Anti-aromaticity
In chemistry, antiaromaticity describes the phenomenon that cyclic delocalization of 4n electrons (n = 1, 2, 3 ...) leads to an energetic destabilization and thus increased reactivity compared to a linear conjugated reference. It is thus the counterpart to the phenomenon of aromaticity ( main article , see there for in-depth discussions), with the cyclic delocalization of 4n + 2 electrons (n = 0, 1, 2 ...) compared to a linear conjugate reference to energetic stabilization and thus reduced Reactivity leads. The phenomena of aromaticity and antiaromaticity can be explained in a simple form using the Hückel ruleillustrate. Compounds which exhibit the phenomenon of antiaromaticity are as antiaromatics , those which exhibit the phenomenon of aromaticity as aromatics , respectively.
While aromatic systems can be found in large numbers due to their high stability, the number of well-known and investigated antiaromatic compounds is comparatively small due to their high reactivity. Since anti-aromatic systems also strive to reduce the unfavorable cyclic delocalization as much as possible - z. B. by taking conformations with reduced conjugation - characteristic physical properties (ring currents, magnetic properties) are usually only weakly pronounced in contrast to aromatics (for a discussion of the magnetic phenomena see under aromaticity ).
Although aromaticity and antiaromaticity are fundamental concepts in chemistry, it is difficult for those skilled in the art to find a short, comprehensive, and complete definition of the terms. The following table summarizes the usual aspects of the two phenomena according to the current state of the discussion:
| property | Aromatic | Reference (olefin) | Anti-aromatic |
|---|---|---|---|
| Delocalization / conjugation | cyclic | linear | cyclic |
| Number of π electrons | 4n + 2 | 2n | 4n |
| Energetic effect of conjugation | stabilization | = Reference | destabilization |
| Extent of delocalization | elevated | = Reference | humiliated |
| Bond lengths | Tendency to Binding length compensation |
alternating | alternating |
| Diamagnetic anisotropy | elevated | - | small |
| magnetic susceptibility exaltation | high | - | low |
| Ring current | diamagnetic | - | paramagnetic |
| NICS values | clearly negative | - | clearly positive |
| Chemical reactivity | electrophilic substitution | addition | addition |
| HOMO-LUMO difference | elevated | = Reference | humiliated |
| Typical representatives | benzene | Butadiene | Cyclobutadiene |
Historical development
While the phenomenon of aromaticity was already known in the second half of the 19th century, antiaromaticity was only discovered among chemists a good 100 years later through the work of Breslow et al. discussed more intensively.
The discussion was based on the observation that the acidity (determined by the rate of the H / D exchange) of cyclopropenes is lower than that of analogous cyclopropanes, although the anion of cyclopropene should be stabilized by conjugation with the neighboring double bond. Since the decreased acidity indicates destabilization, the cyclopropenyl anion was classified as antiaromatic. This is in agreement with Hückel's rule (cyclic delocalization of 4n π electrons). These considerations find an important confirmation in the high acidity of cyclopentadiene: the cyclopentadienyl anion benefits from aromatic stabilization (4n + 2 π electrons), the deprotonation is thus energetically facilitated.
The discussion of antiaromaticity in Breslow et al. was strongly focused on the energetic aspect of anti-aromaticity. Work by Dauben et al., Which was carried out in the same period and dealt with magnetic properties as an aromaticity criterion , grouped compounds such as [16] annulene , cyclooctatetraene and heptalene under the keyword “pseudo-aromatics”. Although the authors have already discussed that weak paramagnetic ring currents predominate in these compounds (aromatics have diamagnetic ring currents, see under aromaticity ) and that these are reduced in their extent due to the alternation of bond lengths compared to the completely delocalized model systems, the concept “antiaromaticity “As we understand it today not yet enforced.
Similar to aromatics, a much broader understanding of the term antiaromaticity has established itself since this early work, which, in addition to energetic criteria, primarily includes the magnetic properties - especially in their effect on NMR phenomena (chemical shifts, NICS, see discussion under aromaticity ) - used for discussion. As with the study of aromatics, powerful quantum chemical methods played a decisive role in the development.
Energetic aspects
Delocalization and conjugation - two fundamental concepts in understanding chemical bonding - are generally associated with energetic stabilization. In the special case of aromaticity with its cyclic conjugation / delocalization of 4n + 2 electrons, the stabilization reaches an unusually high level. The fact that the cyclic conjugation of 4n electrons therefore leads to a destabilization is a phenomenon that is surprising at first glance - but one that follows naturally from the quantum mechanical treatment (see also quantum chemistry ). The different effects of cyclic delocalization can be impressively demonstrated by looking at some isodesmic reactions of cyclopropene and cyclopentadiene with cyclopropyl or cyclopentyl cations and anions (see figure on the left): While the formation of aromatic, cyclic delocalized systems with 2 or 6 π electrons (4n + 2, n = 0, 1, 2… Hückel rule) is clearly exothermic, the formation of the anti-aromatic systems with 4 π electrons (4n, n = 1, 2, 3… Hückel Usually) clearly endothermic. When evaluating the antiaromatic systems, it should be noted that there is already a significant alternation of bonds in the corresponding ions, which leads to a reduction in delocalization. Enthalpies of reaction for the formation of the hypothetical systems with maximum delocalization would therefore be significantly more endothermic.
Individual evidence
- ↑ a b Kenneth B. Wiberg: Antiaromaticity in Monocyclic Conjugated Carbon Rings. In: Chemical Reviews . 101, 2001, p. 1317, doi : 10.1021 / cr990367q .
- ↑ Annette D. Allen, Thomas T. Tidwell: Antiaromaticity in Open-Shell Cyclopropenyl to Cycloheptatrienyl Cations, Anions, Free Radicals, and Radical Ions. In: Chemical Reviews. 101, 2001, p. 1333, doi : 10.1021 / cr990316t .
- ↑ An overview of the current discussion on the subject of aromaticity and antiaromaticity can be found in Chemical Reviews Vol. 101, Issue 5, 2001 and Vol. 105, Issue 10, 2005.
- ↑ a b Hyp J. Dauben, James Dennis. Wilson, John L. Laity: Diamagnetic susceptibility exaltation as a criterion of aromaticity. In: Journal of the American Chemical Society . 90, 1968, p. 811, doi : 10.1021 / ja01005a059 .
- ^ A b R. Breslow, J. Brown, JJ Gajewski: Antiaromaticity of Cyclopropenyl Anions. In: Journal of the American Chemical Society. 89, 1967, p. 4383, doi : 10.1021 / ja00993a023 .
- ^ R. Breslow: Small Antiaromatic Rings. In: Angewandte Chemie International Edition in English . 7, 1968, p. 565, doi : 10.1002 / anie.196805651 .
- ↑ Entry on isodesmic reaction . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03272 Version: 2.3.3.