Explain why the following systems are not aromatic ? 

(i) Due to the presence of a sp3-hybridized carbon, the system is not planar. It does contain six p-electrons but the system is not fully conjugated since all the six p-electrons do not form a single cyclic electron cloud which surrounds all the atoms of the ring. Therefore, it is not an aromatic compound. 

(ii) 

Due to the presence of sp3-carbon, the system is not planar. Further, it contains only four π electrons, therefore, the system is not aromatic because it does not contain planer cyclic cloud having (4n+2) π-electrons. This system, however, behaves as a diene. 

(iii)

Cyclooctatetraene is not planar but is tub-shaped. Further, it has only 8 π-electrons. Therefore, the system is not aromatic since it does not contain a planar cyclic cloud having (4n+2) π electrons. The planar cyclootatetraene is, however, antiaromatic. .