Structure of Pyrrole

Before we talk about pyrrole it is necessary to remember the structure of the cyclopentadienyl anion, which is a six 𝜋-electron aromatic system produced by the removal of a proton from cyclopentadiene. This system serves to illustrate nicely the difference between aromatic stabilisation and reactivity, for it is a very reactive, fully negatively charged entity, and yet is 'resonance stabilised' - everything is relative. Cyclopentadiene, with a pKa of about 14, is much more acidic than a simple diene, just because the resulting anion is resonance stabilised. Five equivalent contributing structures, 28-32, show each carbon atom to be equivalent and hence to carry one-fifth of the negative charge.

Pyrrole is isoelectronic with the cyclopentadienyl anion but is electrically neutral because of the higher nuclear charge on nitrogen. The other importance of the presence of nitrogen in the ring is the loss of radial symmetry, so that pyrrole does not have five equivalent mesomeric forms: it has one with no charge separation, 33,  and two pairs of equivalent forms in which there is charge separation, indicating electron density drift away from the nitrogen. These forms do not contribute equally; the order of importance is, 33>35,37>34,36.

Resonance leads, then to the establishment of partial negative charges on the carbons and a partial positive charge on the nitrogen. Of course, the inductive distribution in pyrrole is a balance of two opposing effects, of which the mesomeric effect is probably the more significant, and this result in a dipole moment directed away from the nitrogen. The lengths of the bonds in pyrrole are in accord with this exposition, thus the 3,4-bond is very much longer than the 2,3-/4,5-bonds, but appreciably shorter than a normal single bond between sp2 hybridised carbons, in accord with contributions from the polarised structures 34-37. It is because of this electronic drift away from nitrogen and towards the ring carbons that five-membered heterocycles of the pyrrole type are referred to as 'electron-rich, or sometimes '𝜋-excessive'.

It is most important to recognise that the nitrogen lone pair in pyrrole forms part of the aromatic six-electron system.

Thank you.