HYDROCARBON

any of a class of organic chemical compounds composed only of the elements carbon and hydrogen. The carbon atoms join together to form the framework of the compound; the hydrogen atoms attach to them in many different configurations. Hydrocarbons are the principal constituents of petroleum and natural gas. They serve as fuels and lubricants as well as raw materials for the production of plastics, fibres, rubbers, solvents, explosives, and industrial chemicals. Many hydrocarbons occur in nature. In addition to making up fossil fuels, they are present in trees and plants, as, for example, in the form of pigments called carotenes that occur in carrots and green leaves. More than 98 percent of natural crude rubber is a hydrocarbon polymer, a chainlike molecule consisting of many units linked together. Hydrocarbons are insoluble in water and are less dense than water, so they float on its surface. They are usually soluble in one another, however, as well as in certain organic solvents. All hydrocarbons are combustible. If burned completely with sufficient oxygen, they produce carbon dioxide and water, releasing heat. If the oxygen is insufficient, the combustion yields mainly carbon monoxide. The structures and chemistry of individual hydrocarbons depend in large part on the types of chemical bonds that link the atoms of their constituent molecules. A carbon atom can form four single bonds, or it can form double or triple bonds. A hydrogen atom can form only one single bond. Hydrocarbons are divided into several classes according to their structure. The two major categories are aliphatic and aromatic. Aliphatic hydrocarbons can be composed of molecules in which the carbon atoms are linked in chains (called acyclic) or in rings (called alicyclic, or carbocyclic). Aliphatic hydrocarbons are also categorized according to the types of bonds between the carbon atoms. If the bonds are all single (called sigma [s] bonds), the compound is said to be saturated. Such compounds are classified as alkanes or cycloalkanes. If two or more bonds connect any two carbon atoms, the hydrocarbon is called unsaturated. The bonds may be double, as in the alkenes or alkadienes, or triple, as in the alkynes. Some compounds contain both types of multiple bonds in the same molecule. The simplest alkanes are methane (CH4, the most abundant hydrocarbon), ethane (CH3CH3), and propane (CH3CH2CH3). These three compounds exist in only one structure each. Higher members of the series, beginning with butane (CH3CH2CH2CH3), may be constructed in two different ways, depending on whether the carbon chain is straight or branched. Such compounds are called isomers; these are compounds with the same molecular formula but different arrangements of their atoms. As a result, they often have different chemical properties. Cycloalkanes are ring structures with two fewer hydrogen atoms in the molecule of the corresponding alkane. Many have more than one ring. Six-membered rings are of particular interest because they occur in many natural products, especially the steroids. Cyclic structures also can be isomers where two molecules differ only in the spatial arrangement of substituent groups. The main commercial sources of alkanes are petroleum and natural gas. Individual higher alkanes and cycloalkanes usually are synthesized by reactions designed for a specific product. These saturated hydrocarbons can also be synthesized from corresponding unsaturated molecules, by hydrogenation (addition of hydrogen). Saturated hydrocarbons are relatively inert; i.e., at room temperature they are unaffected by most acids, alkalies, and oxidizing or reducing agents. The unsaturated alkenes, also called olefins, have two fewer hydrogens per molecule than do the alkanes. Their most common structural feature is a carbon-carbon double bond. Generally there are more alkene isomers than alkane isomers. The first two members of the alkene series are ethene (ethylene), CH2=CH2, and propene (propylene), CH3CH=CH2. The double bond characteristic of the alkenes consists of a sigma component and a pi (p) component. The electrons of the pi component are weakly held by the positive nucleus and are thus a site of chemical reactivity, making alkenes much more reactive than alkanes. Alkenes with low molecular weights are produced commercially by cracking natural gas or petroleum (the breaking of their chemical bonds) or from mixtures of hydrocarbons derived from them. Ethylene, by far the most important alkene industrially, is used to make several products, including polyethylene, polystyrene, and ethylene oxide (used to make ethylene glycol antifreeze and various other products). Propylene and butene are also manufactured on a large scale and used to produce chemicals for solvents or starting materials. Alkenes are generally physically similar to alkanes or cycloalkanes with equal numbers of carbon atoms. However, while alkanes react mainly by substituting elements, alkenes react mainly by adding them. Alkenes are converted to alcohols by hydrogenation in the presence of an acid catalyst. Ethanol, for example, is produced this way from ethylene. Alcohols are also added to alkenes in the presence of acid to make ether from ethylene. An important alkene reaction takes place when one alkene molecule is added to the double bond of another. This process is used to synthesize polymers. Polymerization is, for example, the process by which ethylene is converted to polyethylene. Other unsaturated aliphatic hydrocarbons include the alkynes that are characterized by a carbon-carbon triple bond, which consists of one sigma component and two pi components. The simplest and commercially most important of the series is ethyne (acetylene), HCCH. Acetylene can be hydrated to make acetaldehyde, an important precursor of other chemicals, or added to hydrogen cyanide to produce acrylonitrile, a valuable monomer used to manufacture synthetic fibres. Alkynes also are useful as intermediates in organic chemistry. Compounds with more than one carbon-carbon double bond are called dienes or polyenes. They occur widely in nature and are commercially important. The physical properties of polyenes are similar to those of alkanes and alkenes with an equal number of carbon atoms. The outstanding characteristic is the intense colour of those which have alternating single and double bonds. The most important commercial diene is 1,3-butadiene, used to manufacture synthetic rubber. The aromatic hydrocarbons have distinctly different properties from the aliphatic compounds. Some of these substances have pleasant aromas, which accounts for their name. Most aromatic substances can be derived from benzene (C6H6), which is a six-ring hydrocarbon that possesses a special stability owing to the complete delocalization of its pi electrons. Aromatics are derived from benzene by replacing one or more of the hydrogen atoms with other atoms or groups. The bonds between the carbon atoms are neither single nor double but are of a type called a resonance hybrid. There are a number of nonbenzenoid aromatic compounds, but the benzenoid compounds form a more important class. Aromatic-hydrocarbon production is usually based on petroleum. Most are produced by catalytic hydrogenation of alkanes. The most characteristic reaction of aromatic hydrocarbons is substitutioni.e., the replacement of the hydrogens by other atoms or groups. The reaction may be repeated several times to produce polysubstituted compounds. There are many aromatic hydrocarbons with more than one benzene ring. Commercially important aromatic hydrocarbons have fused or condensed rings in which two or more carbon atoms are shared by several rings. Most condensed hydrocarbons are crystalline solids. Many are present in coal tar, of which naphthalene is the most common. Fused-ring systems appear in many synthetic dyes and in numerous natural products, including the steroid hormones.