Aromatic compounds comprise a class of hydrocarbons that include a six-member, unsaturated carbon ring in which the pi bond valence electrons are completely delocalized or conjugated. These compounds are stable and abundant in both natural and synthetic forms. The simplest of the aromatic compounds is benzene (C6H6), a flammable carcinogen, yet an industrially important chemical. The name aromatic is based on the strong aromas of many of the larger aromatic compounds. Diamonds and graphite, while not considered aromatic compounds, demonstrate delocalized electron sharing over very long atomic distances.
The carbon-carbon covalent bond, the basis of organic chemistry, shares two electrons between two adjacent carbon atoms as a single bond, or four electrons between two carbons in a double bond. A conjugated system has a series of alternating single and double bonds that can be represented by two or more Lewis structures. Conjugation or resonance occurs when there available p-orbitals, or d orbitals in larger molecular weight compounds, in which to spread the available valence electrons. Conjugation can occur in linear, branched, or cyclic configurations between bonds of carbon, oxygen or nitrogen atoms.
Aromaticity occurs when the electrons in the carbon chain are even more delocalized by forming a six-carbon ring with the equivalent of three each of alternating single and double bonds. If benzene behaved as a molecule with three double bonds, chemists would expect the molecule’s double bonds to be shorter than the single bonds, but benzene’s carbon bond lengths are all equal and coplanar. Benzene and other aromatic compounds do not undergo addition reactions as alkenes do. Alkenes add groups across their double bonds, while aromatic compounds substitute a hydrogen atom for a group.
The energy released when cyclohexene is hydrogenated to cyclohexadiene by adding hydrogen to the double bond is 28.6 kcal per mole. Hydrogenation of cyclohexadiene with two double bonds releases 55.4 kcal/mole or 27.7 kcal per mole H2. Benzene releases 49.8 kcal per mole or 16.6 kcal per mole H2 upon complete hydrogenation. The remarkably low value is a measure of the stability of the aromatic structure.
Chemists explain benzene’s planar morphology, equal carbon bond lengths and the low energy of its double bonds by concluding the 2p orbitals are distributed across all six carbons. The delocalized pi orbitals are visualized as forming a torus above and below the plane of the carbon skeleton ring. This configuration explains all of its characteristics and supports the concept of shared pi orbitals in other conjugated systems.
Aromatic compounds often exert a vapor pressure, and many of the gaseous molecules are detectable by human noses. Cinnamon bark, wintergreen leaves and vanilla beans all have aromatic compounds humans can smell. Synthesis of these or similar compounds also is the basis of artificial food flavoring.
Some very interesting aromatic compounds consist of polycyclic structures sharing one or more sides of the six-member carbon ring with an adjoining carbon ring. Naphthalene (C10H8) has two joined benzene rings; three rings joined linearly is called anthracene (C14H10), while six benzene rings in a circle, with a very high level of electron delocalization, is called hexhelicene (C26H16). With the increase in the number of rings, the hydrogen-to-carbon ratio decreases, the material becomes more stable, harder, and the melting point increases. As the ratio approaches zero, the compound is essentially another form of carbon. Graphite consists of sheets of delocalized ring structures with sp2 hybridized carbon atoms and diamonds are hybridized sp3 in three-dimensional interconnecting cage-like structures all due to aromaticity.