Antiaromatic molecules are cyclic systems containing alternating single and double bonds, where the pi electron energy of antiaromatic compounds is higher than that of its open-chain counterpart. Therefore antiaromatic compounds are unstable and highly reactive; often antiaromatic compounds distort themselves out of planarity to resolve this instability. Antiaromatic compounds usually fail the Hückel's rule of aromaticity.

Examples of antiaromatic systems are cyclobutadiene (A), the cyclopentadienyl cation (B) and the cyclopropenyl anion (C). Cyclooctatetraene is a 4n system but neither aromatic or antiaromatic because the molecule escapes a planar geometry.

By adding or removing an electron pair via a redox reaction, a π system can become aromatic and therefore more stable than the original non- or anti-aromatic compound, for instance the cyclooctatetraenide dianion. The IUPAC criteria for antiaromaticity are as follows:

1. The molecule must have 4n π electrons where n is any integer.
2. The molecule must be cyclic.
3. The molecule must have a conjugated pi electron system.
4. The molecule must be planar.

However, most chemists agree on the definition based on empirical (or simulated) energetic observations.

It is observed that the energy difference between aromatic and antiaromatic compounds diminishes with increasing size . For instance the 12-pi system diphenylene is an antiaromatic compound but stable and even commercially available. The low energy penalty for antiaromaticity is also demonstrated in certain pyrazine-dihydropyrazine pairs:

The compound on the left is a 14 pi-electron aromatic compound (NICS value -26.1 ppm) which can be reduced in a strongly exothermic reaction to the 16 pi-electron antiaromatic compound (NICS +27.7 ppm) on the right . The dihydropyrazine slowly converts back to the pyrazine under the action of oxygen. It shows that other electronic factors can overpower aromaticity.

Antiaromaticity is also observed in a chemical equilibrium between these two porphyrin derivatives :

A regular porphyrin is an 18 electron aromatic compound (not counting two non-contributing double bonds) but on substituting a pyrrole ring by a meta-phenylene ring aromaticity is lost due to lack of conjugation. In this system the phenylene group is also a phenol and structure A is found to interconvert with 20 electron antiaromat B via phenol-keto tautomerism. Antiaromaticity is evident from NMR spectroscopy with the inner NH protons shifting downfield by 10 ppm to 21 ppm. The NICS values compare +0.7 for A (non-aromatic) and +5 (antiaromatic) for B and other in silico experimentation predicts that B is actually more stable than A.

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