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Upsilon
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Synthesizing propylene carbonate
Disclaimer: I know next to nothing about organic chemistry. I am indulging in this experiment for the sole purpose of having propylene carbonate to
conduct more inorganic chemistry experiments. I am specifically interested in its use as a polar, aprotic solvent.
Doing research, I have found several scholarly articles about the ability to synthesize propylene carbonate from propylene glycol, urea, and a ZnO
catalyst - all relatively cheap and readily available. Methods from article to article vary, but the basic reaction is the same. While I know the
stoichiometric amounts of both chemicals I need, I really have no idea how or why this reaction happens. You can explain it in layman's terms if you
like, but I can't promise that I'd take terrible interest in it.
Essentially I need to know about all the nuances of this reaction and what factors I can adjust to get higher yield in less time. I don't know how
much catalyst to use, what temperature this should take place at, etc.
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gdflp
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It's not as easy as those papers make it seem, see here for a discussion about the synthesis. There are significant problems with the synthesis and some experimentation is needed to optimize it
and prevent tar formation.
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Upsilon
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I see. The problems he had, if I understand correctly, were from distillation to separate the PC from the PG? For my use of it as a solvent, would I
not necessarily need to separate the two? As long as I can drive the reaction to be dominantly PC, then it should still work as a solvent. The PG
still present would be inert for my purposes and not really affect anything other than the amount of the PC/PG mixture needed to dissolve a certain
quantity of substance.
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gdflp
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Quote: Originally posted by Upsilon  | | I see. The problems he had, if I understand correctly, were from distillation to separate the PC from the PG? For my use of it as a solvent, would I
not necessarily need to separate the two? As long as I can drive the reaction to be dominantly PC, then it should still work as a solvent. The PG
still present would be inert for my purposes and not really affect anything other than the amount of the PC/PG mixture needed to dissolve a certain
quantity of substance. |
Not if you want an aprotic solvent. Propylene glycol has a very similar pK<sub>a</sub> to water, and thus the mixture will not be
aprotic, rather it will act similar to water in this regard. They will need to be separated if you want to use this as a solvent for any strong
bases, or reactive metals, e.g. alkali metals.
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Upsilon
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Hm, I wouldn't have expected PG to be that protic. Looks like I've got some work to do then. Thanks for the info.
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byko3y
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gdflp, could you give us a source of your pka value for propylene carbonate? Because acidic solvents react with alkali metals, as
well as water does, so they would be not suitable for electrolysis.
Upsilon, the reaction is pretty simple, it can be compared to amide solvolysis, ammonia just needs to be driven away so the
equilibrium
Urea + Propylene glycol <=> propylene carbonate + 2NH3
will be shifted to the right.
I'm sure a lot of metal oxides could catalyze the reaction, ZnO is just more effective.
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gdflp
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I don't have a pK<sub>a</sub> for propylene carbonate, I was talking about the pK<sub>a</sub> of 1,2-propanediol which is 14.9.
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clearly_not_atara
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Why not make dioxane? The reaction is rather simple, ethylene glycol/H2SO4, and the product is quite versatile, not quite the same (less polar/lower
boiling) but certainly gives you more to work with than the standard water/etoh/chcl2/toluene palette everyone uses. Dioxane can form peroxides in
storage, though, so be careful.
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gdflp
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Dioxane is not polar, and thus it is incapable of dissolving many inorganic salts which dissolve readily in propylene carbonate. I believe that this
is what the OP was interested in, hence mentioning the polar aprotic solvent.
DCM and dioxane are quite similar in terms of solvent properties, the only major difference is the boiling point.
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Oscilllator
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| Quote: Originally posted by Upsilon | | Hm, I wouldn't have expected PG to be that protic. Looks like I've got some work to do then. Thanks for the info. |
Upsilon unless you have a fractionating column or a novel idea for a separation method it may not be worth your while to try this. IIRC the consensus
reached on the other thread was that vacuum fractionation would be the best way to achieve separation. When I attempted this I think I may have pushed
the distillation a little hard, so I would recommend adjusting the distillation speed to be quite slow (0.5 drops/sec max) so as to get better
separation.
It's also worth pointing out the importance of the preparation of the catayst. I think one reason I may not have achieved full conversion was improper
preparation of the catalyst. Note that UnintentionalChaos' prep did not use a calcined Zinc oxide catalyst and perhaps as a result of this he did not
isolate any propylene carbonate.
If you can, I would recommend following the patents directions exactly (heating to 500°C for 4 hours) rather than heating the oxide to ~1000° like I
did or to heating at 200° like UnintentionalChaos did. Certainly there is a big difference between these two methods, as my ZnO did not dissolve in
the reaction mixture whereas UnintentionalChaos' did.
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Upsilon
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Hello again all, apologies for neglecting this thread but I haven't actually had the time to try this out yet (though I will soon). I just had a
thought - above it was mentioned that PG will be reactive towards alkali metals. What exactly is the reaction going on when this happens? Because if
it forms a solid product (a salt) then, after some simple distillation to drive off a lot of the PG, you could run an NaCl electrolysis in the PC/PG
mixture. The sodium metal will react with any remaining PG, and afterwards it would be extremely simple to separate the PC from the salt formed. This
idea has gotten me very excited, who'll be the first to burst my bubble?? 
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byko3y
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Can't you just distill the propylene glycol off?
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Upsilon
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Based on what has been posted earlier this is probably much harder than it sounds. Apparently a fractioning column is needed, and that the
distillation foams a lot. Not to mention that introducing this high heat can form tar. This electrolysis method would be a lot cleaner, if I can get
any confirmation. I'm still about to try it anyway.
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gdflp
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| Quote: Originally posted by Upsilon | | Hello again all, apologies for neglecting this thread but I haven't actually had the time to try this out yet (though I will soon). I just had a
thought - above it was mentioned that PG will be reactive towards alkali metals. What exactly is the reaction going on when this happens? Because if
it forms a solid product (a salt) then, after some simple distillation to drive off a lot of the PG, you could run an NaCl electrolysis in the PC/PG
mixture. The sodium metal will react with any remaining PG, and afterwards it would be extremely simple to separate the PC from the salt formed. This
idea has gotten me very excited, who'll be the first to burst my bubble??
|
Propylene glycol is an alcohol, and sodium metal is basic. The sodium metal will replace both acidic
protons with sodium ions, releasing hydrogen gas and forming disodium propane-1,2-diolate. If the mixture is rich in propylene glycol, you would
likely form a slush of the alkoxide in propylene carbonate, which would be difficult to filter and might need to be processed several times to prevent
the solution from becoming too full of crystals. You may also encounter troubles with chlorination of the alcohol from the produced chlorine. It's
worth a shot, but separating the glycol with a couple vac distillations is likely going to be less troublesome.
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BromicAcid
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Here is a thread where I was working toward ethylene carbonate, same procedure, different diol.
http://www.sciencemadness.org/talk/viewthread.php?tid=2948
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Upsilon
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| Quote: Originally posted by gdflp | | Propylene glycol is an alcohol, and sodium metal is basic. The sodium metal will replace both acidic protons with sodium ions, releasing hydrogen gas
and forming disodium propane-1,2-diolate. If the mixture is rich in propylene glycol, you would likely form a slush of the alkoxide in propylene
carbonate, which would be difficult to filter and might need to be processed several times to prevent the solution from becoming too full of crystals.
You may also encounter troubles with chlorination of the alcohol from the produced chlorine. It's worth a shot, but separating the glycol with a
couple vac distillations is likely going to be less troublesome. |
I'm aware of the possible effect of chlorine so that's why I'd separate it into 2 cells with a salt bridge. I'd use water as the solvent at the anode
so not to waste any valuable PC. It should be pretty easy to tell once all of the PG is gone, since visible sodium metal will theoretically start
forming.
As for having too high saturation of PG, I don't know. I used the 1.5 PG:urea ratio and 1.5g ZnO catalyst as outlined in the papers. I didn't really
calcinate the ZnO like above mentioned, since I'm only doing a test run currently. I put the ZnO in a crucible and held a butane torch to it for
several minutes.
I am in the middle of the reaction currently and my ZnO has dissolved which I don't assume is good. The mixture has also turned yellow, which I also
assume is not good. It is still bubbling very vigorously though.
UPDATE: The yellowness has indeed turned into the tar described in the other thread.
Note also that I'm not trying to distill it as the reaction progresses. I am simply heating the mixture in open air.
Well, the contents all turned into yellow mass with no liquid left. Pretty safe to say it was a failure.
[Edited on 17-10-2015 by Upsilon]
[Edited on 17-10-2015 by Upsilon]
[Edited on 17-10-2015 by Upsilon]
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clearly_not_atara
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What if you react carbon dioxide with the mono-alkoxide in some inert solvent (acetone?) and isolate the monocarbonate salt, then cyclize that
somehow?
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halogen
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The formaldehyde acetal of glycol should be vulnerable to radical oxidation. There's an effect that activates to radicals the carbon next to ether or
alcohol. Something about a lone pair. I don't know. But you might look up the oxidation of the acetal, called dioxolane, it is a solvent, with
something with lots of oxygen, like permanganate.
Oh.
"Formation of Cyclic Carbonates. In the absence of water, chlorohydrins such as 2-chloroethanol and l-chloro-2-propanol react with an alkah carbonate
or bicarbonate to produce cycHc carbonates such as ethylene carbonate [96-49-1] and propylene carbonate [108-32-7] ia yields of up to 80% [c.73]"
Oh; again.
https://books.google.com/books?id=jsMFRVsqtmEC&pg=PA82&a...
These two from the first page of a google search. Told ya so. 
[Edited on 18-10-2015 by halogen]
F. de Lalande and M. Prud'homme showed that a mixture of boric oxide and sodium chloride is decomposed in a stream of dry air or oxygen at a red heat
with the evolution of chlorine.
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byko3y
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halogen, Chlorohydrins are toxic and carcinogenic.
Upsilon, what are you doing on the chemistry forum without distillation apparatus with fractioning column? You can't purify any
solvent you use, including the propylene carbonate that might have contained some amount of acetic acid or other crap. AFAIK ZnO should not become
dissolved during the reaction. Contamination of urea might also be a reason for the problem. | Quote: | | I am simply heating the mixture in open air. |
You should blow some inert gas through the reaction in this
case, because otherwise amides will be formed instead of the propylene carbonate. In fact it's much easier to apply a slight vacuum (300 mmHg;
propanediol-1,2 reflux).
Also, you haven't mentioned what procedure exactly were you using. I'm talking about
Synthesis of propylene carbonate from urea and propylene glycol - doi:10.1016/S0167-2991(04)80318-5
[Edited on 18-10-2015 by byko3y]
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Upsilon
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Did I ever say I didn't have one? But, true, I do not have a vacuum distillation setup. That's exactly why I asked about using electrolysis to purify
the PC, and theoretically it is possible.
| Quote: Originally posted by byko3y |
You can't purify any solvent you use, including the propylene carbonate that might have contained some amount of acetic acid or other crap.
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I'm not sure what you mean? The only contaminants I can forsee are the adverse products of the reaction that I have already witnessed.
| Quote: Originally posted by byko3y |
You should blow some inert gas through the reaction in this case, because otherwise amides will be formed instead of the propylene carbonate. In fact
it's much easier to apply a slight vacuum (300 mmHg; propanediol-1,2 reflux).
|
Again, this organic stuff is over my head. However if inert gas is involved, then the whole prospect of propylene carbonate is useless to me; I asked
about it because I was interested in electrolyzing alkali metal salts in it, as opposed to setting up an inert atmosphere to electrolyze the molten
salt in. I suppose there could still be some benefits, but requiring an inert atmosphere for the reaction is a huge blow to its usefulness to me.
Though the URL is not working for me, that's the process I'm using - urea + propylene glycol + ZnO catalyst -> propylene carbonate + ammonia
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clearly_not_atara
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Why not just go for acetonitrile instead? It's leagues easier to make than either propylene carbonate or DMF, never mind DMSO.
AcOH + CO(NH2)2 >> AcNH2 + pyrolysis >> MeCN
EtBr + aq NH3 + I2 >> MeCN
IIRC at least the first route has some reports here.
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Oscilllator
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Upsilon I am afraid this is going to be a big problem. The main reason I stopped attempting synthesis of this compound was that I did not have a
proper aspiration pump, only a water aspirator. This was a big problem as it meant I couldn't really run the distillation for very long without
wasting hundreds of litres of water.
I would strongly recommend a cheap vacuum pump as your next chemistry purchase as they are tremendously useful.
I would also recommend heating the catalyst for a bit longer - try and get it hot enough to see that cool yellow color.
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Ozone
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When last I checked, dioxane is an ether that is miscible with water. That makes it both polar and aprotic (like THF). It can solvate/complex many
inorganic compounds (whether or not it is good for the OP remains to be seen).
I've also used it to solvate polar compounds for GC (to avoid water).
Otherwise, see:
your approach? http://www.sciencedirect.com/science/article/pii/S1001841708... and http://ir.sxicc.ac.cn/bitstream/0/1501/1/573-576.pdf (especially)
From CO2 and 1,2-propylene glycol: http://pubs.acs.org/doi/abs/10.1021/ie070789n
From urea and 1,2-propylene glycol: http://pubs.acs.org/doi/abs/10.1021/ie049948i
It can also be done from propylene oxide and CO2, but that seems to require a supercritical fluid cell.
I'm not bothering to cover the obvious use of phosgene.
Good luck,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Waffles SS
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Synthesis of Alkylene Carbonates in Ionic Liquid
| Quote: |
We now propose a new synthesis of alkylene carbonates from urea and alkylene glycols, which requires neither elevated pressure nor high temperature.
The reactions are carried out in a two-phase system consisting of a ionic liquid and a chlorinated organic solvent.As ionic liquid we used a mixture
of equimolar amounts of a metal chloride, urea, and glycol,which formed a colorless transparent solution on heating above 50°C. The yields attained
80%. The reactions were carried out with a number of glycols and metal chlorides.
V. A. Kuznetsov, M. G. Pervova, and A. V. Pestov |
Attachment: kuznetsov2013.pdf (153kB)
This file has been downloaded 385 times
[Edited on 23-10-2015 by Waffles SS]
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Upsilon
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Wait, dimethyl sulfoxide is marked as polar and aprotic, and is readily available for quite cheap. Is there a reason this isn't used?
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