Nonaromatic and Antiaromatic Cyclic Polyenes

Some cyclic conjugated polyenes do not satisfy the Hiickel rule and are not aromatic. Two examples are cyclobutadiene and cyclooctatetraene. Both are cyclic polyenes with alternating single and double bonds, but neither is aromatic. 

Cyclooctatetraene, which has eight 71 electrons, seems to fit the category of antiaromatic polyenes since it has 4 ti electrons ( = 2). Nevertheless, cyclooctatetraene is a stable molecule, and reacts like an alkene. For example, it undergoes addition reactions with bromine and is easily hydrogenated. Cyclooctatetraene is not planar. It is not antiaromatic because it exists in a tub conformation, so its 71 orbitals cannot overlap to form a continuous 7t system, which for 871 electrons would be very unstable. Therefore, cyclooctatetraene does not exhibit the general characteristics of either aromatic or antiaromatic compounds. It is nonaromatic. 

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