Many types of cosmetics,

including theatrical make­up, contain ethylene glycols. These substances can be synthesized from the alkene ethene (ethylene, C₂H₄).

Red oil dropletsare float­ing off a fiber strand as a re­sult of the action of a deter­gent in the liquid. Many detergents are synthesized using long-chain alkenes (having between 4 and 20 carbon atoms) as starting materials.

Benzeneis the simplest ar­omatic compound. It is usu­ally the presence of at least one benzene ring that char­acterizes other compounds as aromatic. The structure of benzene is unusual. Instead of having alternate long sin­gle and shorter double carbon-carbon bonds in the six-membered ring, all the bonds are of equal length. The "spare" pi-electrons (that would otherwise be needed for the double bonds) are spread evenly round the ring. These pi-electrons comprise two doughnut-shaped clouds above and below it. Repre­sentations of the structure include the old Kekule for­mula (A) and the modern Robinson formula (B).

Aromatic hydrocarbons

In oil refineries,such as the one shown below, a combination of fractional distillation and selective dis­solving techniques is used to isolate aromatic hydro­carbons from their sources. Fossil fuels, principally oil and coal, are the chief sources of aromatic hydro­carbons.

All aromatic hydrocarbons are composed solely of carbon atoms and hydrogen atoms in various arrangements. What sets most of them apart from other hydrocarbons is the presence in their molecular structures of at least one benzene ring.

A benzene ring consists of six carbon atoms linked together in a planar hexagonal struc­ture. This is a six-sided (hexagonal) structure, all of which is in one plane (planar). Each car­bon atom of the benzene ring is bonded to a single hydrogen atom. The ability of chemists to replace these hydrogen atoms with other

groupings has given rise to the vast number of aromatic hydrocarbons we know today. Many also occur naturally in the coal and oil depos­its that lie beneath the earth's crust. Others are widespread in nature itself.

Some of the most commonly used aromatic hydrocarbons include toluene and the xylenes. Every year chemists isolate millions of tons of these compounds from petroleum (crude oil) and, to a lesser extent, from coal. They are im­portant industrially, especially as solvents. A solvent is a substance that is able to dissolve other substances. For example, gasoline is a solvent liquid able to dissolve grease spots. Aromatic hydrocarbons are also the starting materials for the manufacture of plastics. They are also used by laboratory chemists for con­version into other, more valuable compounds.

Benzene

The simplest aromatic compound is benzene itself, the building block from which aromatic chemistry sprang. Known since 1825, benzene is a colorless liquid at room temperature. It is 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. A feedstock is a principal material in any chemical process that produces petroleum products. There are various techniques for separating benzene and other aromatic hydro­carbons from coal, tar, and petrochemical feedstocks. These techniques include distilla­tion (purifying) methods and the use of sol­vents to dissolve the required products selec­tively.

In the second half of the nineteenth cen­tury, the potential importance of products ob­tained from benzene became evident. Scien­tists noted the presence of the benzene nucleus in many naturally-occurring drugs and dyes. They then searched for ways to make them synthetically. But chemists investigating

Organic chemistry: Aromatic hydrocarbons 75

benzene were puzzled by its structure until as late as the 1920s. Many models were put for­ward to explain its molecular makeup. Nowa­days, its structure has been rationalized using the concept of delocalized electrons.

Structures of most organic molecules are drawn in terms of single, double, or triple bonds between carbon atoms, each atom pos­sessing the ability to form four bonds. But the properties and reactions of benzene do not conform with those of any single structure drawn in this conventional way. Instead, the six carbon atoms in benzene are considered to be held together by two-electron (single) bonds of equal length. The third electron in each car­bon atom forms the bond with hydrogen. The fourth electron in each carbon atom (a total of six in the entire benzene ring) is not bonded with other electrons. These six electrons, which would otherwise be required for three double bonds, are evenly distributed around the hexagonal ring. These six delocalized elec­trons, known as pi-electrons, give rise to a doughnut-shaped electron cloud. This cloud is distributed above and below the plane of the ring atoms. As a result, all of the bonds be­tween carbon atoms in benzene are the same length. They are shorter than a carbon-carbon single bond in a simple aliphatic (chainlike) compound, but longer than an ordinary carbon-carbon double bond.

The greater mobility of the pi-electrons ac­counts for the unique properties of benzene and its derivatives. For example, the reaction between bromine and a simple aliphatic al-kene, such as ethene (ethylene), is an addition reaction. The molecule of bromine attaches (adds) itself to the ethene. The reaction be­tween bromine and benzene, however, is a substitution. One of the hydrogen atoms is re­placed by a bromine atom.

Since its discovery, benzene has fulfilled

many roles industrially. It is an excellent sol­vent. It can also be converted into a vast range of derivatives—products derived (obtained) from a source—in this case, benzene. Styrene, a compound formed by adding benzene to ethene, helps in the making of polystyrene. Many other plastics, rubbers, and resins are built 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 (for making dyes, certain medicines, and plastics), benzaidehyde (used in flavoring agents), and benzoic acid (used as an antiseptic and as a preservative).

Despite its undoubted usefulness, benzene must be treated with caution because it may cause cancer. As such, it has been banned from the classroom in most Western coun­tries. It is used with great care in the labora­tory and in industry.

Benzaidehyde CHO

This unusual buildingis a

house made almost entirely of polyurethane foam, a polymer derived from tolu­ene. To build the house, a thick layer of the foam was sprayed on the inside of large hemispherical molds. The molds were removed after the foam had hard­ened. The exterior surface was then painted with sun­screen paint to block ultra­violet light, which would otherwise cause the poly­urethane to break down. The interior surface was covered with a fire-proofing material, because polyure­thane gives off poisonous fumes when burned.

76 Organic chemistry: Aromatic hydrocarbons

Toluene

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, able to dissolve other substances. 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), antisep­tics, dyes, and perfumes. It is possible to con­vert toluene into a vast number of other deriv­atives. The major sources for toluene are coal tar and naphtha. It is extracted in the same way as benzene.

Xylenes

These are liquids at room temperature. They

are similar to toluene in structure, except that a further 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. They are all 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 Terylene (Dacron). Again, they can be pre­pared from tars or naphtha using the same techniques as those for preparing benzene and toluene.

Date: 2015-12-11; view: 1887