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Cyclooctatetraene

It's a mouthful, so I like to abbreviate it as "COT."

It's also a fascinating case of antiaromaticity.

Yeah, I just said "fascinating" and "antiaromaticity" in a nerd voice in my head, too...but it's true! I'll definitely cover this in more detail in a lecture, but the gist is this: molecules, when the meet certain conditions, experience a special type of stability called aromaticity; in other words, a molecule that is especially stable is "aromatic." That is the organic chemistry definition of the word aromatic--don't confuse this with the common definition of the word (which is essentially "of good/pleasing smell").

Benzene is probably the most well-known aromatic molecule. It contains an uninterrupted, planar, cyclic system of 6 pi electrons. Without getting into crazy detail here, if we change the amount of pi electrons to 8 (exactly as depicted with COT), we obtain a special case of instability called antiaromaticity (not to be confused with nonaromaticity, which is something completely different and irrelevant to what I'm discussing here).

Because molecules that are antiaromatic are so unstable, they will "do" things to make themselves more stable (basic physics: anything in a high energy state will move to a low energy state if possible/allowed). In this case, to relieve its antiaromaticity, COT will adopt a nonplanar (aka, a "boat") configuration as seen above.

This is not normal!

This contortion increases many different types of strain within the molecule, which result in instability (e.g., the bond angles for these type of bonds usually like to be 120°, but the angles in COT vary between 114-127°--accordingly, this is called bond angle strain). However, the instability of antiaromaticity outweighs anything else, which results in the structure that we see above.

There are other cases of antiaromaticity causing "weird" things to happen (check out cycolobutadiene if you're curious), but I just happen to think that COT is the most interesting of them all.