Orbital Derivation of Hiickels Rule

Benzene is aromatic because it has a filled shell of equal-energy oibitals. The degenerate orbitals tt2 and 773 are filled, and all the electrons are paired. Cyclobutadiene, by contrast, has an open shell of electrons. There are two half-filled orbitals easily capable of donating or accepting electrons. To derive Hiickel s rule, we must show under what general conditions there is a filled shell of orbitals. 

Recall the pattern of MOs in a cyclic conjugated system. There is one all-bonding, lowest-lying MO, followed by degenerate pairs of bonding MOs. (There is no need to 

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