Heterocyclic Compounds

HETEROCYCLIC COMPOUNDS, an extensive 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 importance belongs to systems with O, S, and especially N. Among the numerous and diverse representatives of this class of organic chemistry, five- and six-membered rings possess special stability and strength of the cycle. According to chemical properties, heterocyclic compounds are divided into 1) saturated, 2) unsaturated, and 3) unsaturated of the aromatic compound type (see). Saturated ones, such as: tetrahydrofuran, pyrrolidine, piperidine do not essentially differ in their chemical properties from non-cyclic compounds. Both the degree of mobility of H 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 one or more methylene groups are located. Thus, for example, pyrroline exhibits all the properties of a typical unsaturated compound: it easily adds bromine, is oxidized by an alkaline permanganate solution, etc. If, however, only methine (-CH=) groups are located in the nucleus of the heterocyclic compound, then these compounds in their chemical behavior approach benzene, i.e., they possess specific properties of aromatic compounds. As heterocyclic compounds with the properties of substances of the benzene series, the following simplest heterocycles can be cited:

S furan

thiophene (furfuran) syn syn \/ N thiazole

pyridine In the thiophene group, the similarity with 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 the phenomena of isomerism taking place in the cycles, as well as due to the existence of diverse combinations with other cyclic systems: benzene, naphthalene, phenanthrene, etc. To the latter, for example, belong simple type compounds: syn syn

acridine or alkaloid molecules (see), sometimes reaching extreme complexity. The source for obtaining certain 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, etc.).