What ring size is stable? How can I tell if it should be planar?

1 Answer
Truong-Son N.
Feb 2, 2016

Don't be fooled: 1,3,5,7-cyclooctatetraene is actually NOT planar.

Here is the "tub"-shaped structure elucidated by Kaufman, Fankuchen, and Mark using X-ray Diffraction in 1948:

https://upload.wikimedia.org/https://upload.wikimedia.org/

Typically, a 6 or 5-membered ring is stable.

However, on either side of the spectrum, you can have an unstable 3 and 4-membered ring, or a nonstandard 7, 8, or even 9-membered ring.

For instance, cyclopropene, a three-membered ring, is VERY unstable, having a ~51^@ angle on the opposite side of the double bond instead of an expected 60^@, because the double bond is shorter than the single bonds.

Cyclobutadiene, a four-membered ring, has been proposed to not be a perfect square, but rectangular.

As a general rule of thumb, if a conjugated polyene ring has more than \mathbf(6) members, start doubting that it is planar and do some light research to see if any chemists have already elucidated a structure for the compound using techniques like X-ray Crystallography or X-ray Diffraction.

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