All aromatic hydrocarbons are composed solely of carbon atoms and hydrogen atoms in various arrangements. What sets them apart from other hydrocarbons is the presence of at least one benzene ring in their molecular structures.
A benzene ring consists of six carbon atoms linked together in a six-sided, two-dimensional structure. Each carbon atom of the ring is bonded to a single hydrogen atom. Chemists can replace these hydrogen atoms with other groupings, creating the vast number of aromatic hydrocarbons known today. Many also ‘ occur naturally in deposits of coal and oil.
Toluene and the xylenes are among the most useful aromatic hydrocarbons. Every year, chemists isolate millions of tons of these compounds from petroleum and coal. They are important industrially, especially as solvents and as the starting materials for the manufacture of plastics. Aromatic hydrocarbons are also converted into other, more valuable compounds.
The simplest aromatic compound is benzene itself, the building block from which aromatic chemistry developed. Known since 1825, benzene is a colorless liquid at room temperature, one of the chief ingredients in coal tar. It can also be isolated from petrochemical feedstocks, such as naphtha. Naphtha, in turn, is derived from petroleum, coal tar, or natural gas. Benzene and other aromatic hydrocarbons are separated from coal, tar, and petrochemical feedstocks by various techniques, such as distillation (purifying) and the use of solvents.
The potential importance of products obtained from benzene became evident late in the 1800’s, when scientists noted the presence of the benzene nucleus in many naturally occurring drugs and dyes. They then searched for ways to make these products synthetically.
Since its discovery, benzene has fulfilled many roles in industry. It is an excellent solvent. It can also be converted into a vast range of derivative products. Styrene, a compound formed by adding benzene to ethene, is the monomen for polystyrene. Many other plastics, rubbers, and resins are manufactured from benzene derivatives. It is also the basis of many substances essential to the synthetic chemist. Among the better-known benzene derivatives are nitrobenzene (used for making perfumes); aniline (used in dyes, certain medicines, and plastics); benzaldehyde (used in flavoring agents); and benzoic acid (used as an antiseptic and preservative). But, despite its great usefulness, benzene must be treated with caution because it may cause cancer.
A liquid at room temperature, toluene is identical to benzene except that one of the hydrogen atoms has been replaced with a methyl group. A methyl group is a molecule of methane without one of its hydrogen atoms. Thus, toluene is also called methylbenzene. Like benzene, from which it is easily derived, toluene is a good solvent It is also used as a starting material for the high explosive TNT (trinitrotoluene) and for assorted plastics. In addition, toluene is used in the manufacture of preservatives (for cosmetics, beverages, and food), as well as in antiseptics, dyes, and perfumes. Toluene can be converted into a vast number of other derivatives. It is obtained from coal tar and naphtha in the same way as benzene.
These are also liquids at room temperature and similar to toluene in structure, except that an additional hydrogen atom has been displaced from the ring by a methyl group. Thus, they are also called dimethylbenzenes. Three types of xylene exist: orthoxylene, metaxylene, and paraxylene, depending on the positions of the two methyl groups about the ring. All are useful solvents and are starting materials in the production of some types of products obtained from benzene. For example, paraxylene is the starting material for the synthetic fiber Dacron. Xylenes can be prepared from tars or naphtha in the same way as benzene and toluene.
Condensed aromatic hydrocarbons
Benzene rings can be fused together to form a group of compounds called condensed-ring systems. The simplest of these is naphthalene, which consists of two benzene rings joined together. Other common condensed-ring hydrocarbons include anthracene, which has three benzene rings fused together, and pyrene, which has four fused rings. These and many much larger condensed hydrocarbons are extracted from coal tar.
The best-known and simplest condensed-ring hydrocarbon is naphthalene, a solid at room temperature. Anthracene is also a solid at room temperature. Both can be isolated from coal tar.
Products of both substances are used in the manufacture of dyestuffs and in the production of plastics and polyester resins. These resins are useful in the manufacture of paints, film, and synthetic fibers. Certain compounds of naphthalene and anthracene, however, have been banned in many countries because they cause cancer.
Coal carbonization is the major source of anthracene, naphthalene, and most other condensed aromatics. In the absence of air, coal is heated to a temperature of about 1650° F. (900° C), causing it to break down into coal gas, coal tar, and coke. Coal gas is used as fuel gas, coke is used in making steel, and coal tar contains many organic compounds, including condensed hydrocarbons. The most important compounds—benzene, naphthalene, and anthracene—are extracted using a combination of different techniques.
Today, coal gas has largely given way to natural gas as a domestic heating fuel, and less coke is required to make steel because blast furnaces are more efficient As a result, coal carbonization is declining. However, other sources of condensed hydrocarbons are available.
All condensed-ring hydrocarbons have an unfortunate characteristic: They cause cancer. Benzfajpyrene is the most dangerous in this respect
Condensed-ring hydrocarbons are released when organic materials are heated to high temperatures. This happens, for example, in the carbonization of coal, when it is burned at very high temperatures. These compounds have been linked to especially high incidences of skin cancers among people who work with coal tar.
Cigarette smokers also suffer the effects of poisoning from condensed-ring compounds. These compounds are in the tar deposited in smokers’ lungs. Doctors have long known about the link between heavy smoking and lung cancer.