[CandiedMicrobe]

My professor built some models to show the stability of some planar molecules, but I am not comfortable with it since I'm not sure short of looking at each Cycloalkene that I will know which are stable or not in a planar conformation.

Disclaimer:This is only important to me at the moment for evaluating aromatic and related molecules. I am aware that there are other things to test for, but I was particularly curious about the planar requisite.

Anyone have any tips? Is there a way to evaluate...planarity?

All I can think of is that there must be something about angles between the molecules and possibly that it would be beneficial to have sp2 members for the most part.

[Honclbrif]

Usually its safe to assume that sp² hybridized atoms will be planar, but the bonds within the plane can be distorted from their ideal 120^o angle.

If you're looking at a conjugated system, its usually safe to assume that will be planar too. The exceptions to this are things which can't be due to steric clash (e.g 2,4-dimethylpenta-1,3-diene), or antiaromaticity (cyclooctatetraene). In the case of aromatics, bond angles can be significantly distorted to achieve aromaticity, but once you get over about 8 ring members (cyclooctatetraenide dianion), you've got to start playing games with cis/trans geometry to get annulenes to hold together.