Unlike most amines, pyrrole is not basic and unlike most conjugated dienes it does not undergo electrophilic addition reactions. Each of the four carbon atoms contributes one pi electron, and the SP 2 hybridized nitrogen atom contributes two more, its lone pair of unshared electrons in the P orbital. The six pi electrons occupy P-orbitals with lobes above and below the plane of the flat ring. Be sure that you can compare this structure with that of benzene. Like benzene, pyrrole undergoes substitution of a ring hydrogen atom on reactions with electrophiles.
Substitution normally occurs at the position next to nitrogen.
Substituted pyrrole rings form the building blocks from which many plant and animal pigments are constructed. Among these is heme, an iron-containing tetrapyrrole found in hemoglobin. Another example is pyridine, a nitrogen-containing heterocyclic analogue of benzene. Like benzene, pyridine is a flat molecule with bond angles of 120 degrees and with bond lengths of 1.39 units, a value which is intermediate between single and double bonds. Pyridine is aromatic with six pi electrons in a cyclic, conjugated pi-orbital system.