Hydrocarbons, the simplest class of organic chemical compounds, are composed of the elements carbon and hydrogen only. Because they are the principal constituents of petroleum and natural gas, and thus serve as fuels and lubricants, hydrocarbons are tremendously important economically. They are also the raw materials for the production of plastics, fibers, rubbers, solvents, explosives, pharmaceuticals, and many other industrial chemicals. All hydrocarbons are combustible, and, when burned with sufficient oxygen, produce carbon dioxide, water, and heat. Due to the remarkable ability of carbon atoms, unlike those of any other element, to bond to other carbon atoms, in chains, rings and networks of many atoms, there are an enormous number of different hydrocarbon molecules. There are four major types of hydrocarbons: alkanes, alkenes, alkynes, and aromatics. They differ by ratio of hydrogen to carbon atoms in the molecule, and by the number and types of bonds between the carbon atoms. The "valence" or connecting power of carbon is four, and a carbon atom can be connected, or "bonded," to four other atoms, as in methane, CH4. The central carbon atom is connected to each hydrogen atom by a "single" bond. The true shape of the methane molecule is "tetrahedral," a three dimensional structure in which each hydrogen is equidistant from the other three hydrogen atoms, and the HóCóH angle is 109E 28". Whenever a carbon atom is connected to four other atoms, the carbon always approximates this kind of shape. Carbon atoms can be connected in chains. Such structures can be made of literally thousands of CóC bonds: each carbon atom is always connected to four other atoms, either to 1 or more other carbon atoms, or to hydrogen atoms. Such hydrocarbon molecules are called "alkanes" (or "cycloalkanes" if in a ring.). Chemists have developed a remarkable naming ("nomenclature") system (the IUPAC system) that attaches a unique name to each possible structure, as shown. Names are composed of a syllable that denotes the number of carbon atoms in the longest chain (meth = 1, eth = 2, prop = 3, but = 4, pent = 5, hex = 6, etc.) and a syllable denoting special bonding features of the molecule (ane = all single bonds; ene = a double bond; yne = a triple bond). Alkenes, the second kind of hydrocarbon, contain at least two carbon atoms linked by two bonds: a "double bond." The remaining bonds are to hydrogen atoms. The introduction of a double bond affects the shape of the molecule greatly. All atoms that are connected to the six bonds of the two doubly bonded carbon atoms lie in the same plane; that portion of the molecule is flat. Alkynes, the third kind of hydrocarbon, contain at least two carbon atoms connected by a "triple" bond. The atoms directly connected to the two triply bonded carbon atoms are forced into a straight line. This feature restricts the size of the ring which can contain a triple bond to nine or larger carbon atoms. Aromatics are hydrocarbons that contain at least one "benzene" ring. The particular arrangement of double and single bonds connecting the six carbon atoms of benzene give it special properties that are different from those of any other alkene. There is only one way to connect two or three carbon atoms (barring rings). However, like a four-link chain, there are two ways to connect four carbon atoms. Called "isomers" (same units), such compounds have the same molecular formula but different arrangements (connectivity) of their atoms, and have different chemical properties. Butane is called a "straight chain" alkane; isobutane is a "branched chain" alkane. There are 3 ways to connect 5 carbon atoms, 5 ways to connect 6, 9 ways to connect 7: the number of isomers rapidly increases with the number of carbons, hence the enormous number of possible organic structures. An important consequence of the "tetrahedral" shape of carbon atoms is the existence of "stereoisomers." Any molecule that has an "asymmetric carbon," (in brackets here) that is, one that has four different things attached, can exist in two, mirror-image forms. They are identical except for the way the atoms are arranged in space. Alkanes are very unreactive, except with oxygen and the halogens. With oxygen they burn. With halogens, they undergo a "substitution" reaction, in which a halogen atom replaces a hydrogen atom. Alkenes are much more reactive. Halogens, water (in the presence acids), and hydrogen (in the presence of a catalyst, such as finely divided platinum) add to the double bond. The reaction with hydrogen produces an alkane; the double bond is said to be "saturated" by the addition of hydrogen, all carbons are now singly bonded to either carbon or hydrogen. A catalyst is a substance that makes the reaction go faster, but is not itself changed. The alkyne triple bond acts as if it were two double bonds, and can add one or two molecules of halogen, for instance. The simplest and commercially most important alkyne is ethyne (acetylene), HC/CH. Aromatic hydrocarbons, despite the presence of double bonds, do not act like alkenes. They undergo a substitution reaction with halogens, like alkanes, but only in the presence of a catalyst such as ferric chloride. 1,4-dichlorobenzene is used in mothballs. Complex structures may be assembled which include all types of bonds and structures. Compounds with two carbon-carbon double bonds are called dienes. The most important commercial diene is 1,3-butadiene, used to manufacture synthetic rubber. Beta-carotene is a polyene (actually, an undecaene), and is the yellow pigment of carrots and other vegetables. The aromatic hydrocarbons sometimes have pleasant aromas, which accounts for their name. Most aromatic substances can be derived from benzene. Many aromatic hydrocarbons have more than one benzene ring. Commercially important aromatic hydrocarbons have fused or condensed rings in which several rings share two or more carbon atoms. Most condensed hydrocarbons are crystalline solids, and 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. Petroleum and natural gas are the main commercial sources of alkanes, alkenes, and aromatics. Gasoline and diesel fuel are mixtures of alkanes (mainly octanes). Ethene (ethylene), easily the most important alkene industrially, is used to make several products, including polyethylene, and ethylene oxide (used to make ethylene glycol antifreeze). Propylene and butene are also manufactured on a large scale, and are used to make plastics (e.g. polypropylene). Benzene, also a component of petroleum, is used in aviation fuel, and to make polystyrene and dyes.óWilliam F. Berkowitz, Ph. D., Professor Emeritus of Chemistry, Queens College