Editing Conjugated system (section)

==Conjugated cyclic compounds==
[[File:Benzene Orbitals.svg|class=skin-invert-image|thumb|250px|Basis p orbitals of benzene.]]
[[File:Benzene MO diagram.png|thumb|Benzene π molecular orbitals according to Hückel theory. Molecular orbitals are frequently described as linear combinations of atomic orbitals, whose coefficients are indicated here by the size and shading of the orbital lobes.]] [[Cyclic compound]]s can be partly or completely conjugated. [[Annulenes]], completely conjugated monocyclic hydrocarbons, may be aromatic, nonaromatic or antiaromatic.

===Aromatic compounds===
Compounds that have a monocyclic, planar conjugated system containing [[4n+2 rule|(4''n'' + 2) π-electrons]] for whole numbers ''n'' are [[aromaticity|aromatic]] and exhibit an unusual stability. The classic example [[benzene]] has a system of six π electrons, which, together with the planar ring of C–C σ bonds containing 12&nbsp;electrons and radial C–H σ bonds containing six electrons, forms the thermodynamically and kinetically stable ''[[benzene ring]]'', the common core of the benzenoid aromatic compounds. For benzene itself, there are two equivalent conjugated contributing Lewis structures (the so-called Kekulé structures) that predominate.<ref>{{Cite journal|last1=Rashid|first1=Zahid|last2=van Lenthe|first2=Joop H.|date=March 2011|title=Generation of Kekulé valence structures and the corresponding valence bond wave function|journal=Journal of Computational Chemistry|volume=32|issue=4|pages=696–708|doi=10.1002/jcc.21655|issn=1096-987X|pmid=20941739| s2cid=16526798 }}</ref><ref>While the two Kekulé resonance forms contribute to most (>90%) of the π bond energy, there are also a number of other minor contributors to the wavefunction in the valence bond treatment, including the three Dewar resonance forms, and even smaller contributions from various ionic and singlet diradical forms. See article by Rashid and van Lenthe for a recent computational treatment.</ref>  The true electronic structure is therefore a quantum-mechanical combination (resonance hybrid) of these contributors, which results in the experimentally observed C–C bonds which are intermediate between single and double bonds and of equal strength and length.  In the molecular orbital picture, the six p atomic orbitals of benzene combine to give six molecular orbitals.  Three of these orbitals, which lie at lower energies than the isolated p orbital and are therefore net bonding in character (one molecular orbital is strongly bonding, while the other two are equal in energy but bonding to a lesser extent) are occupied by six electrons, while three destabilized orbitals of overall antibonding character remain unoccupied.  The result is strong thermodynamic and kinetic aromatic stabilization.  Both models describe rings of π electron density above and below the framework of C–C σ bonds.

===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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