Aromaticity

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