cyclooctatetraene
/ SY-kloh-OK-tuh-TET-ruh-een /
Cyclooctatetraene, often shortened to COT, is an eight-membered carbon ring, C8H8, bearing four double bonds. At first glance it looks like a bigger version of benzene, so you might guess it is aromatic. It is not, and the way it cleverly avoids being antiaromatic is one of the most elegant demonstrations of how aromaticity rules actually play out in three dimensions.
Counting its pi electrons gives eight, and eight is 4n with n=2, the antiaromatic count. If COT were forced to be flat and fully conjugated, it would be a destabilized antiaromatic molecule. Nature has a way out: the ring simply refuses to stay flat. It folds into a non-planar tub shape, like a shallow boat, in which the eight p orbitals can no longer all line up and overlap continuously. With the pi loop broken, conjugation is interrupted, and the molecule becomes an ordinary nonaromatic polyene with four essentially separate, alternating double bonds. By bending out of plane, COT trades the threat of antiaromaticity for the comfortable neutrality of a normal alkene-like compound.
Cyclooctatetraene therefore behaves like a typical set of double bonds, undergoing addition reactions just as alkenes do, with none of benzene's special stability or reluctance. Its story rounds out the trio: benzene shows aromaticity, cyclobutadiene shows antiaromaticity, and COT shows how a molecule escapes antiaromaticity altogether by sacrificing planarity. It is a vivid reminder that the planarity requirement in the aromaticity rules is not a formality but a physical degree of freedom molecules can and do exploit.
Add two electrons to COT, however, and you get the COT dianion with ten pi electrons (4n+2, n=2); this dianion does flatten out and becomes aromatic, the flip side of the same rule.
Eight pi electrons buckle the ring; ten flatten it into an aromatic dianion.
COT is nonaromatic, not antiaromatic, precisely because it puckers; a molecule only suffers antiaromaticity if it is locked flat, and the tub shape is exactly the molecule choosing not to be.