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Related to Aromatic Compounds: Aliphatic Compounds
a class of organic cyclic compounds in which all the atoms participate in forming a single conjugated system; the π-electrons of this system form a stable, or interlocking electron cloud (shell).
The term “aromatic compound” came into use because the first representatives of this class to be discovered and studied had pleasant odors. The simplest aromatic compound is benzene (I). Naphthalene (II), anthracene (III), phenanthrene (IV), and other compounds containing condensed benzene rings, as well as various derivatives, belong to the class known as aromatics.
Aromatic compounds are designated a special class for a number of reasons. Benzene, C6H6, which formally contains three double bonds, should have the same properties as a highly unsaturated compound. But benzene and other aromatic compounds are not altered when treated with cold potassium permanganate, nor do they add bromine as do olefins containing double bonds. A characteristic of aromatics is the ready substitution of hydrogen atoms attached to carbon atoms of the benzene ring when aromatics are treated with various electrophilic reagents. Thus, treating benzene with nitric acid yields nitrobenzene:
C6H6 + HNO3 → C6H5NO2 + H2O
Analogous processes of electrophilic substitution occur during sulfonation, halegonation, and acetylation of aromatic compounds, which react more like saturated than unsaturated compounds. It should be kept in mind, however, that the ease of substitution reactions and the difficulty of addition reactions has only a quantitative character. At certain conditions, benzene adds three molecules of chlorine to form hexachlorocyclohexane, C6H6Cl6; hydrogenation of naphthalene results in the addition of five molecules of hydrogen to yield decahydronaphthalene.
Aromatic compounds are very stable; they are formed from other classes under severe conditions. Thus benzene can be formed by treating acetylene with activated carbon at 650°C; it is also formed in the dehydrogenation (“aromatization”) of cyclohexane (V).
Substituted aromatic compounds have particular characteristics. Phenol, for example, has more acidic properties than do alcohols, but in this respect nitrophenols approximate carboxylic acids. Aromatic amines are significantly less stable than aliphatic amines; for aromatic amine—for example, aniline C6H5NH2—the characteristic reaction with nitric acid is diazotization which yields diazo compounds, widely used in the manufacture of dyes.
Aromatic compounds are extremely numerous and have practical significance. Thus, aromatic nitro compounds, sulfonic acids, phenols, and amines are intermediates in the synthesis of many dyes and pharmaceuticals. Phenols, styrene, and terephthalic acid are used in synthesizing polymers. The explosive trinitrotoluene is derived from toluene.
The special characteristics of this class of compounds can be explained by the fact that aromatics do not actually contain alternating single and double bonds; all bonds in benzene are equal and completely uniform. The distances between carbon atoms in benzene (1.4 angstroms [Å]) are between the interatomic distances for single (1.54Å) and double (1.34 Å) bonds. Therefore the preferred representation of the structure of benzene is not the usual formula (I) but (Ia). In benzene and other aromatic compounds it is characteristic for the π-electrons to form a stable, closed electron shell (cloud).
Later it was found that many other nonbenzene compounds have characteristics similar to aromatic compounds. The unsaturated five-member heterocyclics like furan, thiophene, and pyrrole are among the most important. Six-member heterocyclics such as pyridine also have properties of aromatics.
Also known are aromatic nonbenzene compounds whose skeleton is composed only of carbon atoms; to this class belong such stable organic ions as the cation of tropylium (VI), the anion of cyclopentadiene (VII), and dipolar compounds like azulenes (VIII) and others.
Several inorganic compounds like borazole (IX) and phos-phonitrile chloride (X) have properties of aromatics. Comparative aromaticity of benzene and nonbenzene compounds as manifest in a compound’s ability to take part in elec-trophilic substitution reactions, corresponds to the series: anion VII > pyrrole > benzene → pyridine > tropylium. The ability of a compound to take part in nucleophilic substitution reactions changes in the reverse order.