If the nitrogen atom of an amine is incorporated as part of a ring, the resulting amine is said to be a heterocyclic compound one of a group of compounds whose rings contain at least one element other than carbon. Heterocyclic compounds containing oxygen, sulfur, or other elements are also known; however, heterocyclic nitrogen compounds are the most common.

Saturated heterocyclic amines behave much the same as their acyclic analogs, just as n-hexane and cyclohexane are similar; pyrrolidine and piperidine (Fig. 11-13) both undergo all the reactions of typical aliphatic secondary amines. If a heterocyclic ring is highly unsaturated, however, it may acquire unusual properties all its own. For this reason, heterocyclic chemistry is often treated as a special subject. Often heterocyclic compounds have aromatic properties reminiscent of those of benzene. Pyridine and pyrrole are the most common examples (Fig. 11-13). Pyridine is the direct heterocyclic analog of benzene, in which one carbon atom and its attached hydrogen atom have been replaced by a nitrogen atom. Pyridine is still basic, since it has a free electron pair on the nitrogen atom. Similarly, alkylation takes place on the nitrogen atom to give a quaternary salt (Fig. 11-14).

Figure 11-13. Four heterocyclic compounds. The electrons making up the aromatic sextets in pyridine and pyrrole are shown in red.

Figure 11-14. Pyridine is a basic amine, and reacts, like other tertiary amines, with alkyl halides to form quaternary ammonium salts.

Pyrrole also displays some of the properties of an aromatic compound because the two electrons that it possesses on the nitrogen atom can be used to complete an aromatic sextet, when combined with the four electrons of the double bonds (Fig. 11-13). In pyrrole, the unshared pair of electrons on the nitrogen atom is very much a part of the aromatic system, and so the compound is the heterocyclic analog of the cyclopentadiene anion (see Fig. 4-28). As a consequence, pyrrole does not behave as a base, since combination of this electron pair with a proton would disrupt the aromatic system.

A vast number of heterocyclic compounds are known, because nearly any carbon atom in any ring system can be replaced by a nitrogen atom, and oxygen, sulfur, and other elements can similarly be introduced. Four heterocyclic compounds whose ring systems are of special importance in life processes are shown in Fig. 11-15. The first two, imidazole and indole, are found widely in proteins; the last two, pyrimidine and purine, are the basic ring systems found in genes.

Figure 11-15. Four heterocyclic systems of particular biological significance.

The alkaloids, nitrogen-containing plant products, are the largest class of naturally occurring heterocyclic amines. Many alkaloids have powerful physiological actions and are important drugs. Morphine and other opium alkaloids fall into this class of compounds.

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