Antiaromaticity

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 fail Hückel's rule of aromaticity i.e. 4n+2 pi electrons.

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.^{[citation needed]}

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 keto-enol 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 computer simulations predict that B is actually more stable than A.