Editing Conjugated system (section)

===Nonaromatic and antiaromatic compounds===
[[File:All-Z-Cyclooctatetraene_3D_skeletal_formula.svg|class=skin-invert-image|thumb|200px|[[Cyclooctatetraene]]. Adjacent double bonds are not coplanar.  The double bonds are therefore not conjugated.|alt=|left]]

Not all compounds with alternating double and single bonds are aromatic. [[Cyclooctatetraene]], for example, possesses alternating single and double bonds. The molecule typically adopts a "tub" [[Conformational isomerism|conformation]]. Because the p orbitals of the molecule do not align themselves well in this non-planar molecule, the π bonds are essentially isolated and not conjugated. The lack of conjugation allows the 8 π electron molecule to avoid [[antiaromaticity]], a destabilizing effect associated with cyclic, conjugated systems containing 4''n'' π (''n'' = 0, 1, 2, ...) electrons. This effect is due to the placement of two electrons into two degenerate nonbonding (or nearly nonbonding) orbitals of the molecule, which, in addition to drastically reducing the thermodynamic stabilization of delocalization, would either force the molecule to take on triplet diradical character, or cause it to undergo [[Jahn–Teller effect|Jahn-Teller distortion]] to relieve the degeneracy. This has the effect of greatly increasing the kinetic reactivity of the molecule.  Because of the lack of long-range interactions, cyclooctatetraene takes on a nonplanar conformation and is nonaromatic in character, behaving as a typical alkene. In contrast, derivatives of the cyclooctatetraene dication and dianion have been found to be planar experimentally, in accord with the prediction that they are stabilized aromatic systems with 6 and 10 π electrons, respectively. Because antiaromaticity is a property that molecules try to avoid whenever possible, only a few experimentally observed species are believed to be antiaromatic. [[Cyclobutadiene]] and cyclopentadienyl cation are commonly cited as examples of antiaromatic systems. 
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