaporation at warm
temperatures to begin erupting suddenly in a furious boiling of orange fumes. That's one thing that I verified. Copper salts have been tried as a
catalyst to reduce the nitric acid content to zero, but my volcanic eruptions have still happened while using copper salts in the reaction mixture. I
don't know what to say about that, other than the catalytic method isn't apparently straightforward. It would seem logical to neutralize the excess
nitric acid with calcium carbonate, but calcium nitrate is very soluble in all the generic alcohols and in acetone, so it will carry over with the
glyoxal.
One idea to remove excess nitric acid, would be to first neutralize the reaction with calcium carbonate. After extraction with alcohol and then
evaporation of the alcohol, proceed to an electro-dialysis stage in water. If calcium nitrate ionizes in alcohol (or with minimal water), then you
can probably do this purification in pure or concentrated ethanol. The concept is simple. The anode and cathode compartments are packed with
activated carbon, or some other electrically conductive carbon with high surface area (high surface area is important). The electrodes could be
graphite. You're basically making an electric double-layer capacitor (a supercapacitor). This is intentionally a non-faradaic reaction. You don't
want charges crossing the electrode/electrolyte interface, so the cell potential needs to be kept low, below about 0.5 volts. The positive and
negative ions will adsorb on the surface of their respective electrodes, but I "DON"T THINK" the glyoxal should have any particular charge, so it can
be flushed from the various compartments as soon as the cell current drops off close to zero.
[Edited on 4-23-2016 by WGTR]
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morsagh
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So what about separation by bisulfite after neutralization by CaCO3?
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WGTR
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It would have to be barium bisulfite instead of the sodium salt, if you intend to break the adduct later and precipitate the leftover salts.
Obviously, you can't just simply distil off the glyoxal like you can with formaldehyde or acetaldehyde. Dilute sulfuric acid can be used to break the
adduct and precipitate the barium, but it has to be done carefully. Excess acid will be painful to remove. Historically, lead acetate was used to
remove traces of sulfate, and then, I think, dilute hydrogen sulfide was used to remove traces of lead.
Glyoxylic acid can also form a bisulfite adduct, so make sure that the reaction is neutralized fully before extracting with ethanol.
Dr. H. Debus wrote an article using some of these methods, and he was an early researcher, if not the discoverer of glyoxal. You can find it on
Google Books, as The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, Vol. XIII, 4th series, published 1857. pages 39-49.
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Nicodem
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Threads Merged
24-4-2016 at 01:09 |
morsagh
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I think that in synthesis of curcubituril sodium salts won´t interfere... Or yes?
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Quaff
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heat sucrose in conc. h3po4 to 100c using oil bath for control. Employ a sweep gas, N2 or CO2, and what distills off is predominantly a mixture of
glyoxal and methylglyoxal. these are good reducing agents in precious metal recovery
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