Benzene and Other Rings

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of benzene and other ring systems are discussed, and the special characteristic of aromaticity is described. The benzene structure and ring structures in general are common in natural products and are very useful in electrophilic reactions.

The Nature of Benzene and Other Rings

The term "cyclic" refers to a basic molecular structure of hydrocarbon molecules in which the carbon atoms are bonded together to form a closed loop or ring structure, rather than as a two-ended linear chain. Of these, cyclohexane, cyclopentane, and their derivatives are by far the most common of the saturated hydrocarbon compounds, while benzene and its derivatives form the basis of a vast number of unsaturated compounds.

The cyclic nature of these compounds relates directly to the unique electronic nature of the carbon atom. The carbon atom has four electrons in its valence electron shell, two in the 2s orbital and one each in two of the three p orbitals in the 2p subshell. This arrangement is stabilized at a lower intrinsic energy when the 2s and all 2p orbitals combine, or hybridize, to form four equivalent sp³ hybrid atomic orbitals. The sp³ orbitals are geometrically arranged around the carbon atom nucleus so that they are oriented toward the four apexes of a tetrahedron. This places any two of them at a 109.5-degree angle to each other. As it happens, this particular angle and the physical size of the carbon atom are nearly ideal for the formation of a five- or six-membered ring of carbon atoms. Both configurations are very stable, with essentially no physical strain on the bonds between carbon atoms. Accordingly, a great many compounds are formed of five- and six-membered rings of carbon and other, very similar atoms, such as nitrogen and oxygen. This includes glucose; all of the carbohydrates that derive from glucose and other sugars, including the ribose and deoxyribose that form the structural backbone of RNA and DNA, respectively; and all animal and plant steroids.

In other compounds, the 2s orbital and just two of the three 2p orbitals hybridize to form three equivalent sp² hybrid atomic orbitals. These are arranged in a plane, radiating outward from the nucleus of the carbon atom. Any two of the three sp² orbitals are at an angle of 120 degrees to each other. The third 2p orbital remains as a p orbital, oriented perpendicularly to the plane of the sp² orbitals. When two carbon atoms with this orbital arrangement are adjacent to each other in a molecule, their remaining unhybridized p orbitals are able to overlap side by side, forming a secondary covalent bond called a pi (π) bond. The combination of a pi bond with the standard sigma (σ) bond, in which orbitals overlap head on, is significantly stronger than a sigma bond alone. With regard to molecular structure, the 120-degree bond angle between sp² orbitals precisely matches the internal angles of a hexagon. As a result, six sp² hybridized carbon atoms in a ring make for a very stable planar molecular structure, characteristic of the compound benzene (C₆H₆). Accordingly, a great many naturally occurring compounds are based on the benzene ring, from the simple amino acid phenylalanine to highly complex plant biopolymers called "lignins." In fact, it would be fair to say that far more naturally occurring compounds incorporate benzene rings in their structures than do not.

Ring structures range in size from three-carbon units and their heteroatomic analogs to very large single rings. In many of these, the benzene ring is present as a phenyl group (−C₆H₅). Heteroatomic analogs are also very common. One of the most interesting and potentially most valuable of compounds based on the benzene ring is graphene, perhaps the most advanced material known to modern chemistry. Paradoxically, it is also a naturally occurring substance that people have been writing with for literally hundreds of years. Graphene, for all its technological potential, is the basic structural component of graphite, or pencil lead. The structure of graphene consists of an immense array of carbon atoms bonded together in six-membered rings, each of which is structurally identical to the benzene ring.

Aromaticity

The prevalence of the six-membered benzene ring in nature is due to a special electronic characteristic of its bond structure. Historically, benzene, discovered in 1825 by Michael Faraday (1791–1867), was long known to be a hydrocarbon of the alkene class (unsaturated hydrocarbons with at least one C=C double bond), though it did not exhibit the chemical behavior characteristic of alkenes, and especially not of its homologous six-carbon compounds. Before there was a usable theory of atomic structure, and well before any modern analytical apparatus, scientists were greatly puzzled by the behavior of benzene and at a loss to explain its unexpected stability. Legend has it that German chemist Friedrich August Kekulé (1829–96), while pondering the structure of the benzene molecule, dreamed of a snake swallowing its own tail or of six monkeys holding hands and dancing in a circle. This clue allegedly led to his 1865 proposal that the six carbon atoms of the benzene molecule were joined together in a circle, alternating three double bonds with three C−C single bonds. For many years, scientists questioned whether these were normal single and double bonds or whether there was something special about them.

In 1872, Kekulé further proposed that the C=C and C−C bonds were constantly changing places, causing the molecule to oscillate between two different configurations. While considered outrageous at the time, this later proved to be not far from the truth. Following the development of methods for testing the bonds between atoms, it was discovered that all six carbon-carbon bonds in benzene are identical in length and therefore in bond strength. In fact, the electrons that form the bonds exhibit a form of delocalization, meaning that they are not restricted to a single atom or bond but rather follow orbitals that encompass several adjacent atoms. Thus, while the molecule is best represented with alternating double and single bonds, functionally both types of bond behave more like one-and-a-half bonds (which do not otherwise exist). At low temperatures, as the energy of the molecule decreases, the activity of the electrons in the bonds also decreases, allowing the bonds to differentiate into normal single and double bonds; at normal temperatures, the equalization of the bonds produces an enhanced stability in the molecule, a characteristic known as "aromaticity."

The term "aromatic" may have originally been ascribed for benzene’s sweetish, oily aroma, but it has since come to refer to the behavior of electrons in pi-bond systems rather than the compound’s effect on the sense of smell. Not all compounds with alternating single and double bonds are aromatic in nature. The compound 1,3,5,7-cyclooctatetraene is a hydrocarbon with alternating single and double bonds throughout its eight-membered ring structure, but it does not exhibit aromaticity. Rather, aromaticity seems to be reserved for compounds that have only certain numbers of pi electrons (electrons engaged in the pi-bond system), in accord with what is known as Hückel’s rule, named for German chemist and physicist Erich Hückel (1896–1980). The rule states that compounds only exhibit aromaticity if they have 4n + 2 pi electrons, where n equals zero or any positive integer.

Benzenes are part of a larger class of aromatic hydrocarbons, also called "arenes." The name "benzene" refers specifically to an aromatic compound with six carbon atoms arranged in a ring and one hydrogen atom bonded to each carbon atom. In benzene derivatives, one or more of the hydrogen atoms have been replaced with substituents. While six-membered arenes are the most common, those with more or fewer carbon atoms exist as well. Aromatic compounds in which one or more of the carbon atoms has been replaced by another atom are called "heterocyclic aromatics."

Nomenclature of Benzene Derivatives

Benzene derivatives are named by first identifying the various substituents (either single atoms or functional groups) bonded to the ring. The International Union of Pure and Applied Chemistry (IUPAC) provides rules for nomenclature of organic compounds that cover all possible combinations of substituents on the benzene molecule. The position of each substituent is specified numerically, with the primary substituent being assigned position 1 on the benzene ring. For example, a benzene ring bearing a cyclohexyl group (−C₆) and a neopentyl group [−CH₂−C(CH₃)₃] separated by two carbon atoms on the ring would be named 1-cyclohexyl-4-neopentylbenzene.

Because certain substituents are commonly found in many different benzene derivatives, the name of the base derivative can also be used to describe the same derivative with additional substituents. For example, toluene (IUPAC name methylbenzene) is a benzene ring in which one of the hydrogen atoms has been substituted with a methyl group (−CH₃), while anisole (methoxybenzene) is a benzene ring with a methoxy functional group (O−CH₃). The location of the functional group does not need to be identified in such compounds if it is the only substituent, but if one or more additional substituents are added, their positions in relation to the compound’s characteristic functional group can be identified either by number or by the prefixes o- (ortho-), m- (meta-), and p- (para-). If the characteristic functional group is on the carbon atom designated number 1 (C-1), then the ortho position is one atom away, on the C-2 atom; the meta position is two away, on C-3; and the para position is on C-4, directly across the benzene ring from the original group. Thus, a toluene molecule with a cyclohexyl group directly across the benzene ring from the methyl group can be named p-cyclohexyltoluene rather than the more unwieldy 4-cyclohexyl-1-methylbenzene. The ortho/meta/para system can quickly become confusing, however, as it requires the use of m'- and o'- identifiers for substituents in the C-5 and C-6 positions, respectively. For this reason, the numerical system, which would name the molecule 4-cyclohexyltoluene instead, is generally preferred.

Formation of Benzene Rings

The extra stability associated with the benzene ring makes its formation by dehydration and related reactions very easy. In fact, the ready formation of the benzene-ring structure often leads to undesired products during synthesis reactions. Various condensation reactions with aldehydes, ketones, and esters are often used to form six-membered ring structures in molecules, and subsequent dehydration reactions produce the conjugated double-bond system of the benzene ring.

Faraday originally isolated benzene by distilling coal tar. While this method is still used, the vast majority of benzene is derived from petrochemicals, most commonly via catalytic re-forming. In this method, various straight-chain hydrocarbons are vaporized and mixed with hydrogen gas before being exposed to a catalyst, typically platinum and aluminum oxide, under conditions of high heat and pressure. The process causes the hydrocarbons to re-form into arenes, after which the benzene is separated from the rest of the reaction products, again by distillation.

Applications of Benzene and Other Rings

Benzene is used as a solvent in many applications because of its ability to dissolve nonpolar compounds and its general inertness. However, benzene is also an electron-rich material due to the cloud of pi electrons on each "face" of the molecule, making it attractive to electrophiles (electron acceptors) such as Lewis acids. The prime examples of this electrophilic reactivity are the Friedel-Crafts alkylation and acylation reactions. In the Friedel-Crafts alkylation reaction, the presence of a Lewis acid catalyst can cause an alkyl or aryl halide to give up an alkyl or aryl group (a saturated hydrocarbon or arene derivative, respectively) to replace a hydrogen atom on the benzene ring. A similar substitution occurs in the Friedel-Crafts acylation reaction, with an acyl group (carboxylic acid derivative) from an acyl chloride or an acid anhydride replacing the hydrogen atom.

The electron-rich character of benzene and similar cyclic compounds also permits the formation of organometallic "sandwich" compounds, such as ferrocene and dibenzenechromium. In these compounds, the metal atom coordinates with the pi electrons of the cyclic system in such a way that the metal atom is sandwiched between two planar arenes. One of the more infamous such compounds is methylcyclopentadienyl manganese tricarbonyl (MMT), known as a "half-sandwich" compound because the manganese atom is only bonded to an arene on one side. MMT was used to replace the tetraethyllead in leaded gasoline until it became associated with various health hazards. US regulations still permit low levels of MMT in fuel, but it is no longer commonly used.

PRINCIPAL TERMS

• aromatic hydrocarbon: a hydrocarbon in which the carbon atoms form a ring with alternating double and single bonds, distributed in such a way that all bonds are of equal length and strength; also called an arene.
• cyclohexane: a saturated hydrocarbon composed of six methylene bridges (−CH2−) bonded in a six-membered ring structure; has the molecular formula C₆H₁₂.
• cyclopentane: a saturated hydrocarbon composed of five methylene bridges (−CH2−) bonded in a five-membered ring structure; has the molecular formula C₅H₁₀.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• 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.

Bibliography

Acheson, R. M. An Introduction to the Chemistry of Heterocyclic Compounds. 3rd ed. New York: Wiley, 1976. Print.

Hendrickson, James B., Donald J. Cram, and George S. Hammond. Organic Chemistry. 3rd ed. New York: McGraw, 1970. Print.

Herbert, Richard B. The Biosynthesis of Secondary Metabolites. 2nd ed. London: Chapman, 1989. Print.

Mann, J., et al. Natural Products: Their Chemistry and Biological Significance. New York: Wiley, 1994. Print.

Morrison, Robert Thornton, and Robert Neilson Boyd. Organic Chemistry. 6th ed. Englewood Cliffs: Prentice, 1992. Print.

O’Neil, Maryadele J., ed. The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals. 15th ed. Cambridge: RSC, 2013. Print.