2012: Advent Chemistry: Cubane

by on December 18, 2012

The cubane molecule in 3D ball-and-stick form

This is cubane, so named because it’s an alkane and, well, it’s cubic.

It doesn’t actually occur in nature. It’s a purely synthetic molecule, and if I were to guess I’d say that it was invented, not because of its possible uses, but because it’s cool. It’s a cube! How awesome is that, a cubic molecule?

The picture is a ball-and-stick model, which doesn’t show all the details but makes the three-dimensional structure much easier to see. The black balls are carbon atoms and the white are hydrogen. Although you can’t see it in this format, all the bonds are single (sigma) bonds, so this is an alkane.

It’s made up of interconnected carbon rings. They’re four-membered rings, which is quite unusual.

Carbon most likes being part of a six-membered ring, usually made up of other carbons, but ‘heteroatoms’ like oxygen and nitrogen can be included. It doesn’t mind being part of a five-membered ring. But numbers smaller than five are quite unusual. They get unstable, due to something called ‘ring strain’. Put simply, ring strain is what you get when the shape of a ring forces the bonds to be at a smaller angle to each other than they want to be. Bonds are the negative, electron-rich part of a molecule, and they repel each other just like nuclei do. Push them too close together and they try to push apart again.

It takes energy to overcome this repulsion and form a strained molecule. So the resulting molecule is higher in energy than its unstrained equivalent, simply by virtue of existing, just as a compressed spring holds more energy than a relaxed one. High energy situations are unstable ones, because the nature of the universe is such that things tend to move down the enegy gradient. Hot things cool. Fast things slow down. Springs go ‘poing’.

Strained rings want to open.

Actually, it turns out that cubane is pretty stable. It’s a very strained ring system, but there’s nothing it can easily turn into. No bits of it can drop off and produce a lower total energy. Some molecules will fall apart very easily, turning into more stable things, but cubane doesn’t have any easy ways to do that, so it doesn’t. If it did, it would, and synthesising cubane would be an impossible task unless you worked at horribly low temperatures – and if you work at low temperatures to stop your product falling apart, it can be too cold for the reaction to happen in the first place. It’s not easy to synthesise something unstable.

Leave a Reply