Structures of Heteroaromatic Compounds 7from cyclopentadiene, is a 6-electron aromatic system. Five equivalent contributing structures show each carbon
atom to be equivalent and hence to carry one fi fth of the negative charge.
Pyrrole is related to the cyclopentadienyl anion through the replacement of a CH by an NH, thus pyrrole is isoelec-
tronic with the cyclopentadienyl anion. It is electrically neutral because of the higher nuclear charge on nitrogen.
Pyrrole does not have fi ve equivalent mesomeric forms: it has one with no charge separation, and four contributors
in which there is charge separation, indicating electron density drift away from the nitrogen, in direct contrast with
pyridine (page 5).
Resonance in pyrrole thus leads to the establishment of partial negative charges on the carbons and a partial positive
charge on nitrogen. The electronic distribution in pyrrole is a balance of two opposing effects, inductive (towards
the electronegative nitrogen) and mesomeric (away from nitrogen), of which the latter is the more signifi cant. It is
because of this net electronic drift away from nitrogen and towards the ring carbons that fi ve-membered hetero-
cycles of the pyrrole type are sometimes referred to as ‘-excessive’. The polarised contributors imply a permanent
polarisation of the -electron system that shows itself in the dipole moment of pyrrole, which is directed away from
the nitrogen in spite of the intrinsic polarisation of the -skeleton towards the more electronegative nitrogen, as
shown in pyrrolidine. The resonance energy of pyrrole is about 90 kJ mol–1.
A signifi cant aspect of pyrrole reaction chemistry is the relative acidity of the pyrrole N-hydrogen. Its removal as a
proton, using a strong base, produces the pyrryl anion in which there are two pairs of electrons associated with the
nitrogen: one pair is part of the aromatic sextet and the other, in the plane of the ring, is available for interaction with
electrophiles without disrupting the 6-electron aromatic system.