Heterocyclic Compounds

organic compounds containing a ring made up, in addition to carbon atoms, of atoms of other elements (heteroatoms), most often nitrogen, oxygen, and sulfur, and less frequently phosphorus, boron, and silicon. The diversity of types of heterocyclic compounds is very great, since they can differ in the number of ring atoms and by the nature, number, and positions of the heteroatoms, by the presence or absence of substituents or condensed rings, and by the saturated, unsaturated, or aromatic nature of the heterocyclic ring. The chemistry of nonaromatic heterocyclic compounds is very much like that of their open-chain analogues; there are some differences because of the effects of strain bonds in the ring and steric effects associated with the cyclic structure. Thus, ethylene oxide (I) and tetrahydrofuran resemble ethers, whereas ethylenimine (III) and pyrrolidine (IV) resemble acyclic secondary amines:

Aromaticity appears in heterocyclic compounds (mainly those with five and six members) that, like other aromatic compounds, contain a closed system of (4n + 2) π-electrons. The chemistry of such heterocyclic compounds, which retains a certain resemblance to the chemistry of aromatic compounds in the benzene series, is basically determined by the specific nature of each heterocyclic ring. Among the most important aromatic heterocyclic compounds are furan (V), thiophene (VI), pyrrole (VII), pyrazole (VIII), imidazole (IX), oxazole (X), thiazole (XI), and pyridine (XII). Heterocyclic compounds condensed with benzene rings—coumarone (benzofuran; XIII), benzopyrrole (indole; XIV), thianaphthene (benzothiophene; XV), benzothiazole (XVI), benzopyridine (or quinoline; XVII), isoquinoline (XVIII), and dibenzopyridine (acridine; XIX)—are also of great importance:

The aromatic nature of furan, thiophene, and pyrrole and of their benzo derivatives is due to the participation of the unshared electron pair of the heteroatom in formation of a closed system of six π-electrons. In an acid medium the heteroatom adds a proton, and the system ceases to be aromatic. Consequently such heterocyclic compounds as furan, pyrrole, and indole do not withstand the action of strong acids (thiophene is resistant to acids because of the smaller affinity of sulfur for a proton):

In six-membered heterocyclic rings the nonshared electron pair of the heteroatom does not participate in formation of an aromatic system of bonds. Therefore pyridine is a much stronger base than pyrrole, and it forms stable salts with acids:

Some important heterocyclic compounds—for example, pyridine and its homologues, quinoline, isoquinoline, indole, acridine, and carbazole—can be produced from coal tar, and furfurol is produced by hydrolysis of plant wastes (sunflower-seed hulls, straw, and so on). However, synthetic methods, which are very varied and specific, are the most important; they are treated in articles dealing with individual heterocyclic compounds. In synthesis, open-chain compounds are most frequently used as starting materials. Inter-conversions are known for some five-member heterocyclic ring systems. Thus, furan, pyrrole, and thiophene are inter-converted at 450° C over Al₂O₃ under the action of H₂O, NH₃, or H₂S, respectively.

The role of heterocyclic compounds in the active lives of plants and bodies of animals is exceptionally great. Substances such as the chlorophyll of plants and the heme of blood, the components of nucleic acids, coenzymes, some irreplaceable amino acids (such as proline and tryptophane), almost all alkaloids, penicillin and some other antibiotics, a number of vitamins (such as cobalamine [vitamin B₁₂] and nicotinic acid and nicotinamide [vitamin PP]), and plant pigments (anthocyanins) are also heterocyclic compounds. Many widely used synthetic medicinals, such as antipyrine, aminopyrine, analgin, Acrichine, Aminazine, and norsulfazole, are heterocyclic compounds. Heterocyclic compounds are widely used in various branches of industry as solvents, dyes, and rubber vulcanization accelerators.