CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 18 - Professor J. Michael McBride
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
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