Antiaromaticity

Conjugated ring systems having 4n π-electrons (e.g. 4, 8, 12 etc. electrons) not only fail to show any aromatic properties, but are less stable and more reactive than expected. 

 1,3,5,7-cyclooctatetraene is non-planar and adopts a tub-shaped conformation. 

The compound is readily prepared, and undergoes addition reactions typical of alkenes. 

Catalytic hydrogenation of this tetraene produces cyclooctane.

 Planar bridged annulenes having 4n π-electrons have proven to be relatively unstable.

 Examples of 8 and 12-π-electron systems are shown below, together with a similar 10 π-electron aromatic compound.

The simple C₈H₆ hydrocarbon pentalene does not exist as a stable compound .

 The 12-π-electron analog heptalene is extremely reactive (more so than cyclooctatetraene).

Azulene is a stable 10-π-electron hydrocarbon  is a stable blue crystalline solid that undergoes a number of typical aromatic substitution reactions.

 The unexpected instability of 4n π-electron annulenes has been termed "antiaromaticity".

 Similarly, cyclopentadienyl cation (4 π-electrons) and cycloheptatrienyl anion (8 π-electrons) show very high reactivity when forced to form.