Anti-aromatic

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Three antiaromatics with 4 π electrons (from left to right): Cyclopropenide anion (three mesomeric boundary structures), cyclobutadiene (two valence tautomers ) and cyclopentadienyl cation (five mesomeric boundary structures). The π electrons are marked in blue .

In chemistry, compounds that show the phenomenon of anti-aromaticity are called anti-aromatics . With anti-aromatics, the cyclic delocalization of 4n electrons (n ​​= 1, 2, 3 ...) leads to an energetic destabilization ( Hückel rule ). This is in exact contrast to aromatics , in which the cyclic delocalization of 4n + 2 electrons leads to an energetic stabilization (Hückel rule, for more detailed discussion see aromaticity ). The energetic destabilization of the anti-aromatics is accompanied by increased reactivity.

Basically, the phenomena aromaticity and antiaromaticity are encountered in organic and inorganic systems. In organic chemistry , they are mostly discussed in terms of planar, cyclic hydrocarbon compounds with conjugated double bonds . Antiaromatics then have 4n π electrons. To avoid the energetically unfavorable situation, on the one hand conformations are adopted in which the cyclic delocalization is reduced as much as possible, on the other hand reactions occur in which the cyclic conjugation is canceled. For perfectly cyclically delocalized anti-aromatic systems, the Hückel rule predicts a diradical structure (triplet), from which the high reactivity of the anti-aromatic can easily be explained.

A prime example of an antiaromatic compound is cyclobutadiene , which is only stable in a solid matrix at very low temperatures (≤ 20 K ). Tri- tert -butylcyclopentadiene, on the other hand, is stable for a few hours at 20 ° C (steric shielding). The complex cyclopentadiene iron tricarbonyl is stable at room temperature and serves as a source for the generation of cyclopentadiene .

Individual evidence

  1. ↑ Joachim Buddrus: Fundamentals of Organic Chemistry , 4th edition, de Gruyter Verlag, Berlin, 2011, p. 425, ISBN 978-3-11-024894-4 .
  2. ^ Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 73, ISBN 3-342-00280-8 .
  3. ↑ Joachim Buddrus: Fundamentals of Organic Chemistry , 4th edition, de Gruyter Verlag, Berlin, 2011, p. 426, ISBN 978-3-11-024894-4 .