Aromaticity

Fields of Study: Organic Chemistry

ABSTRACT

The electronic property of aromaticity results from the hybridization and overlap of atomic orbitals in the formation of conjugated bond structures. Only those conjugated compounds that conform to the 4n + 2 rule are aromatic.

The Nature of Aromaticity

The term "aromatic" does not refer to the effect a compound has on the sense of smell; rather, it describes a characteristic of the electrons in the pi (π) bonds of the molecule. Atoms with their valence electrons in the p orbitals, especially carbon atoms, have the unique ability to form a type of extra chemical bond between adjacent atoms. A carbon atom has four electrons in its valence shell, distributed as 2s²2p²: two electrons in the s orbital and one each in two of the three p orbitals. The s orbital is spherically symmetric about the atomic nucleus, but the p orbitals are figure-eight shaped and at right angles to each other, causing the electrons in these orbitals to experience very asymmetrical forces of repulsion. Because the s and p orbitals belong to the same electron shell, they are sufficiently close in energy that they are able to combine to form four equivalent hybrid sp3 orbitals. These typically form four sigma bonds to as many other atoms, characteristic of saturated hydrocarbons.

The carbon atom can also hybridize the s orbital with just two of the three p orbitals to produce three equivalent sp2 hybrid atomic orbitals, oriented in a plane about the nucleus at 120-degree angles to each other. The third p orbital remains a p orbital, perpendicular to the plane of the sp2 orbitals.

The three sp2 orbitals form sigma bonds to other atoms by end-to-end overlap of the orbitals on adjacent atoms. The lone p orbital does not take part in these bonds, but in order for the carbon atom to achieve its octet of valence electrons, one more electron is required. This in turn requires a second sp2 hybridized atom next to the first. In this relative position, the lone p orbitals on the two atoms can overlap side by side to form a pi-bonding molecular orbital containing two electrons.

When two pi bonds are adjacent to each other, spanning four carbon atoms, as C=C−C=C, they are said to be in conjugation with each other, or forming a conjugated system. Because all four pi bonds overlap equally well, the four p orbitals effectively form a single pi-bonding molecular orbital across all four carbon atoms. The electrons in that molecular orbital can therefore occupy any space along its entire span rather than being restricted to a bond between just two atoms, a phenomenon known as delocalization. In the case of cyclic conjugated systems such as benzene, it is called cyclic delocalization.

Benzene and Other Aromatic Compounds

Benzene, discovered in 1825 by Michael Faraday (1791–1867), was long known to be a type of hydrocarbon compound, but before the development of the modern atomic theory, the structure of the benzene molecule was a mystery. Scientists in the nineteenth century could not reconcile the composition of benzene with the known behaviors of other hydrocarbons. Legend has it that German chemist Friedrich August Kekulé (1829–96), while studying the compound, fell asleep one night before the fireplace in his home and had a dream of a snake eating its own tail; another version of the story has him dreaming about six monkeys holding hands and dancing in a circle. The dream supposedly made Kekulé realize that the structure of benzene was cyclic, not linear. He envisioned the benzene molecule as a ring of six carbon atoms with alternating single (C−C) and double (C=C) carbon-carbon bonds.

Despite this, Kekulé’s breakthrough did not solve the mystery of benzene, since the compound simply would not take part in the same kinds of reactions that other carbon-carbon double-bonded compounds underwent with ease. Only when twentieth-century technology enabled close study of the individual bonds in the benzene molecule was the mystery solved. The structure of the benzene molecule had been previously hinted at in the concept of resonance. Kekulé suggested, but could not prove, that the benzene molecule oscillated between two equivalent "resonance structures." It was eventually determined that the molecule exhibits a hybrid structure that is the midpoint between the two resonance structures, with all six carbon-carbon bonds in the molecule being equivalent in length and bond strength rather than alternating between single and double bonds. Molecular orbital theory finally provided a satisfactory description of the benzene molecule as a ring of six carbon atoms in a ring with six sigma bonds and a pi cloud encompassing all six of the carbon atoms through their respective p orbitals.

The descriptive term "aromatic" was used to indicate the special nature of the conjugated double-bond system of the benzene molecule, perhaps because of benzene’s sweet, oily aroma. This bond arrangement provides somewhat more stability than would be provided by single and double bonds alone. In other compounds with conjugated double-bond systems, some were found to have this aromatic character while others did not. Observations of their chemical behavior resulted in the formulation of the 4n+2 rule, or Hückel’s rule, which states that only conjugated systems in which the number of pi electrons is equal to 4n + 2, where n = 0 or a positive integer, exhibit the property of aromaticity.

Reactions of Aromatic Compounds

Aromatic conjugated systems are somewhat more stable and unreactive than nonaromatic compounds, but conjugation does "transmit" electron-withdrawing or -donating influences from other substituents. As electron-rich compounds, benzene and other aromatic compounds are subject to various substitution reactions involving electrophiles (chemical species that attract electrons), such as Friedel-Crafts alkylation and acylation reactions.

PRINCIPAL TERMS

• benzene: an organic compound with the molecular formula C₆H₆, consisting of a six-membered carbon ring with a hydrogen atom bonded to each carbon; in theory, the carbon-carbon bonds alternate between single and double bonds, but in fact they are all equal due to the electronic property of aromaticity.
• conjugation: the overlap of p orbitals between three or more successive carbon atoms in a molecular structure, creating a system of alternating double and single bonds through which electrons can move freely.
• cyclic delocalization: a property of some ring molecules, such as benzene, in which the overlap of orbitals between the atoms that make up the ring allows their electrons to move freely about the molecule.
• hybrid structure: a representation of molecular structure that averages a number of possible molecular structures that are equivalent in terms of the arrangement of orbitals and electrons within them.
• pi bond: a covalent chemical bond formed when parallel p orbitals of two adjacent atoms overlap in a side-by-side manner to form two molecular orbitals.
• resonance: a method of graphically representing a molecule with both single and multiple covalent bonds whose valence electrons are not associated with one particular atom or bond; two or more diagrams, or resonance structures, depict each possible arrangement of single and multiple bonds, while the true structure of the molecule is somewhere between the different resonance structures, an intermediate form known as the resonance hybrid.

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