Molecules: You'd better learn to live with them.
Crown ethers are an unusual series of molecules that have the ability to complex small cations, notably alkali metals such as sodium and potassium. These cyclic molecules, upon complexing a cation, can allow a salt to go into organic solvent that otherwise wouldn't. This makes reactions that are otherwise impossible accessible.
A good example is how 18-crown-6 (shown below, an 18-membered ring with 6 oxygens/ether linkages) can help potassium permanganate (a beautiful purple oxidizing agent based on manganese (VII)) go into organic solvent. Normally, you can only get permanganate into very polar solvents (maybe just water, I forget). The use of 18-crown-6 with potassium permanganate in organic solvent generates a potassium-crown ether complex, allowing the permanganate ion to go into solution as well.
Things like this are useful because you have to transform most organic molecules in organic solvents for a number of reasons, not the least of which is the fact that the molecules themselves are insoluble in water. So it's the KMnO4 that has to get into organic solvent somehow. There's a few other options, one of which I'll touch on tomorrow.
See you tomorrow.
Comments
Crown ethers are awesome for these purposes. In many cases you can even use them catalytically (phase transfer catalysis!) which is nice because they are a little bit pricey. Some of the crown ethers can even selectively bind one particular cation in the presence of others, like 12-crown-4 will preferentially bind Li+ in the presence of Na+ and K+!
Posted by: movies | September 19, 2006 11:43 AM
movies - thanks for the addition tidbit. I am curious (not close to synthetic chemistry) if these crown complexes exhibit ionophoric behavior in the presence of bilipid membranes - such as gramicidine (but on a much smaller scale); perhaps exchanging, e.g., K+ with H+ to collapse a gradient.
It would seem quite likely since the overall field is significantly dampened considering they are soluable in organic solvents. Just a thought!
Posted by: Louis | September 20, 2006 10:38 AM