Aromaticity explains why benzene behaves unlike an alkene and why some cyclic pi systems are unusually stable while others distort or react to avoid destabilization.
Apply the aromaticity checklist: cyclic, planar, fully conjugated, and 4n+2 pi electrons.
Classify systems as aromatic, antiaromatic, or nonaromatic with correct attention to conformation.
Explain why benzene undergoes substitution rather than ordinary addition reactions.
Recognize aromatic heterocycles and decide whether a lone pair does or does not participate in the pi system.
Checkpoint Questions
Q: Why is cyclooctatetraene nonaromatic rather than antiaromatic?
A: Because it puckers into a nonplanar tub conformation, which breaks continuous overlap and avoids the destabilization of a planar 8-pi-electron ring.
Q: Does the pyridine lone pair count toward aromaticity?
A: No. It resides in an sp2 orbital orthogonal to the aromatic pi sextet.