From the results of the Circle Method, we find that we get a closed shell, stable system only for cyclic polyenes with (4n+2) p electrons (n=0,1,2).


Such systems have the highest delocalization energies and are said to be aromatic.


The classic example is benzene:


p electron energy = 2(a+2b) + 4(a+b) = 6a+8b


Compare to equivalent, non-delocalized model:



delocalization energy = p electron energy - energy of 3 isolated double bonds


= 6a+8b - 3(2a+2b)

= 2b

150 kJ mol-1


This is the energy by which the delocalized p electrons in benzene are more stable than those in 3 isolated double bonds.



Other examples of aromatic molecules