Molecule of the Day

Cubane, unsurprisingly, is a cube-like structure with formula C₈H₈:

It may not look like much, but bonds to sp³ carbons (saturated, no multiple bonds, to a first approximation) at 90 degree angles are really, really hard. This is because of the geometry of the orbitals in which you find the electrons. 90 degrees may not sound that far off, but chemical bonds are formed by these orbitals overlapping. If you draw a tetrahedron with a bowling pin facing the center at each vertex, you have a pretty good image of the orbitals in sp³ carbon:

Get out your clay (you do have clay, right?), and make a few of these. Notice how well you can overlap them if you don’t interrupt the 109.5 degree angle. Notice how terribly you can overlap them if you force them into a 90 degree angle. Overlapping orbitals is how chemical bonds come to be, and cubane doesn’t have very good orbital overlap. This is why cubane is so hard to make.

Cubane is a great example of a concept that is esoteric to the uninitiated. It has terrible thermodynamic stability: the carbon atoms would much prefer to be in other bonds. One would think it would fall apart. However, it doesn’t. Because it’s forced into this rigid structure, you have to break a few bonds for it to really start falling apart (and realize the energy associated with the carbon being involved in other, more thermodynamically at peace, compounds). This involves climbing a big hill (breaking the necessary bonds for it to fall apart) in order to get to tumble down a really huge hill (realizing the energy associated with breaking down into more thermodynamically favorable compounds). This kind of stability is known as kinetic stability.

Such stability can be used to good effect in explosives: octanitrocubane, not synthesized until 1999 (cubane was first prepared in the sixties), has this property of metastability, making it one of the most potent explosives known, while remaining almost entirely shock-insensitive. I am told you can bang on it with a hammer with essentially no effect (contrast nitroglycerin, which can detonate with “rough handling” - which includes stirring).

See you tomorrow!