Question

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

Answer 1

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^@`, 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.