Cyclic conjugated pi system

MOLECULAR ORBITAL THEORY FOR CYCLIC CONJUGATED PI SYSTEMS... 

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

Since the degenerate pair must be completely filled or completely empty in order for the molecule to be aromatic, stable arrangements arise when 2, 6, 10,. .. or 4n + 2 pi electrons are in the cyclic conjugated pi system. Unstable half-filled degenerate shells will occur with 4, 8, 12,. .. or 4n pi electrons. The 4n systems tend to distort from planarity to diminish pi overlap and this destabilizing antiaromatic conjugation. 

The racetrack analogy states that the electrons of benzene are low in P.E. because they reside in a box that is effectively very large due to the fact they can race endlessly around a track of conjugated p orbitals. We call this racetrack ofp orbitals a cyclic conjugated pi system. 

Pattern of molecular orbitals in a cyclic conjugated system. In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has AN+2) pi electrons, it will have a filled shell. If it has (47V) electrons, there will be two unpaired electrons in two degenerate orbitals. 

An electrocyclic reaction is an intramolecular reaction in which a new a (sigma) bond is formed between the ends of a conjugated tt (pi) system. This reaction is easy to recognize—the product is a cyclic compound that has one more ring and one fewer TT bond than the reactant. 

After understanding the usefulness of unsaturated compound, or conjugated system, we hope to explore the unique structure of aromatic compounds, including why benzene should not be called 1,3,5-hexatriene because it is more stable than a typical triene, and seemingly unreactive. Called aromatic initially because of its fragrance, aromaticity now refers to the stability of compounds that are considered aromatic, not only benzene. Any cyclic compound with 4n+2 pi electrons in the system is aromatic. The stability of aromatic compounds arises because all bonding orbitals are filled and low in energy. 

---