Aromatic Compounds

AROMATIC COMPOUNDS, a class of organic compounds, the characteristic features of which are 1) six-membered cyclic (closed) structure and 2) a special system of affinity distribution within the molecule, giving great strength to the cycle. The simplest substance of this class is benzol, the carbon skeleton of which is schematically depicted in the form of a hexagon-"core". -C5 4 »C- % C / Aromatic compounds also include homologs of benzol and condensed systems of the naphthalene, phenanthrene, chrysene type, consisting of two, three, four mutually fused cores of benzol, as well as many heterocyclic compounds (see), possessing "aromatic character," i.e., a certain complex of common specific properties. These properties, distinguishing aromatic compounds from aliphatic and alicyclic compounds (see), in essential consist of the following: 1) hydrogen atoms have great mobility, and therefore are easily replaced. In this respect, the reactions of nitration and sulfuration are especially characteristic, i.e., processes associated with the exchange of hydrogen for a nitro-group (NO₂) and sulfogroup (SO₃H):C_eH₅H + HNO₃=C_eH₆NO₂ + + H₂O; C_eH₆H + H₂SO₄ = C_eH_BSO₃H + H₂O; 2) the properties of various reactive groups located at the "core" undergo certain changes: halogens lose the ability to easily exchange for other groups, the acidity of hydroxyl increases, while the basic properties of the amino-group, on the contrary, decrease. The behavior of aromatic amines under the action of nitrous acid is especially characteristic; in this case, so-called diazo compounds (see) are formed. Being unsaturated, aromatic compounds nevertheless do not exhibit the typical properties of this class. In nature, aromatic compounds are less widespread than compounds of the fatty series, although some of them enter as constituent parts into proteins, alkaloids and other substances of the plant and animal world.

S. Medvedev. Aromatic hydrocarbons, obtained synthetically from products of dry distillation of coal (coal tar), constitute the main part of aromatic compounds. During fractional distillation of coal tar, as the temperature gradually rises, the following are distilled off: 1) light oils (up to 150°), 2) medium oils (150-220°), 3) heavy oils (220-260°), 4) anthracene oil (260-400°), 5) residue-coal tar. With further fractional sublimation of each of the listed groups, new substances are obtained. Thus, from light oils, pure benzol (C_vH_v), technical benzol I and II, toluene (methylbenzol-C₆H₅.CH₃), xylene [dimethylbenzene C₄H₄(CH₃)₂] and others are obtained; from the group of medium oils, pyridine, naphthalene, phenols, cresols, etc. are obtained. These substances (mainly from the group of light oils) are the starting materials for obtaining numerous compounds which find extensive application in the most diverse branches of industry: chemical, metalworking, textile, ph&rmaceutical and many others. Most of the initial and many derivative substances possess toxic properties; these substances mainly act on the blood, partly on the central nervous system, sometimes on the skin. Due to their poisonous properties, volatility, and ability of many of them to penetrate the body through intact skin, these hydrocarbons cause a large number of acute and chronic poisonings, both in the initial (chemical) industry and in many others where these substances are used. The number of industrial poisons from the group of aromatic hydrocarbons is very large. First place should be given to benzol and its numerous derivatives: nitrobenzene C_vH_v.NO₂, dinitrobenzene C_vH₄(NO₂)₂, aniline (aminobenzene C_vH₅.NH₂), which in turn is the starting material for obtaining a huge quantity of diverse dyes, many of which also possess toxic properties (e.g., phenylenediamines). Further, toluene, phenol (C_vH₅.OH), numerous explosive substances-trinitrophenol (picric acid), trinitrotoluene, tetranitromethylaniline (tetryl), etc. (see corresponding words) should be mentioned.