Antiaromaticity

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Antiaromatic or pseudoaromatic 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.

The criteria for antiaromaticity are as follows:

1. The molecule must have 4N(pi) 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.

Antiaromatic compounds usually fail the Hückel's rule of aromaticity.

The IUPAC definition of antiaromatic compound is:

Compounds that contain 4n (n≠0) π-electrons in a cyclic planar, or nearly planar, system of alternating single and double bonds.

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

Anti-aromatic compounds become more stable by gaining or losing an electron pair in the π system, making them aromatic. Examples are cyclobutadiene (A), the cyclopentadienyl cation (B) and the cyclopropenyl anion (C). Cyclooctatetraene is a 4n system but neither aromatic or antiaromatic because the molecules escapes a planar geometry.