Why are antiaromatic compounds less stable than aromatic compounds?

[avistein]

In aromatic compounds ,it is stabled because $\pi$ electrons are delocalised.But why in antiaromatic compounds delocalisation of $\pi$ electrons destablizes the compound and make it less stable than the corresponding single chain compound also. Should not the delocalisation have stabled it?

 

[DrDu]

Delocalization also stabilizes anti-aromatic compounds, however, the statilization is not as large as in aromatic compounds. You also have to take in mind not only the absolute stability but also the stability relative to deformations leading away from equal bond lengths. For this, what is most relevant is the sigma system, not only the pi system.
There exists voluminous literature on that topic, a nice read is e.g.:

http://onlinelibrary.wiley.com/doi/10.1002/jcc.20470/full

 

[avistein]

. For this, what is most relevant is the sigma system, not only the pi system.

That means delocalisation of $\pi$ electrons in sigma system does not stablize the system?

 

[DrDu]

It does stabilize the system energetically but not necessarily geometrically.

 

[LudusRex]

The stability of a compound is determined by the overall energy of the molecule, which is influenced by the arrangement of its electrons. In aromatic compounds, the \pi electrons are delocalized, meaning they are spread out over the entire molecule. This results in a lower overall energy and greater stability.

On the other hand, in antiaromatic compounds, the \pi electrons are also delocalized, but the arrangement of these electrons results in a higher overall energy. This is because in antiaromatic compounds, the \pi electrons are forced into a planar, cyclic arrangement, which causes them to be in a higher energy state compared to the corresponding linear, non-cyclic compound.

Additionally, in antiaromatic compounds, the \pi electrons are not fully delocalized due to the presence of nodes or regions of zero electron density in the molecule. This further destabilizes the compound.

Overall, the delocalization of \pi electrons in antiaromatic compounds does not result in stability, but rather in destabilization due to the specific arrangement and distribution of these electrons.