Although is has a Huckel number, the molecule cannot adopt the necessary planar conformation. If the double bonds are all cis or one is trans the angle strain excessive (fig a). The isomer with two trans double bonds cannot be flat because two of the H atoms interfere with each other(fig b)
Note that the polygon rule in the circle only predict characticity of system, but cannot conclude. If you know the structure of system, planar or coplanar, you can determine characticity of cyclopolyene systems.
[10]-Annulene is not aromatic although it has ten electrons available for delocalizaton. However J (fig c) is aromatic. Why???
As stated above, [10]-Annulene with two trans double bonds cannot be flat because two of the H atoms interfere with each other, however, for J, the interference of the H atoms is replaced by bridge C, so ring system is planar, and aromaticity.
Happy fun and reply if you don't agree!!!
.gif "Matcuoi (88)")
bluemonster
Exercise about aromatic characticity
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Linear Mode
