Antiaromatic (or pseudoaromatic) molecules are cyclic systems containing alternating single and double bonds, without having properties common to aromatic compounds. Antiaromatic compounds fail the Hückel rule of aromaticity. The pi electron energy of antiaromatic compounds is higher than that of its open-chain counterpart. Antiaromatic compounds are unstable and highly reactive.

The IUPAC definition of antiaromatic compound:

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

Examples

• Cyclobutadiene
• Cyclooctatetraene