[science123]

Both cycloheptatrienyl anion and oxepin have 8 pi electrons if we count one lone pair of oxygen. Someone told me that the oxygen of oxepin is sp3 so we don't count it's pi electons but even oxygen of furan is sp3 but still count it's pi electrons. Any ideas? This is very confusing topic for me in organic II so far.

[Hybrid]

by counting lone pair of electron now it have even no. "4" so that is why oxepin and anion are not aromatic.

[science123]

by counting lone pair of electron now it have even no. "4" so that is why oxepin and anion are not aromatic.

I don't understand what you mean by even no. 4. My confusion is when to count the lone pairs. If we count the lone pairs of oxygen making 8 pi electrons, then oxepin should be anti-aromatic not non-aromatic. Is there any rule that we only count lone pairs when the result would be aromatic like the case of furan but not when counting lone pairs would make it anti-aromatic like oxepin.

[jake.n]

Anti-aromaticity is very unfavorable, and molecules tend to do whatever they can to avoid being anti-aromatic.  For example, if a change in hybridization or a change in conformation can lower the potential energy by reducing the anti-aromaticity, then it will happen.

[Hybrid]

in any case it not categorized as aromatic cpd as it fail to obey Huckel's rule

[science123]

Anti-aromaticity is very unfavorable, and molecules tend to do whatever they can to avoid being anti-aromatic.  For example, if a change in hybridization or a change in conformation can lower the potential energy by reducing the anti-aromaticity, then it will happen.

Can change in hybridization occur only for neutral molecules like oxepin or thiepine? Cycloheptatrienyl anion is considered anti-aromatic. Why can't there be a change in hybridization of one of the carbon to sp3 to break the anti-aromaticity?

[Hybrid]

science123, i suggest reading aromaticity chapter in Mcmurry organic chemistry, it will help u in understanding stability of aromatic cpd and bases of aromaticity and planarity and also search to see the resonance structures of oxepin and why its very unstable and what is the 3D structure in space