Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous overlap of p orbitals, cyclopentadiene does not comply with aromaticity criteria. However, the removal of one hydrogen with both or one or no electrons from the CH2 group drives the conversion of sp3 carbon to sp2, thereby converting cyclopentadiene into cyclopentadienyl cation, radical, and anion, respectively, having a vacant p orbital. Among all the three species, cyclopentadienyl cation and radical with four and five π electrons, respectively, do not meet the criteria of 4n + 2 π electrons and are not aromatic. On the other hand, cyclopentadienyl anion with 6 π electrons, a continuous overlapping ring of p orbitals, and delocalized π electron density becomes aromatic. The electrostatic potential map of the anion corroborates the delocalization of π electrons throughout the ring. Moreover, the five resonance structures of cyclopentadienyl anion and the occupancy of bonding molecular orbitals by all 6 π electrons in the frost diagram validate the unusual stability of the anion.
