Heterocyclic compounds: nomenclature and classification

Heterocyclic compounds (Greek heteros is another, the second) is a class of organic cyclic compounds in which, in addition to carbon atoms, there are also atoms of other chemical elements, so-called heteroatoms: oxygen (for example, furan and pyran), nitrogen (pyrrole and pyridine ), Sulfur (thiophene, thiophenaldehyde, thiopyran, thiazole), selenium (selenophene), etc.

Heterocycles are formed by two or more carbon atoms, as well as heteroatoms. According to Bayer's theory, stable heterocycles will only occur when the deviation of the valences of the atoms forming the cycles is the smallest from the angle 109 ° 28 '. For example, heterocyclic compounds with trinomial heterocycles, whose molecules exhibit the greatest deviation of 24 ° 44 '(ethylene oxide, ethylene oxide and ethyleneimine), will be most unstable.

Heterocycles are widespread in nature. These include amino acids (tryptophan, carnosine, histidine), imino acids (proline and hydroxyproline), purine (adenine and guanine) and pyrimidine (thymine, uracil and cytosine) bases, biologically important substances of living matter (heme, hemin chlorophyll), alkaloids Caffeine and atropine), antibiotics (penicillin, gramicidin C, streptomycin), medications (norsulfazole and caffeine), sulfonamides (norsulfazole, streptocid), organic solvents (pyridine), carbohydrates, nucleic acids, proteins, hormones, vitamins, and many others Important substances.

Classification of chemical compounds

The classification is based on the structure of heterocycles in which the carbon atoms are bound to a heteroatom (heteroatoms) and hydrogen atoms. Heterocycles and their derivatives are divided into groups depending on the number of atoms forming the cycle (three-, four-, five-, six-membered, etc.). In each such group there are subgroups with one, two, and three heteroatoms.

Heterocyclic compounds in most cases are named according to the historical nomenclature (pyridine, pyrrole, acridine). From the historical names, the names of their derivatives are formed (pyridine-4-carboxylic acid, methylpyridine). To indicate the position of the substituents, the atoms of the heterocycles are numbered. Numbering is carried out from a heteroatom or denoted by the letters of the Greek alphabet, starting from the carbon atom adjacent to the heteroatom - alpha, beta, gamma, etc.

Heterocyclic compounds: nomenclature

When naming heterocyclic compounds according to the IUPAC nomenclature, the number of atoms in the heterocycle, its structure, placement of substituents and other features (for example, furfural is systematically named furan-2-carbaldehyde) is taken into account. The names of heterocyclic compounds are indicated in the description of the methods for their preparation, properties and values.

To heterocycles include oxides (for example, ethylene oxide), anhydrides of dibasic saturated carboxylic acids, etc. The stability of the previously considered heterocycles depends on the number of carbon atoms in the molecule of the heterocycle, the heteroatom itself and its location in the heterocycle. The least stable are heterocyclic compounds, consisting of three and four cycles, which are easily broken and become acyclic compounds. There are many heterocyclic compounds, the cycles of their molecules are stable, similar to the cycle of the benzene nucleus. They are the main structural element of many bio-compounds that are of great importance for industry, medicine and veterinary medicine.

Chemistry of heterocyclic compounds

The chemical properties of five-membered (thiophene and its derivatives) heterocycles are due to the presence of a pi-excess electron system in their molecule, which increases their aromaticity. Compared with benzene, thiophene more easily enters the electrophilic substitution reaction.