Editing Antiaromaticity (section)

==Definition==
The term 'antiaromaticity' was first proposed by [[Ronald Breslow]] in 1967 as "a situation in which a cyclic delocalisation of electrons is destabilising".<ref>{{cite journal|last1=Breslow|first1=Ronald|last2=Brown|first2=John|last3=Gajewski|first3=Joseph J.|title=Antiaromaticity of cyclopropenyl anions|journal=Journal of the American Chemical Society|date=August 1967|volume=89|issue=17|pages=4383–4390|doi=10.1021/ja00993a023}}</ref> The [[IUPAC]] criteria for antiaromaticity are as follows:<ref name="IUPAC definition">{{cite journal|last1=Moss|first1=G. P.|first2=P. A. S. |last2=Smith |first3=D. |last3=Tavernier|title=Glossary of class names of organic compounds and reactivity intermediates based on structure|journal=Pure and Applied Chemistry|year=1995|volume=67|pages=1307–1375|doi=10.1351/pac199567081307|url=http://iupac.org/publications/pac/67/8/1307/|doi-access=free}}</ref>

#The molecule must be cyclic.
#The molecule must be planar.
#The molecule must have a complete conjugated π-electron system within the ring.
#The molecule must have 4''n'' π-electrons where ''n'' is any integer within the conjugated π-system.

This differs from [[aromaticity]] only in the fourth criterion: aromatic molecules have 4''n'' +2 π-electrons in the conjugated π system and therefore follow [[Hückel’s rule]]. Non-aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring.

{| border="1" cellpadding="5" cellspacing="0" align="center" 
|+ '''Comparing aromaticity, antiaromaticity and non-aromaticity'''
|-
! style="background: #efefef;" | 
! style="background: #efefef;" |Aromatic
! style="background: yellow;" |Antiaromatic
! style="background: #efefef;"|Non-aromatic

|-
| Cyclic?
| Yes
| style="background: yellow;"| Yes
| rowspan="3" style="border-bottom: 1px solid grey;" valign="top" | Will fail at least one of these

|-
| Has completely conjugated system of p orbitals in ring of molecule?
| Yes
| style="background: yellow;"|Yes

|-
| Planar?
| Yes
| style="background: yellow;"|Yes

|-
| How many π electrons in the conjugated system?
| 4n+2 (i.e., 2, 6, 10, …)
| style="background: yellow;"|4n (4, 8, 12, …)
| N/A
|-
|}

Having a planar ring system is essential for maximizing the overlap between the ''p'' orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic.<ref name="Conformational Criterion">{{cite journal|last=Podlogar|first=Brent L.|author2=William A. Glauser |author3=Walter R. Rodriguez |author4=Douglas J. Raber |title=A Conformational Criterion for Aromaticity and Antiaromaticity|journal=Journal of Organic Chemistry|year=1988|volume=53|issue=9|pages=2127–2129 |doi=10.1021/jo00244a059}}</ref>

An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity. Being highly reactive is not "indicative" of an antiaromatic compound, but merely suggests that the compound could be antiaromatic. An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound.<ref name=Antiaromaticity>{{cite journal|last=Breslow|first=Ronald|title=Antiaromaticity|journal=Accounts of Chemical Research|date=December 1973|volume=6|issue=12|pages=393–398|doi=10.1021/ar50072a001}}</ref>

In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The [[potential energy]] of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation.<ref name="Conformational Criterion" /> 
This procedure is recommended because there have been multiple instances in the past where molecules which appear to be antiaromatic on paper turn out to be not truly so in actuality. The most famous (and heavily debated) of these molecules is cyclobutadiene, as is discussed later.

Examples of antiaromatic compounds are [[pentalene]] (A), [[biphenylene]] (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, [[cyclobutadiene]], is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization.<ref name="WuMo2012"/> Cyclooctatetraene appears at first glance to be antiaromatic, but is an excellent example of a molecule adopting a non-planar geometry to avoid the destabilization that results from antiaromaticity.<ref name="Modern Physical Organic Chemistry"/> Because antiaromatic compounds are often short-lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation.<ref name="WuMo2012"/>