Aromatic Compound

Delocalisation is what separates an aromatic ring from an ordinary cyclic alkene. For a planar ring in which every atom contributes a p orbital, the pi electrons occupy molecular orbitals spread over the whole ring rather than fixed double bonds, and the resulting stabilisation is substantial: benzene is roughly 150 kJ/mol lower in energy than a hypothetical cyclohexatriene with three isolated double bonds. Huckel's rule expresses the requirement numerically, aromatic character arising when the delocalised system holds 4n+2 pi electrons. The chemical consequence is a marked preference for substitution over addition, since addition would destroy the delocalised system that confers the stability. Heteroatoms may occupy ring positions, giving pyridine, furan and thiophene, and fused rings give naphthalene and the larger polycyclic aromatics.