[Swedish Architect]

What feature of an anti-aromatic system makes them so unstable? It may be necessary to add more why aromatic systems are stable as well. This is purely interest.

Thanks 

[AlphaScent]

What do you think?

[Swedish Architect]

OK, so the general rule is that they must be planar, both aromatic and anti-aromatic (although anti-aromatics bend etc. to escape the unfavourable interaction), and conjugated.
In an aromatic substance, such as benzene, the out-of-plane p-orbitals interact, to form 6 MOs. The 6 electrons occupy the lowest energy, thus an overall bonding effect.
I would think this would be similar if they have 4n electrons??
If I look at cyclooctatetraene it has 8 pi electrons. These I think will interact to form 8 MOs, but only 4 filled.
So it has the lowest MO filled- contribute to bonding effect. Next two MO's - bonding, each with one node, between two lobes. And the next two MO's have one electron each. Is this the case in all anti-aromatic systems??
If it has unpaired electrons, that makes it a free radical?? Therefore highly reactive. This explains why anti-aromatic substances are unstable, because the free radicals are reactive, and unstable. Thanks for getting me thinking!!