Heterocyclic compounds (Greek heteros - another, second) - a class of organic cyclic compounds, in the cycles of which, in addition to carbon atoms, there are also atoms of other chemical elements, the so-called heteroatoms: oxygen (for example, furan and pyran), nitrogen (pyrrole and pyridine ), sulfur (thiophene, thiophenaldehyde, thiopyran, thiazole), selenium (selenophen), etc.
Heterocycles are formed by two or more carbon atoms, as well as heteroatoms. According to Bayer’s theory, stable heterocycles will be only in those cases when the deviation of the valencies of the atoms forming the rings is the smallest from the angle 109 ° 28 '. For example, the most unstable will be heterocyclic compounds with three-membered heterocycles, in the molecules of which the greatest such deviation is observed - 24 ° 44 '(ethylene oxide, ethylene thioxide and ethyleneimine).
Heterocycles are widespread in nature. These include amino acids (tryptophan, carnosine, histidine), imino acids (proline and oxyproline), 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), medicines (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 the molecule of which carbon atoms are bonded to a heteroatom (heteroatoms) and hydrogen atoms. Heterocycles and their derivatives are divided into groups depending on the number of atoms forming a 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. The numbering is carried out from the heteroatom or is 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, the arrangement of substituents and other features are taken into account (for example, furfural has the systematic name furan-2-carbaldehyde). The names of heterocyclic compounds are indicated in the description of methods for their preparation, properties and values.
Heterocycles include oxides (for example, ethylene oxide), dibasic saturated carboxylic acids anhydrides, etc. The stability of the previously considered heterocycles depends on the number of carbon atoms in the heterocycle molecule, 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, their molecular cycles 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 system of electrons in their molecule, which increases their aromaticity. Compared to benzene, thiophene more readily enters into electrophilic substitution reactions.