Lecture 18 - Linear and Cyclic Conjugation Theory; 4n+2 Aromaticity

Despite the substantial change in the energy of individual orbitals, the overall pi-electron energy and orbital shape changes little upon linear conjugation of two double bonds. Conjugation energy of polyenes and allylic systems may be predicted by means of a semicircle mnemonic. The much greater stabilization in "aromatic" conjugated rings, and Hückel's 4n+2 rule, derive from alternating stabilization and destabilization of successive orbitals when the ends of a conjugated chain overlap as it is closed to form a ring. A circle mnemonic predicts orbital energies for conjugated rings. This aromaticity concept is generalized to heteroaromatic compounds like furan and imidazole, to polycyclic compounds like naphthalene, and to hydrocarbon ions like cyclopentadienide.