Heterocyclic Compounds

HETEROCYCLIC COMPOUNDS, a vast class of organic compounds with a cyclic molecular structure, in the ring of which include not only carbon atoms, but also atoms of other elements (heteroatoms). Cyclic compounds are known in which the role of heteroatoms is played by phosphorus, arsenic, selenium, antimony, bismuth, silicon, and certain metals, for example, mercury, but the most essential significance belongs to systems with oxygen, sulfur, and especially nitrogen. Among the numerous and diverse representatives of this class of organic chemistry, five- and six-membered rings possess exceptional stability and strength of the cycle. According to their chemical properties, heterocyclic compounds are divided into 1) saturated, 2) unsaturated, and 3) unsaturated of the aromatic compound type (see). Saturated ones, such as, for example: tetrahydrofuran, pyrrolidine, piperidine, in their chemical properties do not substantially differ from non-cyclic compounds. Both the degree of mobility of hydrogen atoms and the properties of heteroatoms, and in substituted compounds—the character of reactive groups—are fully analogous to the properties of these atoms or groups in aliphatic or alicyclic compounds (see). The same applies to unsaturated heterocyclic compounds in the cycle of which there is one or more methylene groups. Thus, for example, pyrroline exhibits all the properties of a typical unsaturated compound: it easily adds bromine, is oxidized by an alkaline permanganate solution, and so on. If, however, in the "nucleus" of a heterocyclic compound there are only methine groups, then these compounds in their chemical behavior approach benzene, i.e., possess the specific properties of aromatic compounds. As heterocyclic compounds with the properties of substances of the benzene series, the following simplest heterocycles can be cited: furan, thiophene, thiazole, pyridine.

In the thiophene group, the similarity to benzene extends not only to chemical, but also to physical properties. The groups of pyrrole, pyrazole, furan, pyridine, thiazole, and other similarly constructed cycles are also chemically very close to aromatic compounds. Due to the presence of heteroatoms (especially nitrogen), heterocyclic compounds are physiologically active substances, and therefore it is to them that a large number of medicinal substances and poisons belong.

The number of heterocyclic compounds increases extraordinarily due to isomerization phenomena taking place in the cycles, as well as due to the existence of various combinations with other cyclic systems: benzene, naphthalene, phenanthrene, and so on. To the latter, for example, belong simple-type compounds: acridine.

or molecules of alkaloids (see), sometimes reaching extreme complexity. The source for obtaining some heterocyclic compounds can be the products of dry distillation of various natural substances. Thus, for example, pyridine is isolated from coal tar, pyrrole from bone distillation products, thiophene from coal and shale tar. Others are prepared synthetically and used as medicinal substances (antipyrine, pyramidon, salipyrine) or dyes (indigo) or are substances of plant (alkaloids, anthocyanins, chlorophyll) or animal origin (hemin, uric acid, tryptophan, skatole, and so on).