I know that crown ethers have the interesting property of strongly complexing alkali metals, which allows them to be used in organic solvents (eg. as "dissolving-activity enhancing agents" for potassium permanganate or potassium tert-butylate).

Having never used such molecules myself, I was wondering: is there a procedure employed in order to remove the complexed metal, hence allowing a recycling of that ether?

I am sorry if the question might be trivial, but I couldn't find an aswer online.

  • 4
    $\begingroup$ In my experience it is rare to try to recover chemicals used in the lab. I've only known it to be done with precious metals, silver gold and platinum. $\endgroup$ – MaxW Aug 25 '18 at 3:42
  • $\begingroup$ You can always recover the lighter ones by (vacuum) destillation. Most chemistry labs would just scale down the reaction until recycling in no longer worthwhile. But, if you need liters of the stuff, one would sure think about recycling. Mind, worthwile would mean that over a year, it would save thousands of Euros or dollars. Because you already cost ten times as much. ;-) $\endgroup$ – Karl Aug 25 '18 at 8:19
  • $\begingroup$ Given that crown ethers are very expensive (edit: apparently not quite so), I imagine if they were economically recyclable the procedure would be widely divulged, but it doesn't seem to be the case. That said, the Wikipedia article for 18-crown-6 says that it can be dried using NaK, which destroys any water and forms the electride [K(18-crown-6)]Na. This effectively "spends" the crown ether, so it makes no sense to use this process if there weren't a way to recover the metal-free crown... $\endgroup$ – Nicolau Saker Neto Aug 25 '18 at 13:14

Crown ethers are typically not recycled because they remain in the organic phase where normally also all the byproducts are (so the purification is very complicated) and they are still complexing the alkali metal after the reaction. You want to recycle them to again complex some alkali metal, but then you have first to break down the previous complex, which is not straightforward.

To recycle crown ethers you must use a different approach. The trick is to immobilize them on some solid support. Typically, you have some crown ether functionalized monomer and polymerize it, or you have some reactive polymer and introduce the crown ether at the end. A review can be found here.

These solid phase crown ethers are used in the same way as the better known ion exchange resins. The problem is that you will have to synthesize them, because they are not readily available for purchase.


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