Aromatic chemistry is the chemistry of aromatic compounds. An aromatic compound is an organic compound containing one or more benzene rings, so named due to its tendency to exhibit a pleasing aroma. Because of the structure of benzene, aromatic chemistry typically involves substitution reactions.
Benzene, C6H6, is a planar and symmetrical six-carbon closed ring, most commonly represented on paper as a hexagon with a circle in the center. Each corner of the hexagon represents a carbon atom, and each carbon atom has one hydrogen atom attached. Benzene has been estimated as a combination of two resonance structures, each of which has alternating double and single carbon bonds from one carbon atom to the next. In reality, benzene does not have alternating double and single carbon bonds. Experimental data have shown that the bond lengths within the ring are all equivalent, indicating symmetry to the bonding. Scientists assume that the extra (pi) electrons are delocalized, which is why the bonding is represented on paper as a symmetrical circle.
In addition to bond length data, chemical reactivity of benzene provides further evidence against actual double bonds within the benzene ring, because benzene does not behave chemically like an alkene. An alkene is more likely to undergo addition reactions, by which the pi electrons in the double bond are pulled away and used in the addition of two new bonds, leaving the carbons single-bonded to one another by way of the remaining sigma electrons. When this occurs, the carbon-carbon bonds rotate, and the planar structure of the alkene shifts into a three-dimensional shape.
Alternatively, benzene is noted for its stability, which is attributed to the delocalization of electrons; the pi electrons in the carbon bonds are much less likely to form new reactions. When benzene undergoes a typical chemical reaction, it retains its planar shape, because the carbon-carbon bonds are unaffected. Instead of addition reactions, aromatic compounds are more likely to undergo substitution reactions, by which the hydrogen atoms are replaced by substituent groups such as a methyl group (resulting in the aromatic benzene derivative, toluene) or chloride (resulting in the aromatic benzene derivative, chlorobenzene).
It is possible to free the pi electrons from the carbon-carbon bonding within a benzene ring, forming non-planar, single-bonded cyclohexane. However, a catalyst and extreme conditions, including high temperature and pressure, are required, and even then, the reaction occurs very slowly. Aromatic chemistry is most commonly defined by substitution reactions at the hydrogen sites around the benzene ring, which are the most readily occurring types of reactions seen with aromatic compounds.