An aromatic compound is an organic compound containing a molecular framework with a specially stabilised system of delocalised electrons, most characteristically a cyclic, planar, conjugated system that satisfies the aromaticity criteria associated with quantum mechanical electron delocalisation. The word aromatic originally referred to the noticeable smells of some early compounds in this class, but modern chemistry uses aromatic to describe electronic structure rather than odour.
Benzene is the classic example, but aromatic compounds include many other hydrocarbons and compounds containing heteroatoms such as nitrogen, oxygen or sulfur. Aromaticity is therefore a structural and electronic property rather than simply a synonym for benzene or for compounds with pleasant smells. In the simplest textbook treatment, a cyclic molecule is aromatic when it is planar, continuously conjugated and contains 4n plus 2 pi electrons, where n is a nonnegative integer. This is known as Hückel's rule. The resulting delocalisation lowers the energy of the molecule relative to appropriate hypothetical localised structures and often produces enhanced thermodynamic stability.
The electronic basis of aromaticity is the delocalisation of pi electrons around a cyclic conjugated framework. In a conventional aromatic ring, adjacent atoms possess p orbitals that can overlap continuously around the ring, allowing pi electrons to occupy molecular orbitals extending over several atoms rather than being confined to individual carbon carbon double bonds. Benzene contains six pi electrons distributed across the entire six carbon ring. The six electrons occupy bonding molecular orbitals in a particularly stable arrangement.
Hückel's rule expresses the electron counting condition for many common aromatic systems as 4n plus 2 pi electrons, with n equal to zero, one, two and so forth. Thus two, six, ten and fourteen pi electrons can support aromaticity when the other structural requirements are met. The rule is not an arbitrary numerical recipe; it arises from the quantum mechanical energy levels of cyclic conjugated systems and the pattern of orbital occupation required to produce a closed shell of bonding molecular orbitals.
Aromatic compounds can be divided into several broad categories. Aromatic hydrocarbons, often called arenes, contain only carbon and hydrogen, with benzene, toluene, ethylbenzene and naphthalene being familiar examples. Heteroaromatic compounds contain one or more heteroatoms within the aromatic ring, such as nitrogen in pyridine and pyrrole, oxygen in furan or sulfur in thiophene. Fused aromatic systems contain rings that share atoms and bonds, producing structures such as naphthalene, anthracene and phenanthrene.
Aromaticity can also occur in ions, including certain cyclic cations and anions, provided their orbital structure, planarity and electron count support aromatic delocalisation. There are also antiaromatic systems, which are cyclic, planar and conjugated but have 4n pi electrons and consequently tend to be destabilised when the electrons remain delocalised. Nonaromatic systems fail one or more necessary conditions, such as continuous conjugation or planarity, and therefore do not receive the characteristic aromatic stabilisation.
Aromatic compounds commonly undergo reactions that preserve the aromatic electron system whenever possible. Electrophilic aromatic substitution is a central reaction type for benzene and many related compounds. In such a reaction, an electrophile temporarily interacts with the aromatic ring, forming an intermediate in which aromaticity is disrupted, followed by loss of a proton that restores the aromatic system. Nitration, sulfonation, halogenation and Friedel Crafts alkylation or acylation are important examples.
This behaviour contrasts with the typical addition reactions of alkenes, where addition across a carbon carbon double bond can permanently remove the local pi bond. Aromatic compounds can undergo addition under sufficiently forcing conditions, but such reactions often sacrifice aromatic stabilisation and therefore require appropriate energetic or catalytic circumstances. The tendency to restore aromaticity is a major factor controlling the chemistry of aromatic molecules.
Substituents attached to aromatic rings strongly influence their chemical behaviour. Electron donating groups can increase electron density in particular regions of a ring and often affect both reaction rate and the position at which electrophilic substitution occurs. Electron withdrawing groups can reduce ring electron density and can also influence substitution patterns. In monosubstituted benzene derivatives, substituent effects are often discussed using the terms ortho, meta and para to describe positions relative to the existing substituent.
Some substituents favour substitution at the ortho and para positions, whereas others favour substitution at the meta position. These effects arise from combinations of inductive effects, resonance effects and the relative stability of reaction intermediates. Substituents also influence physical properties such as polarity, boiling point, solubility and biological activity. Consequently, aromatic chemistry is central to the design and transformation of pharmaceuticals, dyes, polymers, agrochemicals and numerous industrial chemicals.
Aromaticity is important in biological chemistry because aromatic rings occur in many biomolecules and biochemical compounds. The amino acids phenylalanine, tyrosine and tryptophan contain aromatic side chains, although their biological behaviour differs because of the different substituents and ring systems involved. Nucleobases such as adenine, guanine, cytosine, thymine and uracil contain conjugated heterocyclic systems with aromatic or aromatic-like electronic characteristics that contribute to their structural and electronic properties. Many natural products, neurotransmitter-related compounds, pigments and pharmaceuticals contain aromatic rings.
Aromaticity can affect molecular rigidity, electron distribution, stacking interactions and the ability of a molecule to participate in noncovalent interactions. These properties can be important in molecular recognition, enzyme binding and the behaviour of drugs within biological systems. Aromatic structures therefore connect fundamental physical organic chemistry with biochemistry, medicinal chemistry and molecular biology.
The term aromatic should not be interpreted as meaning that a compound has a strong smell, nor does every aromatic compound have an obvious odour. The historical name arose because several early members of the class were fragrant, but modern classification is based on electronic structure. Aromaticity is also not simply equivalent to having alternating single and double bonds drawn around a ring.
Lewis structures are useful representations, but the actual electrons in an aromatic system are delocalised and cannot generally be assigned permanently to particular bonds. Benzene, for example, is often drawn with three alternating double bonds, but its six carbon carbon bonds have equivalent bond lengths that are intermediate between idealised single and double bonds. Resonance structures are therefore representations of one delocalised electronic structure rather than separate molecules rapidly switching between distinct arrangements.
Aromatic compounds are widespread in industry and environmental chemistry. Benzene derivatives are used as starting materials for polymers, solvents, pharmaceuticals, detergents, dyes, resins and many other chemical products. Some aromatic compounds are toxic, persistent or carcinogenic, and their environmental behaviour can depend on volatility, water solubility, chemical stability and biological degradation. Polycyclic aromatic hydrocarbons, which contain several fused aromatic rings, can arise during incomplete combustion of organic material and include compounds of major environmental concern.
At the same time, aromatic chemistry is indispensable in synthetic chemistry because aromatic rings provide stable structural frameworks that can be selectively functionalised. Understanding aromaticity therefore requires combining molecular orbital theory, structural chemistry, reaction mechanisms and practical considerations concerning toxicity, environmental persistence and industrial use.
