18-Crown-6

18-Crown-6 is an example of a crown ether, which is a large cyclic polyether. Crown ethers were discovered by Charles Pedersen while working at DuPont in 1960, but weren't reported in the scientific literature until 1967. Pedersen's important discovery led to a flurry of activity in the fields of guest-host chemistry and supramolecular chemistry. For this accomplishment, he (along with Donald Cram and Jean-Marie Lehn) received the Nobel Prize in Chemistry in 1987.

Crown ethers are available in a variety of sizes and are named according to the number of atoms in the ring. For example, 18-crown-6 derives its name from the 6 oxygen atoms in the 18-membered ring. The other 12 atoms in the ring are carbon atoms.

The picture above shows 18-crown-6 as an electrostatic potential map, where colors are used to indicate calculated charge distributions. Red represents electron-rich regions and blue represents electron-poor regions. Intermediate levels of electrostatic potential are shown by orange, yellow, and green. The inner red area in the picture is due to the oxygens with partial negative charge and the outer blue area is due to the hydrocarbon portion of the molecule with partial positive charge.

The picture below (left) shows 18-crown-6 as the more familiar ball-and-stick model and the picture below (right) shows the space filling model. The oxygens are shown as red atoms, carbons as dark gray atoms, and hydrogens as light gray atoms.

Crown ethers are useful as complexing agents that help polar inorganic salts (like KF) dissolve in nonpolar organic solvents (like benzene). The potassium ion fits inside the ring of 18-crown-6, much like a grape fits in the hole of a donut. The oxygen atoms use their lone-pair electrons to hold onto the potassium ion, separating it from its anion.

18-Crown-6 has complexed (or solvated) a potassium ion in the image below. This solvated potassium ion dissolves in nonpolar organic solvents, bringing along with it the anion to maintain electrical neutrality. This in turn enhances the nucleophilic character of the anion, as long as it's not solvated by the nonpolar solvent. The upshot of all of this is that reactions that would not normally take place in nonpolar solvents can in fact be carried out with the help of crown ethers.

Last revised: March 14, 2005