Annulenes

Eight electron systems [8]-annulene (Cyclooctatetraene, COT)

• COT is a 4n system, Huckel rule predicts it is not aromatic. If the molecule were to be planar it should be actually anti aromatic.
• The planar system would have a bond angle which is considerably different from 1200 thus the planar form would have a lot of angle strain and anti aromatic. The molecule is actually tub shaped without any angle strain. It has the same pi electron energy as its open chain analogue.
• It is therefore classified as NON aromatic. The single bond length is 1.46 A and the double bond is 1.33 A

Ten electron systems [10] annulenes

From electron count a ten-electron system should be a Huckel system and aromatic but the molecules listed below are not planar so none of them are aromatic.

• In A there is no angle strain, but the 1,6-H atoms have a lot of steric hindrance between them so the molecule is forced out of planarity. The molecule has not been prepared.
• B is a regular decagon and the angle should be 144 for greater than 120 so there is lack of planarity.
• C also has some angle strain. How ever 10 electron planar, aromatic systems are known.

• By overcoming the steric hindrance of the two hydrogen atoms aromatic [10] annulenes have been prepared.
• The following compound undergoes nitration, bromination and acylation as well