Molecular orbital description

•	The special stability (aromaticity) of benzene comes from having six π electrons. These six electrons fully occupy the three molecular bonding orbitals and are therefore delocalised over the entire conjugated system. This closed shell structure is the reason for the observed stability of benzene.

 

•	By comparison, cyclooctatetraene has eight electrons, six of these fill the molecular bonding orbitals and two occupy the degenerate pair of non-bonding orbitals.

 

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Cyclooctatetraene must therefore lose or gain 2 electrons in order to have a closed shell structure. We have seen this already: the 1,3,5,7-tetramethyl-cyclooctatetraene dianion is planar, allowing delocalisation over the ring, whereas the neutral structure adopts a nonplanar tub shape with localised bonds.

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Look at the MO level diagrams above. There is always a single low-energy bonding orbital followed by pairs of degenerate orbitals. Since the single orbital will hold two electrons when full and the degenerate pairs four, we will only have a closed shell of electrons in these π orbitals when they contain 4n+2 electrons.