[sharbeldam]

The rule usually is in my mind, if there is an atom that has atleast one pair of electrons and also is connected to SP2 atom then that makes it sp2 as well (just like in furan the oxygen is sp2 which makes it conjugated and aromatic)

in 1,4-dioxine i have the same situation, both oxygen can give their pairs of electrons (each oxygen gives 1 pair), that makes the molecule conjugated with 8 PI electrons, so why is it not anti aromatic?

[hollytara]

You know about cyclobutadiene: it distorts to a rectangle to minimize its anti aromatic character. 

The same will be true for dioxin - it will twist or modify bond lengths to minimize anti-aromaticity.  In the more common dibenzodioxin molecules, the aromaticity is in the benzene rings.

[sharbeldam]

But is it possible for a student to know by himself? or he needs to memorize the exceptions?

[Marko]

This is no exception. In any situation where a molecule would be antiaromatic, it will "try" to avoid this. By adopting different bond angles or by bending out out of the plane, in other words by changing the geometry.
A
So, yes. The dioxine you show, with 2 double bonds and 2 lone pairs that could be part of the π-system, would be antiaromatic, *if the lone pairs would actually both be in the same plane as the lone pairs. But they don't have to; in fact, bending slightly out of plane would be beneficial even if antiaromaticity didn't play a role.

And as I mentioned, this is general. Any molecule that would be antiaromatic will distort to reduce/remove this. And every molecule with 4n π electrons in a ring will be able to do so. So it's a matter of searching for the possible way; a molecular model kit greatly helps here.

[sharbeldam]

Thanks.
But doesn't that mean that antiaromatic molecules are fictional? since no molecule would be 100% planar?

[Marko]

A molecule will indeed never be fully antiaromatic. What you do see is that the possbility to be antiaromatic (which is avoided as much as possible) influences the energy / stability of the molecule.