Molecular Orbital Theory for Cyclic Conjugated PI Systems

3 MOLECULAR ORBITAL THEORY FOR CYCLIC CONJUGATED PI SYSTEMS 

Chapter 12 Qualitative Molecular Orbital Theory Pericyclic Reactions 

The stabilization of benzene can be placed on a semiquantitative scale. If we define the stabilization of one electron in the pi orbital of ethene as P, then the stabilization of two electrons in a double bond is just 2p. The stabilization of six pi electrons in three double bonds is 6p. Benzene s MO energy levels are at 2p, ip, ip, and -Ip,-ip, -2P the first three are filled with two electrons each for a stabilization of 8p total (2 x 2p + 4 X ip). The six electrons of benzene pi loop are then 2p more stabilized than the six electrons in three isolated double bonds. 

The MO energy levels for any simple cyclic conjugated polyene can be estimated by drawing the regular polygon of the ring point down inscribed in a circle of 2p radius (Fig. 

The vertices of the polygon will correspond to the energies of the MOs on a vertical scale. The center of the circle corresponds to the nonbonding level (zero P) with the bonding levels below it and the antibonding above. 

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Section 12.3 Molecular Orbital Theory For Cyclic Conjugated Pi Systems... 

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