Annulenes

Annulenes are the family of completely conjugated, monocyclic hydrocarbons.The annulenes are named as [n]-annulene where n is an even number that represents the number of C atoms in the ring.Members of the annulene series can be checked for aromaticity or anti aroamticity.[4]-Annulene:

Cyclobutadiene, [4]-annulene, is very unstable and highly reactive, much less stable than its acyclic counterpart, buta 1,3 diene, and consequently considered to be antiaromatic.

[6]-Annulene: (4n+2 system) Benzene, [6]-annulene, aromatic

[8]-Annulene: (4n system)

Cyclooctatetraene, [8]-annulene, is well-known and stable. It behaves as a typical alkene. The tub-shaped conformer has alternating long and short bonds and is more stable than the planar molecule.   

  [10]-Annulene: (4n+2 system):

The sis isomer of cyclodecapentaene, [10]-annulene, would have bond angles of 144^o and therefore would be highly strained. This strain has to be relieved by twisting within the molecule and the planarity required for aromaticity is lost. The two of its hydrogens would be strongly sterically opposed and so the ring is distorted.
 [10]-Annulenes of this type are not  aromatic, consistent with the molecule being twisted out of planarity. 

 [12]-Annulene: (4n system):

Cyclodecahexaene, [12]-annulene, would not be expected to be aromatic since it is not a 4n+2 system. The inner hydrogens interfere with one another sterically and the ring is bent.

 [14]-Annulene: (4n+2 system):

Cyclotetradecaheptaene, [14]-annulene, has 4n+2 pi-electrons and has been shown to be aromatic. The larger ring size permits planarity of the ring. 

 [16]-Annulene: (4n system):

Cyclohexadecaoctaene, [16]-annulene, not  aromatic since it is not a 4n+2 system. The inner hydrogens interfere with one another sterically and the ring is 

 [18]-Annulene: (4n+2 system):

Cyclooctadecanonaene, [18]-annulene, has 4n+2 pi-electrons and has been shown to be aromatic. The molecule can readily achieve the planar  conformation required for aromaticity.