The saturated aliphatic hydrocarbons form a series of organic molecules that contain only hydrogen and carbon, hence the term hydrocarbon. Each carbon atom is linked to four other atoms—the maximum number possible—and is therefore saturated, or completely full. In every member of the series, the molecules are arranged in the shape of straight or branched chains with open ends (aliphatic) rather than in closed rings. Formerly known as paraffins, the members of this series are now called alkanes.
The first member of the alkane series—and the simplest organic compound—is methane, the chief component of natural gas. A molecule of methane consists of a single carbon atom linked to four atoms of hydrogen. Each of its chemical bonds involves two atoms—the carbon atom and one of the hydrogen atoms. Each bond consists of a pair of electrons, one provided by each atom. The removal of one hydrogen atom from methane forms a methyl group. Because of the ability of carbon atoms to form chains, two methyl groups can join to form a compound that consists of two carbon atoms and six hydrogen atoms. This substance, called ethane, is the second member of the alkane series.
Just as ethane can be derived from methane, so can propane be derived from ethane. If one of the hydrogen atoms from the ethane molecule is replaced by a methyl group, a chain of three carbon atoms with eight hydrogen atoms is formed. This is propane. Its middle carbon atom, being linked to two other carbon atoms, can bond to only two hydroqen atoms.
Alkane production and uses
The first few members of the alkanes, from methane to butane, have important domestic and industrial uses. They are obtained chiefly from fossil fuels, mainly natural gas. The ionger-chain alkanes—gasoline, kerosene, oils, greases, waxes, and bitumen—are obtained by distilling (purifying) petroleum.
Methane is used mainly as a fuel, although it is also important in the manufacture of other chemicals, such as methanol. Ethane is used mainly as a chemical feedstock to make ethene (ethylene). Ethene itself can be converted into other useful substances—such as the plastic polyethylene. Propane and butane can easily be compressed into liquids, stored in pressurized containers, and used as a portable fuel supply. They are also used to make other chemicals, such as ethene and propene (propylene).
Syngas and biogas
At the present rate of consumption, known reserves of natural gas may last only until about the year 2030, so chemists have been working on ways of treating coal to make a synthetic gas called syngas. (Reserves of coal are large enough to last several hundred years.l When any carbon-containing material is burned in a limited amount of air, toxic carbon monoxide forms. This gas can be reacted with hydrogen to produce methane. In this and other ways, syngas can be made from coal and other organic material, but the gas is still too expensive.
Methane is also a by-product of the metabolism of some living organisms. In parts of Asia, farmers build “digesters” that make methane from waste organic matter, such as animal dung. In the absence of air, bacteria feed on the waste and give off methane, which is used for heating and lighting in small, rural villages. Today, such biogas systems are also being investigated in developed countries. They may provide a productive means of disposing of large amounts of waste from the intensive raising of livestock.
Hydrocarbons are relatively unreactive compared with other substances, but all of them oxidize, readily combining with oxygen in a process known as combustion. For this reason they have become important in applications that depend on their ability to burn.
They undergo other types of reactions as well, including halogenation (combining with one of the halogens—fluorine, chlorine, bromine, or iodine) and nitration (in which a nitrogen-oxygen group replaces a hydrogen atom).
Under extreme conditions, hydrocarbons will isomerize— break down partially and then recombine into different hydrocarbon isomers. This is the basis of the process used to increase the octane number of gasoline. The octane number is based on the combustion properties of an octane isomer which burns smoothly (2,2,4=trimethyl pantane), which is assigned a value of 100. Most gasoline used today has an octane number between 91 and 93.
As the molecular weight of hydrocarbons increases, they become larger and heavier and change from gases to liquids to solids at room temperature. Petroleum jelly—a soft, solid hydrocarbon mixture obtained from crude oil—is used as an ointment to protect wounds and sores from bacteria. Higher-molecular-weight hydrocarbons form hard waxes, used as protective coatings for floors and furniture or as slow-burning candle wax.
In the simplest hydrocarbons, such as most saturated alkanes, carbon atoms are joined together in a chain. Because of the unique ability of carbon atoms to combine with one another, it is possible for the two ends of a chain to be linked. This forms a “ring,” or cyclic compound (sometimes called alicyclic).
The simplest hydrocarbon that can link together in this way is one containing three carbon atoms. A chain hydrocarbon of this type is called propane, or, in the form of a ring, cyclopropane. Cyclopropane is highly reactive because of the great strain placed on its chemical bonds by the ring structure. As the number of atoms in the ring increases, the strain lessens. Cyclobutane (with four carbon atoms) is more stable than cyclopropane, but less stable than cyclopentane (with five carbon atoms). The six-carbon ring compound, cyclohexane, is very stable and is used as a chemical solvent Many of the cyclic hydrocarbons have properties similar to those of straight-chain hydrocarbons. Thus, cyclohexane and normal hexane are both colorless liquids at room temperature, flammable, and will not mix with water.
Many of the cyclic hydrocarbons are found in crude oil. Some cyclohexane is obtained commercially from distilling gasoline, but most is obtained by treating benzene with hydrogen.
Long-chain hydrocarbons: polymers
Very large hydrocarbon molecules called polymers can be made with chains of hundreds or even thousands of carbon atoms. Polymeric materials are composed of very large molecules (macromolecules) formed by linking together smaller, simpler molecular units called monomers. The process of linking monomers to create polymers is called polymerization.
There can be as few as five or as many as several thousand monomer units in a polymer. Typical examples of polymers are plastics such as polyethylene film, a transparent material used in packaging; polyurethane foam, made into cushions and mattresses; and nylon and polyester fibers, used in textiles. Synthetic resins for paints and adhesives are also polymers. Rubber is a natural polymer isolated from trees native to South America and grown now in Asia, but more than half the rubber used today is synthetic. Other natural polymers include various protein substances; plant carbohydrates, such as starch and cellulose; and genetic material such as DNAand RNA.
Scientists have a broad range of monomers at their disposal. They can modify the monomers and the polymerization conditions, making synthetic polymers that suit particular needs and exhibit special properties. These can also vary depending on the number of monomer units in each macromolecule and the way they link together. Branched-chain polymers are normally stronger than straight-chain polymers, because of the extra linkages between chains. Polymers can also respond differently to the action of heat. Thermoplastic polymers can be molded again if remelted, whereas thermosetting polymers harden permanently once molded.