For questions about the properties and reactions of aromatic compounds. This tag should also be applied to antiaromatic compounds. This tag should not be applied to general questions about resonance, use the [resonance] tag instead.
aromatic from the IUPAC Gold Book
- In the traditional sense, 'having a chemistry typified by benzene'.
- A cyclically conjugated molecular entity with a stability (due to delocalization) significantly greater than that of a hypothetical localized structure (e.g. Kekulé structure) is said to possess aromatic character. If the structure is of higher energy (less stable) than such a hypothetical classical structure, the molecular entity is 'antiaromatic'. The most widely used method for determining aromaticity is the observation of diatropicity in the 1H NMR spectrum.
See also: Hückel $(4n + 2)$ rule, Möbius aromaticity- The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of transition states of pericyclic reactions The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Hückel $(4n + 2)$ rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states.
Hückel $(4n + 2)$ rule from the IUPAC Gold Book
Monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally) hybridized atoms that contain $(4n + 2)$ π-electrons (where $n$ is a non-negative integer) will exhibit aromatic character. The rule is generally limited to $n = 0–5$. This rule is derived from the Hückel MO calculation on planar monocyclic conjugated hydrocarbons $\ce{(CH)_{$m$}}$ where $m$ is an integer equal to or greater than 3 according to which $(4n + 2)$ π-electrons are contained in a closed-shell system. Examples of systems that obey the Hückel rule include:
Systems containing $4n$ π-electrons (such as cyclobutadiene and the cyclopentadienyl cation) are 'antiaromatic'.
Möbius aromaticity from the IUPAC Gold Book
A monocyclic array of orbitals in which there is a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hückel systems; with $4 n$ electrons it is stabilized (aromatic), whereas with $4 n + 2$ it is destabilized (antiaromatic). In the excited state $4 n + 2$ Möbius π-electron systems are stabilized, and 4 n systems are destabilized. No examples of ground-state Möbius π-electron systems are known, but the concept has been applied to transition states of pericyclic reactions [See: aromatic]. The name is derived from the topological analogy of such an arrangement of orbitals to a Möbius strip.
antiaromatic compounds from the IUPAC Gold Book
Compounds that contain $4 n~ (n ≠ 0)$ π-electrons in a cyclic planar, or nearly planar, system of alternating single and double bonds, e.g. cyclobuta-1,3-diene.
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