
In [10]-Annulene - the hydrogen atoms in the 1 and 6 position interfere with each other and force the molecule out of planarity

If this annulene with five cis double bonds were planar, each internal angle would be 144∘. Since a normal double bond has bond angle of 120∘, this would be from ideal. This compound can be made but it does not adopt a planar conformation and therefore is not aromatic even though it has ten π electrons.
