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The term 'antiaromaticity' was first proposed by Ronald Breslow in 1967 as "a situation in which a cyclic delocalisation of electrons is destabilising". [4] The IUPAC criteria for antiaromaticity are as follows: [5] The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π-electron system within ...
Benzene, the most widely recognized aromatic compound with six delocalized π-electrons (4n + 2, for n = 1).. In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π-electrons, where n is a non-negative integer.
In organic chemistry, Baird's rule estimates whether the lowest triplet state of planar, cyclic structures will have aromatic properties or not. The quantum mechanical basis for its formulation was first worked out by physical chemist N. Colin Baird at the University of Western Ontario in 1972.
In contrast to the rarity of Möbius aromatic ground state molecular systems, there are many examples of pericyclic transition states that exhibit Möbius aromaticity. The classification of a pericyclic transition state as either Möbius or Hückel topology determines whether 4N or 4N + 2 electrons are required to make the transition state aromatic or antiaromatic, and therefore, allowed or ...
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Annulenes may be aromatic (benzene, [6]annulene and [18]annulene), non-aromatic ([8] and [10]annulene), or anti-aromatic (cyclobutadiene, [4]annulene). Cyclobutadiene is the only annulene with considerable antiaromaticity, since planarity is unavoidable.
Maleic anhydride is a planar molecule. By virtue of the acid anhydride group, the alkene is electrophilic. On account of its cycle of 4 π electrons in an array of 5 atoms with p orbitals, maleic anhydride was long thought to exhibit antiaromaticity.
Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp 3 hybridized carbon atom. Although this sp 3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties ...