AROMATIC COMPOUND

any of a large class of chemical compounds whose molecular structure includes one or more planar rings of atoms joined by covalent bonds of two different kinds. The term aromatic was first applied about 1860 to a group of hydrocarbons isolated from coal tar and distinguished by their odours, which are much stronger than those of other groups of hydrocarbons; in chemistry, aromaticity has come to denote the chemical behaviour, especially low reactivity, derived from the electronic structure of this class of molecules. Many aromatic compounds are relatives of benzene (q.v.), an organic compound that contains a ring of six carbon atoms; in others, atoms of other elements, especially nitrogen, replace one or more of the carbon atoms. Every atom in the ring shares a pair of electrons with each of its two neighbours: each of these electron pairs is concentrated between the two atoms, forming a bond denoted s (sigma). In addition, two regions of high electron density, parallel to the ring formed by the atoms, constitute a system of p- (pi-) bonds associated not with any pair of atoms but with the entire ring. The essential aspect of aromatic compounds is the number of electrons in the p-bond system, rather than the number or identity of the atoms in the ring. Most commonly, there are six electrons in the system, but there may be any number given by the expression 4n + 2, in which n = 1, 2, 3, etc.