Crown Ethers

Students who memorize organic chemistry consider it torture. But to explore it is a delight. It’s like the difference between being force-fed and grazing at a buffet.

For example, I recently learned about ‘crown ethers‘: beautiful crown-shaped molecules of carbon, hydrogen and oxygen. Above is a ball-and-stick model of a crown ether called 15-crown-5.

Why is it called that? I guess because it has 5 oxygens (in red) and 10 carbons (in black) for a total of 15 heavier atoms. Then there are a bunch of hydrogens poking out. It has perfect 5-fold symmetry.

Yasui Yoshio won the Nobel Prize in 1987 for discovering crown ethers around 1967. He’s usually known as Charles J. Pedersen. He was born in Korea but later moved to Japan and then the US.

One use of crown ethers is to hold metal ions. A sodium atom with one electron missing can snugly fit inside 15-crown-5. So can a transition metal with two electrons missing, like cobalt (Co²⁺), nickel (Ni²⁺), copper (Cu²⁺), or zinc (Zn²⁺).

Other metal ions fit better in larger crown ethers. For example, here is 18-crown-6 holding a potassium ion. Beautiful 6-fold symmetry!

The 6 bonds here come from 6 oxygens connecting to the central potassium atom in purple. I don’t know, but I bet each oxygen donates 1/6 of an electron to the potassium atom, which is missing one.

1/6 of an electron?!?

You can’t actually chop an electron into parts, but you can have a single electron whose wavefunction is smeared out around all 6 oxygen atoms and the central potassium.

So, this ball-and-stick model is pretty misleading! The molecule doesn’t really have sticks in it. Let me show you another picture of it:

This more chubby depiction is called a space-filling model. This makes it clear how the potassium fits snugly inside the crown.

Another thing you can put in 18-crown-6 is hydronium, H₃O⁺:

Hydronium is the ion formed by a water molecule and an extra proton. We see here that its 3-fold symmetry fits nicely into the 6-fold symmetry of 18-crown-6.

But when 18-crown-6 does not have an ion inside it, it folds into a shape that lacks 6-fold symmetry:

Here are some more crown ethers:

They are:

1) 12-crown-4, 2) 15-crown-5, 3) 18-crown-6, 4) dibenzo-18-crown-6, 6) an aza-crown ether, meaning one that contains nitrogen.

For a final burst of fun, here’s an aza-crown ether holding a nickel atom in the center, and two chlorines (in green):

It’s called transNi(cyclam)Cl₂.

The diversity of highly symmetrical structures in chemistry is truly wondrous! And needless to say, I didn’t draw any of the pictures here. I got them from Wikicommons, and you can find out where by clicking on them. Most are in the public domain, thanks to the generosity of their creators.