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

##### 1 Answer
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:

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 ${60}^{\circ}$, 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 $\setminus m a t h b f \left(6\right)$ 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.