Aromatic Compounds

Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
Related to Aromatic Compounds: Aliphatic Compounds

Aromatic 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.


Karer, P. Kurs organicheskoi khimii. Leningrad, 1960. (Translated from German.)
References in periodicals archive ?
The high content of aliphatic hydrocarbons and low content of aromatic compounds reveal that the skeletal structure of oil shale kerogen is mainly composed of long-chain aliphatic alkyls and few aromatic rings.
This paper describes a case study in which three analytical techniques including GC/MS, GC/FID, and sonication were used to identify and characterize oil/grease, total petroleum hydrocarbons and volatile aromatic compounds BTEX in soil and sediment samples.
Some general tasting principles: a tulip glass retains aromas and reveals them with great delicacy; the temperature of the cognac should match the ambient temperature; a white background should be used to examine color and nuances; and the glass should be rotated to aerate and accelerate the release of aromatic compounds.
Bob was a natural product chemist and a world authority in his area of aromatic compounds.
They include a few top tips to ensure super salads: Use completely fresh *vegetables and fruits but make sure they are ripe so that the flavour and aromatic compounds (the fruit sugars) are properly developed.
Nevertheless only a few gram positive genera like Corynebacterium, Streptomyces and Rhodococcus are able to mineralize a wide array of aromatic compounds [15,16,17,18,191.
Furanes are aromatic compounds derived from biomass with a structure similar to that of benzene.
In a consent agreement with EPA, the company has agreed to pay a $94,509 penalty for failing to file required annual reports on polycyclic aromatic compounds and benzo (g,h,i) perylene releases from its facility during calendar years 2002 through 2006.
Total aromatic compounds (%) Semi-skimmed cow's milk HYDROCARBONS 15% KETONES 14% ALDEHYDES 16% ALCOHOLS 16% LACTONES 8% ESTERS 11% NITROGEN AND SULPHUR COMPOUNDS 17% PHENOLS 3% Plain soy drink HYDROCARBONS 10% KETONES 19% ALDEHYDES 32% ALCOHOLS 20% SULPHUR COMPOUNDS 7% AROMATIC COMPOUNDS AND FURANS 12% Note: Table made from pie chart.
In this work, hydrogenation of aromatic compounds in light gas oil derived from Athabasca bitumen was carried out using a single- and two-stage hydrotreating processes.
The test centre includes a range of spray-dryers including the FSD drier (Fluidized Spray Drier), the new Integrated Filter Dryer, a spray dryer using inert drying gas (nitrogen) to prevent oxidisation of the fine aromatic compounds, and Ray-1 batch freeze-dryer.
As they have no unsaturated bonds, aromatic compounds or other impurities, Gemseals are not sensitive to heat, UV or pH, and meet LVP-VOC exemption.