Property of conjugated systems with 4n delocalized electrons
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons)[1] and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.
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.[2]
Cyclooctatetraene is an example of a molecule adopting a non-planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight-electron π system around the ring, but it instead adopts a boat-like shape with four individual π bonds.[3] 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.[2]
^"IUPAC Gold Book: Antiaromaticity". doi:10.1351/goldbook.AT06987. Retrieved 27 October 2013.
^ abWu, Judy I-Chia; Mo, Yirong; Evangelista, Francesco Alfredo; von Ragué Schleyer, Paul (2012). "Is cyclobutadiene really highly destabilized by antiaromaticity?". Chemical Communications. 48 (67): 8437–9. doi:10.1039/c2cc33521b. ISSN 1359-7345. PMID 22801355. S2CID 19920318.
^Anslyn, Eric V. (2006). Modern Physical Organic Chemistry. University Science Books. ISBN 978-1-891389-31-3.
prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor...
The lack of conjugation allows the 8 π electron molecule to avoid antiaromaticity, a destabilizing effect associated with cyclic, conjugated systems...
[4]annulene). Cyclobutadiene is the only annulene with considerable antiaromaticity, since planarity is unavoidable. With [8]annulene, the molecule takes...
atoms with p orbitals, maleic anhydride was long thought to exhibit antiaromaticity. However, a thermochemical study concluded that only 8 kJ/mol of destabilization...
delocalized pi electrons, where n is an integer. Particular instability (antiaromaticity) is conferred by the presence of 4n conjugated pi electrons. The characteristics...
containing 4n π electrons are called antiaromatic and are, in general, destabilized. Molecules that could be antiaromatic will tend to alter their electronic...
associated with aromaticity whereas a paratropic ring current signals antiaromaticity. A similar effect is observed in three-dimensional fullerenes; in this...
signals in a 2 to 1 ratio. The addition of two electrons removes the antiaromaticity; it becomes a planar 10π-electron aromatic species and is thus a bicyclic...
studies have been conducted to assess the consequences of antiaromaticity in boroles. Their antiaromatic character entails strong electrophilicity of the boron...
Masahiko (5 February 2010). "Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene". Symmetry. 2 (1): 76–97. Bibcode:2010Symm...
aluminum clusters, and chemical bonding theory, especially aromaticity/antiaromaticity in all-metal structures, and development of the Adaptive Natural Density...
(4n + 2)-electron Möbius and 4n-electron Hückel transition states are antiaromatic and are thus strongly disfavored. Aromatic transition state theory results...
integer starting at zero. In contrast, molecules with 4n pi electrons are antiaromatic. Polycyclic aromatic hydrocarbons (PAH) Clayden, J.; Greeves, N.; Warren...
4-dithiins. Planar dithiins are 8π e− systems, which would lead to antiaromaticity if the structure was planar. Akin to the behavior of cyclooctatetraene...
of aromatic, suprafacial electron systems; likewise, (4r)a refers to antiaromatic, antarafacial systems. It can be shown that if the total number of these...
thought to adopt a rectangular structure. The compound is the prototypical antiaromatic hydrocarbon with 4 pi electrons (or π electrons). It is the smallest...
structure and conformation. 14. Is bicyclo[6.2.0]decapentaene aromatic or antiaromatic?". J. Org. Chem. 50 (15): 2684–2688. doi:10.1021/jo00215a018. Bornmann...
molecule can be isolated and sold as a salt. On the other hand, the antiaromatic cyclopentadienyl cation (C5H+5) is destabilized by some 40 kcal/mol....
product, or can undergo a 4π electrocyclic ring closure, to form an antiaromatic oxirene. This oxirene can reopen in two ways, to either α-ketocarbene...
Global Information