aromatic compound

Chem.

an organic compound that contains one or more benzene or equivalent heterocyclic rings: many such compounds have an agreeable odor.

[1865-70]

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Any of a large class of organic compounds whose molecular structure includes one or more planar rings of atoms, usually but not always six carbon atoms.

The ring's carbon-carbon bonds (see bonding) are neither single nor double but a type characteristic of these compounds, in which electrons are shared equally with all the atoms around the ring in an electron cloud. The term was first applied с 1860 to a class of hydrocarbons isolated from coal tar and distinguished by odours much stronger than those of other classes of hydrocarbons. In modern chemistry, aromaticity denotes the chemical behaviour, especially the low reactivity, of this class of molecules related to their bonding. The parent compound of this class is benzene (C₆H₆). See also hydrogenation.

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▪ chemical compound

      any of a large class of unsaturated chemical compounds (chemical compound) characterized by one or more planar rings of atoms (atom) joined by covalent bonds (covalent bond) of two different kinds. The unique stability of these compounds is referred to as aromaticity. Although the term aromatic originally concerned odour, today its use in chemistry is restricted to compounds that have particular electronic, structural, or chemical properties. Aromaticity results from particular bonding arrangements that cause certain π (pi) electrons (electron) within a molecule to be strongly held. Aromaticity is often reflected in smaller than expected heats of combustion and hydrogenation and is associated with low reactivity.

  benzene (C₆H₆) is the best-known aromatic compound and the parent to which numerous other aromatic compounds are related. The six carbons (carbon) of benzene are joined in a ring, having the planar geometry of a regular hexagon in which all of the C—C bond distances are equal. The six π electrons circulate in a region above and below the plane of the ring, each electron being shared by all six carbons, which maximizes the force of attraction between the nuclei (positive) and the electrons (negative). Equally important is the number of π electrons, which, according to molecular orbital theory, must be equal to 4n + 2, in which n = 1, 2, 3, etc. For benzene with six π electrons, n = 1.

      The largest group of aromatic compounds are those in which one or more of the hydrogens (hydrogen) of benzene are replaced by some other atom or group, as in toluene (C₆H₅CH₃) and benzoic acid (C₆H₅CO₂H). Polycyclic aromatic compounds are assemblies of benzene rings that share a common side—for example, naphthalene (C₁₀H₈). Heterocyclic aromatic compounds contain at least one atom other than carbon within the ring. Examples include pyridine (C₅H₅N), in which one nitrogen (N) replaces one CH group, and purine (C₅H₄N₄), in which two nitrogens replace two CH groups. Heterocyclic aromatic compounds, such as furan (C₄H₄O), thiophene (C₄H₄S), and pyrrole (C₄H₄NH), contain five-membered rings in which oxygen (O), sulfur (S), and NH, respectively, replace an HC=CH unit.

Francis A. Carey

 

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