Aromatics

• Type of physical science: Chemistry
• Field of study: Chemical compounds

Aromatic compounds are cyclic, carbon-based compounds that are unsaturated, although they do not react like most unsaturated compounds. Benzene (C₆H₆) and related aromatics react with certain substances to produce more complicated aromatic structures, some of which are useful in living organisms, whereas others are mutagenic.

Overview

Chemical compounds are those substances that contain different types of elements. Of the millions of existing chemical compounds, two principal classes emerge: inorganic compounds and organic compounds. Inorganic compounds include those noncarbon compounds that are rarely found in living organisms. Organic compounds are those compounds that are usually found within living organisms. Of ninety-two naturally occurring elements, six (carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus) compose 98 to 99 percent of all living organisms, from viruses to humans. Many organic compounds are also called hydrocarbons because carbon and hydrogen are the two principal elements forming the structures of these compounds, although oxygen, nitrogen, sulfur, and phosphorus do play very important secondary roles in organic structures.

Hydrocarbons include a variety of carbon-based compounds, including alkanes, alkenes, alkynes, alcohols, ethers, aldehydes, ketones, amines, aromatics, alkaloids, and carboxylic acids. The primary elements in each of these compounds are carbon and hydrogen.

Alkanes include variable-length, carbon-backboned compounds having the basic molecular formula C_nH_2n+2, such as methane (CH₄), ethane (C₂H₆), and propane (C₃H₈). Alkenes essentially are alkanes that contain a double bond or bonds in the carbon-carbon backbone chain and that tend to have the molecular formula C_nH_2n, such as ethene (C₂H₄) and propene (C₃H₆). Alkynes essentially are triple-bonded alkanes having the molecular formula C_nH_2n-2, such as ethyne (C₂H₂) and propyne (C₃H₄). The remaining hydrocarbon classes are variations upon alkanes and alkenes, with the addition of nitrogen or oxygen or with the cyclization (circular structure formation) of the carbon-carbon chain. For all these molecules, carbon forms exactly four covalent (shared electron) bonds.

Because of the covalent bonding patterns between carbon atoms of the carbon-carbon chains, hydrocarbons can be saturated or unsaturated. If a hydrocarbon is saturated, all the electrons of the carbon atoms of the compound are tied up in covalent bonds to hydrogen and adjacent carbons. All the covalent carbon-carbon bonds in a saturated compound are single covalent bonds. All alkanes are saturated. Alkenes and alkynes, however, are unsaturated. Some of the electrons of some carbons in these compounds are free and unbound to hydrogen. These electrons end up double-bonded between adjacent carbons in the hydrocarbon chain of alkenes or triple-bonded in alkynes. Such unsaturated hydrocarbons tend to undergo addition reactions, in which they bond to atoms of other substances without losing any atoms.

A special class of unsaturated hydrocarbons is the aromatic hydrocarbons, so named because of their distinct scents. Aromatic hydrocarbons are unsaturated (double-bonded) cyclical compounds (atoms are covalently bonded in a circle). Unlike most unsaturated hydrocarbons, the aromatic hydrocarbons do not undergo additional chemical reactions. Instead, they undergo substitution reactions, in which a compound gains a new atom at the expense of losing one of its own atoms, usually a hydrogen. Aromatics represent a unique hydrocarbon class. The other nonaromatic hydrocarbons are termed “aliphatic compounds” and include alkanes, alkenes, and ethers, among others.

The English physicist and chemist Michael Faraday, who would become famous for his contributions to electricity, discovered the first aromatic compound in 1825. This compound was called benzene, and its structure later would prove to be central to the structure of most aromatic hydrocarbons. Benzene was later discovered to have the molecular structure C₆H₆ and to be cyclic in its carbon arrangement.

Because of its equal numbers of carbons and hydrogens, benzene is highly unsaturated.

The German chemist Friedrich August Kekulé proposed the correct structure for benzene in 1865:

Benzene’s six carbons form a hexagonal structure with alternating single and double covalent bonds. Each carbon is singly bonded to a hydrogen, giving each carbon a total of four covalent bonds, in accordance with valence theory; however, a mirror-image resonance structure of benzene is also possible, in which the electron arrangement reverses the double-bonding pattern. Benzene’s actual structure is probably an intermediate between its two alternative resonance structures, with approximately one and one-half covalent bond energies existing between each of the benzene carbons.

Numerous benzene-related compounds exist, including the multiple benzene ring compounds naphthalene (which forms an ingredient of mothballs) and anthracene. Benzene can also exchange hydrogen atoms for other atoms or molecular groups via substitution chemical reactions. Among commonly substituted atoms are fluorine, chlorine, bromine, and iodine.

Commonly substituted molecular groups include methyl (-CH₃), nitro (-NO₂), amino (-NH₂), and hydroxyl (-OH) groups:

When two molecular groups or atoms are covalently bonded to a benzene ring, the resulting compound is named according to the position of the two groups relative to one another.

Two groups that are bound to adjacent carbons (that is, carbon #1 and carbon #2) are in the ortho-configuration. Two groups that are located on alternate carbons (that is, carbon #1 and carbon #3) are in the meta-configuration. Two groups that are located at opposite carbons (that is, carbon #1 and carbon #4) are in the para-configuration:

When three or more molecular groups are covalently bonded to carbons of the benzene ring, only carbon numbers are indicated (for example, 1,2,4-trifluorobenzene). Other nonbenzene aromatic hydrocarbons exist as well. The heterocyclic aromatics contain carbon plus one or two additional elements in the ring structure of the compound. An example is pyridine, which contains nitrogen.

Aromatic hydrocarbons can be structurally defined by their alternating double-bonded resonance structures and by the quantum mechanical arrangement of electrons in their carbon-carbon single and double covalent bonds. The resonance structures of benzene represent a flip-flop rearrangement of electrons from one conformation to another. The double bonds between alternating carbon pairs can shift by one pair position. The actual benzene structure is a hybrid between the two resonance structures, with one and one-half bonds joining each carbon.

Therefore, aromatic hydrocarbons are hybrids of their various resonance structures.

The German physicist Erich Hückel described the quantum mechanical state of the electrons of benzene in 1931. Benzene has six delocalized π electrons, electrons that are free and unshared with any hydrogen atoms. These six free electrons pair to produce three π electronic orbitals and, therefore, alternating double bonds that give benzene an extremely stable structure.

Hückel studied the π electronic bonding patterns of aromatic and aliphatic unsaturated compounds. He discovered that aromaticity exists when the compound in question has 4n + 2 π electrons. Aromatic hydrocarbons can have 2, 6, 10, 14, 18, 22, or 26 (and so on) delocalized π electrons in their overall structures. The π electronic formula 4n + 2 is known as Hückel’s rule for aromatic hydrocarbons.

The π electron arrangement of aromatic hydrocarbons explains both their high stability and their unique chemical reaction properties. Aromatic hydrocarbons are modified, or pick up atoms, by substitution reactions, in which they exchange a hydrogen atom for an atom of some other element:

Aromatic hydrocarbons do not add atoms of other elements because of the stability generated by their delocalized π electrons. Instead, an exchange must occur between available atoms of the two interacting compounds/molecules.

Aromatic hydrocarbons are subject to considerable modification by substitution reactions. Many varieties of aromatics and aromatically derived compounds exist in nature and in living organisms. Several natural and synthetic polymers (long-chain compounds) incorporate benzene ring subunits. Aromatics are found within the structures of amines, sulfanilamides (for example, sulfa drugs), alkaloids, polyester, certain fatty acids, and certain amino acids found in proteins.

In DNA (deoxyribonucleic acid) and in RNA (ribonucleic acid), nitrogen-containing bases are heterocyclic aromatic compounds found in the nucleotides that store genetic information in living cells. The two major classes of nitrogen-containing bases are the purines (for example, adenine, guanine) and the pyrimidines (for example, cytosine, thymine, uracil). The information content within the sequence of DNA nitrogen bases constituting a gene encodes messenger RNA, which encodes the amino acid sequence of a protein. Every characteristic of every living organism is determined by the order of these nitrogen bases in DNA.

Benzene and related aromatic hydrocarbons, however, can be inserted into the DNA of living cells, thereby replacing the correct nitrogen base and altering the amino acid sequence of the corresponding protein, causing a mutation. Such DNA-altering compounds are called mutagens. The effect of such an aromatically induced mutation could be very serious for the organism, resulting in cancer and death if the affected protein is critical for proper cell functioning. Several benzene-related mutagens, such as proflavin, also are carcinogens (cancer producers).

Applications

Whereas aromatic compounds are important structural and information components of some biological molecules, they also exist naturally in the environment. They can be synthesized in the laboratory. Aromatic hydrocarbons and their derivatives are found in solvents, dyes, rubber, synthetic polymers, cigarettes, drugs, cosmetics, and pesticides. Because of the mutagenic effects of some aromatics, these substances must be handled and disposed of with care. Some aromatics are carefully regulated by the United States Food and Drug Administration, the Department of Agriculture, the Department of Transportation, or the Environmental Protection Agency.

Benzene has been used as a solvent, a solution in which certain substances are dissolved, and is a common by-product of many industrial reactions. Two benzene derivatives, 2-naphthylamine and benzidine, are used in the manufacture of chemical dyes. Styrene is an aromatic-ethylene derivative that, when polymerized, is used as an insulating material (polystyrene) for buildings, coolers, and so forth. Terephthalic acid is an aromatic derivative that, when combined with ethylene and subsequently polymerized, becomes polyester (for example, Dacron), which is used extensively for clothing, other textile products, and food packaging. Aromatic polymer additives include diacetylphthalate, which improves the flexibility of plastics, and phenylsalicylate, which protects plastics from light damage (for example, from ultraviolet light).

The benzene derivative benzo(a)pyrene is produced from the combustion (burning) of several substances, including coal and cigarettes. It is a potent carcinogen. Benzene, 2-naphthylamine, and benzidine are all carcinogens as well. Many benzene-related compounds have been associated with cancer.

Aromatically derived compounds are useful in medicine for combating bacterial and viral infections. These aromatic drugs usually work by blocking the metabolism of the infectious agent, although the larger host organism, that is, the patient, can be affected to a certain extent as well. Aromatic antimicrobial drugs interfere with the replication and expression of DNA or with the biosynthesis of molecules and energy. For example, the aromatic derivative methotrexate is given to cancer patients to replace folic acid in the body; the resulting lack of needed folic acid in the body slows cancer cell growth. The sulfa drugs, which destroy numerous bacterial pathogens (for example, pneumonia and gonorrhea), are aromatica derived compounds. Furthermore, antibiotics are antibacterial, aromatically derived compounds that are produced naturally by certain fungi. Among many such antibiotics used to treat infections are penicillin, streptomycin, tetracycline, and cephalosporin.

Various neurological drugs (for example, analgesics, stimulants, and depressants) such as cocaine, novocaine, phenobarbital, methamphetamine, aspirin, and Tylenol contain aromatic structures. Aromatic hydrocarbons are also used in the cosmetic industry.

Benzophenone is an important ingredient of most sunscreen/sunblock skin lotions. Various perfumes and hair dyes also contain aromatic hydrocarbons.

Various pesticides and poisons contain aromatic structures. The herbicides 2,4-dichlorophenoxyacetic acid, paraquat, and atrazine are aromatics. Insecticides such as DDT (dichloro-diphenyl-trichloroethane) and parathion also incorporate aromatic structures.

While many of these aromatic compounds are found in living organisms or are produced in industry, many have mutagenic and/or toxic effects on the cells of living organisms. Aromatics are used extensively in industry and even in household chemicals and appliances. Those aromatics that are mutagenic cause changes in the hereditary DNA molecule of living cells. These mutations may elicit cancer and/or cell death in somatic body cells or may be passed along to future generations of individuals via germ-line cells (sperm and egg). Caution should be used when one is working with some aromatics, although many aromatics are not dangerous.

Context

Aromatic molecules are found throughout nature. They are found in and produced by living organisms for a variety of purposes. Some are toxic and/or mutagenic, causing damage to living tissue. Because of their extensive use by humans in such items as drugs, clothing, cosmetics, and pesticides, an understanding of aromatic hydrocarbons is very important. There still remain some aspects of aromatic compounds that are not well understood.

Benzene is an extremely stable, cyclic, unsaturated hydrocarbon. Its stability lies in its delocalized π-electron bonds. This stringent bonding pattern, equally distributed around the ring structure, explains the resistance of benzene to undergoing many chemical reactions, particularly addition reactions. The delocalized π electrons are very unreactive. Therefore, benzene primarily undergoes substitution reactions, in which atoms are exchanged between molecules.

The high stability and unique reactivity of benzene and some other aromatics, plus their structural similarity to the nitrogen bases of DNA, make them potential mutagens and carcinogens. Mutagenic aromatics can mimic the nitrogen bases of DNA, thereby replacing some bases and disrupting the DNA coding pattern for a specific gene’s protein product. Many industrial processes generate mutagenic aromatics. The potential impact of aromatics on human health may be immeasurable.

Bruce Ames of the University of California at Berkeley developed a simple experimental test, the Ames mutagenicity test, to identify potential mutagens. A suspected substance is applied to mutant Salmonella typhimurium bacteria. If the substance is mutagenic, it should cause reverse mutations in the already mutant Salmonella genes.

Hundreds of substances have been classified as being mutagenic from this test, including several aromatic compounds.

Aromatic compounds, while identifiable by scent, have characteristic patterns of absorption for various types of electromagnetic radiation. Each aromatic has its own characteristic absorption pattern for ultraviolet and infrared light. For example, benzene absorbs ultraviolet light of 185-, 200-, and 255-nanometer wavelengths.

Much research is being applied toward the development of new aromatic compounds for use in medicine, manufacturing, and agriculture. The high stability and substitution reactivity of these compounds make them very versatile molecules for numerous processes. While some aromatics are mutagens, many others are harmless and beneficial. From neurotransmitter molecules in the human brain to clothing, aromatic hydrocarbons permeate every aspect of daily life.

Principal terms

ADDITION REACTION: a chemical reaction in which all the atoms of a molecule react with and become part of another molecule

ALIPHATIC COMPOUND: a nonaromatic organic compound (a hydrocarbon), such as saturated and unsaturated alkanes, alkenes, and alkynes

AROMATIC COMPOUND: a cyclic compound, such as benzene, with unusual stability from delocalized π electrons and a tendency to undergo substitution reactions

BENZENE: one of the first aromatic compounds discovered; the basis of most aromatic structures

HETEROCYCLIC AROMATIC: an aromatic compound that contains an atom or atoms of elements other than carbon in the cyclic ring structure

HUCKEL’S RULE: a quantum mechanical description of aromatic compounds in which aromatics are classified as cyclic compounds having 4n + 2 delocalized π electrons (that is, 2, 6, 10, 14, 18,… π electrons)

RESONANCE STRUCTURE: two or more almost identical chemical structures that differ from one another only in the positions and bonding patterns of their electrons

SATURATED: a condition in which a hydrocarbon compound has no free electrons; all electrons are shared in single covalent bonds between atoms of the compound

UNSATURATED: a condition in which a hydrocarbon compound has free electrons that pair in double or triple bonds between atoms of the compound

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