any of a class of organic chemical substances that consist of molecules containing one or more rings of atoms with at least one atom being an element other than carbon. The class includes many compounds of biological importance, such as nucleic acids and certain vitamins, hormones, and pigments. Also included are industrially significant pharmaceuticals, pesticides, herbicides, dyestuffs, and plastics. Ordinary organic compounds have a backbone of carbon atoms, which are bonded to one another and also to hydrogen or other atoms, or both. Joining a chain of carbon atoms together results in a ring, or cyclic compound. In heterocyclic compounds one or more of the carbon atoms in the ring is replaced by the atom (called a hetero-atom) of another element. Most frequently, the hetero-atoms are oxygen, nitrogen, or sulfur. As in hydrocarbons, the heterocyclic ring atoms may be saturated (held together by single bonds) or unsaturated (one or more bonds are double or triple). Heterocyclic rings may also be aromatic (having alternating single and double bonds). Many of the naturally occurring heterocycles are aromatic. Because of its geometry, the carbon atom most readily forms rings containing five or six members. A common aromatic heterocycle with five members, in which the hetero-atom is nitrogen, is called pyrrole. Both the plant pigment chlorophyll and the red pigment of blood, hemoglobin, contain pyrrole nuclei. Many of the aromatic heterocyclic compounds consist of a heterocycle fused to a benzene ring. A number of natural and synthetic pigments, such as indigo and the phthalocyanins, are derived from fused rings containing pyrrole nuclei, as is the amino acid tryptophan. A pyrrolidine derivative, vinyl pyrrolidone, is the basis for water-soluble polymers that are used as blood plasma extenders. They are also incorporated into many cosmetic products, such as hair sprays. Five-membered, oxygen-containing heterocycles occur widely in nature, especially in many simple sugars. Treating carbohydrates, which are polymers (long-chain molecules) of simple sugars, in a particular way produces furfural. This is a derivative of furan whose hetero-atom is oxygen. Produced commercially from corncobs and oat husks, furfural is used as an industrial solvent and to make phenol-furfural resins. Furfural can be chemically reduced to give furfuryl alcohol, which can be polymerized to make heat- and alkali-resistant resins that are used to line chemical plants. The simplest six-membered heterocycles are pyridine, with the hetero-atom nitrogen, and pyran, with an oxygen hetero-atom. The pyridine ring occurs in many natural substances, such as vitamin B6 (pyridoxine) and nicotine. Commercially important pyridine derivatives include the weedkiller paraquat and vinylpyridine, a starting material for some synthetic rubbers. A number of important pharmaceuticals also contain the pyridine nucleus. Fused rings (i.e., those that share a carbon atom) of pyridine form the basis of some important alkaloids (e.g., quinine and morphine). Fused tetrahydropyran nuclei occur in a number of important natural products, such as vitamin E (tocopherol) and the anthocyanin pigments, which furnish the reds and blues of many flowers. Some important substances contain smaller or larger rings. Ethylene oxide, for example, is a three-membered heterocycle consisting of two carbon atoms and one oxygen atom. It is produced by partial oxidation of ethylene. Its main use is in the manufacture of ethylene glycol, commonly employed in automobile antifreeze. The important antibiotics, the penicillins and cephalosporins, all contain four-membered heterocycles with nitrogen as the hetero-atom. Many heterocyclic compounds contain more than one hetero-atom in a single ring. In many cases, the hetero-atoms are the same. An example is imidazole, with two nitrogen atoms in a five-membered ring. It is an important constituent of the amino acid histidine. The thiazole ring with one sulfur and one nitrogen atom occurs in vitamin B1 (thiamine), penicillins, and a number of other drugs. Six-membered rings containing two nitrogen atoms-uracil, thymine, and cytosine-are important components of nucleic acids. The pyrimidine ring occurs in barbiturate drugs. Dioxan, a heterocyclic ether, contains two oxygen atoms and is important industrially as a solvent. Fused rings may contain several hetero-atoms. For example, the purine bases adenine and guanine, found in nucleic acids, contain four nitrogen atoms in a ring, as does the alkaloid caffeine. Riboflavin, another of the B-group vitamins, has three fused rings and also contains four-ring nitrogen atoms.

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