Here’s a useful little gimmick for being able to draw molecular orbitals of any cyclic aromatic ring system.
Let’s say you’re asked to draw the molecular orbitals for benzene.
Here’s what you do. Draw a hexagon – and make sure to put a vertex [that’s a pointy part] down. This is important or it won’t work.
Now where every vertex is, draw a horizontal line.
Voila. You have just described the positions of the molecular orbitals for benzene.
Now fill them up with electrons! In the case of benzene we have 6 Pi electrons. So fill the orbitals, lowest energy first (Hund’s rule!), and you’re done.
It also works for other systems too, so long as there’s just one ring. Just make sure that the point is pointing down.
Can you see why cyclobutadiene is especially unstable? Because you have two orbitals with single electrons in them (free radicals!). Likewise for the cyclopentadienyl cation.
This technique is called Frost Circles. It’s also described here.
By the way there’s no real “reason” why this method works. It’s just a coincidence (like how Bode’s law describes the placement of planets in the solar system) that it describes this phenomenon so well.
Tomorrow: we come back to carbocation stability.
Thanks for reading! James