Aromaticity in fused rings

 Aromaticity in fused rings

The criteria for aromaticity in mono cyclic hydrocarbons can be applied for polycyclic hydrocarbons as well. Following are some of the well known examples for this class of

compounds.

As the number of aromatic rings increases, the resonance energy per π electron decreases.

As a result, larger polynuclear aromatic hydrocarbons have a tendency to undergo addition

reaction to an internal ring to give more stable compounds.

Azulene is one of the few non-benzenoids that appears to have significant aromatic

stabilization. It has a noticeable dipole moment (0.8 D). It acts like a combination of

cyclopentadienyl anion and cycloheptatrienyl cation. In contrast, Pentalene and heptalene which posses fused five and seven membered rings respectively are not stable as expected on the grounds of antiaromaticity. Attempted synthesis of the former led to the formation of a dimmer, where as the latter undergo polymerization. It is interesting to note that the conjugate acid of heptalene is very stable, reflecting the stability of resulting Tropylium cation.

Fulvalenes represent another interesting class of compounds to look for potential

aromaticity. Among possible symmetrical structures, pentafulvalene and heptafulvalene have been prepared, but were found to exhibit polyene character. However, when a combination of rings, such as cyclopentadiene and cyclopropene were examined, results were in support of the existence of dipolar resonance structures. The large measured dipole moments of phenyl substituted analog x and reduced barrier of rotation (as revealed by NMR) of the dialkyl substituted analog x’ are manifestations of such effects.