Aromatic compounds have a six-membered carbon ring with delocalized pi electrons, making them stable and abundant. Benzene is the simplest and most important. Aromaticity occurs when electrons are delocalized in a ring of six carbon atoms with alternating single and double bonds. Aromatic compounds have unique properties, including low energy double bonds and substitution reactions. Polycyclic aromatic compounds have multiple rings and increased stability. Aromatic compounds have industrial and flavoring applications.
Aromatic compounds comprise a class of hydrocarbons that includes a six-membered unsaturated carbon ring in which the valence electrons of the pi bond 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, but also an industrially important chemical. The aromatic name is based on the strong aromas of many of the larger aromatic compounds. Diamonds and graphite, although not considered aromatic compounds, exhibit 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 carbon atoms in a double bond. A conjugated system has a series of alternating single and double bonds which can be represented by two or more Lewis structures. Conjugation or resonance occurs when p orbitals and d orbitals in higher molecular weight compounds are available in which to diffuse the available valence electrons. Conjugation can occur in linear, branched, or cyclic configurations between bonds of carbon, oxygen, or nitrogen atoms.
Aromaticity occurs when electrons in the carbon chain are delocalized even further forming a ring of six carbon atoms with the equivalent of three each of alternating single and double bonds. If benzene behaved like 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 cyclohexadien 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 of H2. Benzene releases 49.8 kcal per mole or 16.6 kcal per mole of H2 after 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 low energy double bonds by concluding that the 2p orbitals are distributed across all six carbon atoms. The delocalized pi orbitals are visualized as forming a torus above and below the plane of the carbon backbone ring. This configuration explains all its characteristics and supports the concept of pi orbitals shared in other conjugate systems.
Aromatic compounds often exert vapor pressure and many of the gaseous molecules are detectable by the human nose. Cinnamon bark, wintergreen leaves, and vanilla beans all have flavor compounds that humans can smell. The synthesis of these or similar compounds is also the basis of artificial food flavourings.
Some very interesting aromatic compounds consist of polycyclic structures that share one or more sides of the six-membered carbon ring with an adjacent carbon ring. Naphthalene (C10H8) has two joined benzene rings; three rings joined linearly are called anthracene (C14H10), while six benzene rings in a circle, with a very high level of electron delocalization, are called heselicenes (C26H16). As the number of rings increases, the hydrogen to carbon ratio decreases, the material becomes more stable, harder, and the melting point increases. When 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 sp3 hybridized into interconnected cage-like three-dimensional structures, all due to aromaticity.
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