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.
The SP 2 nitrogen atom and the five carbon atoms contribute one pi electron each to the orbital system. Unlike pyrrole, however, the lone pair of electrons on the pyridine nitrogen atom is not part of the pi orbital system, but occupies an SP 2 orbital in the plane of the ring. Substituted pyridines occur in the B 6 -vitamins pyridoxal and pyridoxine, and these vitamins play important biological roles. Another heterocyclic form is pyrimidine, containing two nitrogen atoms and four carbon atoms in a six member ring. Several examples are cytosine, uracil, and thymine which occur in nucleic acids.