[omegasynthesis999]

Why is fulvene (on wiki) not aromatic? I know it is cyclic and planar. It also has all sp2 orbitals for the atoms in the ring. I would think that it has 6 electrons which is a Huckel number. So why is it not aromatic?

[english]

The 4n+2 pi electrons do not reside within the ring.  Draw the resonance structures, and you'll see that moving the C=CH2 pi electrons into the ring results in the terminal C being electron deficient. 

I would assume that the energy required to maintain this arrangement is more than the stabilization offered by aromaticity.  So you have 5 pi electrons (2 C=C bonds plus 1 electron from the carbonyl C), not 4n+2.

In tetrahydrofuran, however, the O atom can participate in resonance and some resulting structures have O with four bonds, yet it isn't electron deficient.  It may be one of those matters where O can handle the extra bonding electrons due to its higher electronegativity, and thus the energy it takes to maintain that state is sufficiently lower than the stabilization afforded by a cyclic, 4n+2 pi system.

That's what I think.  Honestly I have no idea.

[omegasynthesis999]

That makes sense that there are only 5 pi electrons in the ring...but then why wouldn't 1,4-benzoquinone (wiki) be aromatic? It should have 6 electrons in the ring (4 from the two double bonds and 1 from each C=O for a total of six)?

[Structuralist]

1,4-benzoquinone have 2 C=C bonds and 2 C=0 therefore it have 4 p electrons within the ring

[omegasynthesis999]

I looked at the fulvene again. It turns out it has 4 pi electrons in the ring (therefore antiaromatic). You can't count the C=C bond b/c its exocylcic (you can't split electrons of a pi bond so as to give 5 electrons).

[english]

You can't count the C=C bond b/c its exocylcic (you can't split electrons of a pi bond so as to give 5 electrons).

Okay thanks, I was wondering about that. 

[spirochete]

1,4-benzoquinone have 2 C=C bonds and 2 C=0 therefore it have 4 p electrons within the ring

That can't be a full answer, because a CO pi bond would be able to participate in an aromatic cycle if it were fully in the ring.

I accept that 1,4-benzoquinone isn't aromatic, but I still feel like it should be.  It seems easy to visualize electrons delocalized around the ring, even though you can't draw distinct resonance structures for it because of its symmetry.  In other words, imagine one electron from a CO pi bond pushing one electron out of a CC pi bond, which pushes an electron into an adjacent CC pi bond, etc.

[english]

In other words, imagine one electron from a CO pi bond pushing one electron out of a CC pi bond, which pushes an electron into an adjacent CC pi bond, etc.

The carbonyl pi electrons don't really resonate within the ring.  The only way to do so would make O electron deficient; and O does not want to be electron deficient.