I INTRODUCTION Hydrocarbons, family of organic compounds, composed entirely of carbon and hydrogen. They are the organic compounds of simplest composition and may be considered the parent substances from which all other organic compounds are derived. The hydrocarbons are conveniently classified into two major groups, open-chain and cyclic. In open-chain compounds containing more than one carbon atom, the carbon atoms are attached to each other to form an open chain; the chain may carry one or more side branches. In cyclic compounds the carbon atoms form one or more closed rings. The two major groups are subdivided according to chemical behavior into saturated and unsaturated compounds. See Chemistry, Organic.
II OPEN-CHAIN HYDROCARBONS
The saturated open-chain hydrocarbons form a homologous series called the paraffin (Latin parum affinis,"little affinity") series or the alkane series. The composition of each of the members of the series corresponds to the formula CnH2n+2, where n is the number of carbon atoms in the molecule. Among the members of the series are methane CH4; ethane, C2H6; propane, C3H8; and butane, C4H10. All the members of the series are unreactive; that is, they do not react readily at ordinary temperatures with such reagents as acids, alkalies, or oxidizers. The first four members of the series are gases at ordinary temperature and pressure; intermediate members are liquids; and the heavier members are semisolids or solids. Petroleum contains a great variety of saturated hydrocarbons, and such petroleum products as gasoline, kerosene, heavy fuel oil, lubricating oils, petroleum jelly, and paraffin consist principally of mixtures of paraffin hydrocarbons, which range from the lighter liquid members to the solid members.
A The Alkene Series
The unsaturated open-chain hydrocarbons include the alkene or olefin series, the diene series, and the alkyne series. The alkene series is made up of chain hydrocarbons in which a double bond exists between two carbon atoms. The general formula for the series is CnH2n, where n is the number of carbon atoms. As in the paraffin series, the lower members are gases, intermediate compounds are liquids, and the higher members of the series are solids. The alkene series compounds are more active chemically than the saturated compounds. They easily react with substances such as halogens, adding atoms at the double bonds. They are not found to any extent in natural products, but are produced in the destructive distillation of complex natural substances, such as coal, and are formed in large amounts in petroleum refining, particularly in the "cracking" process. The first member of the series is ethylene, C2H4. The dienes contain two double bonds between pairs of carbon atoms in the molecule. They are related to the complex hydrocarbons in natural rubber and are important in the manufacture of synthetic rubber and plastics; important members of this series are butadiene, C4H6, and isoprene, C5H8.
B The Alkyne Series
The members of the alkyne series contain a triple bond between two carbon atoms in the molecule. They are very active chemically and are not found free in nature. They form a series analogous to the alkene series. The first and most important member of the series is acetylene, C2H2.
III CYCLIC HYDROCARBONS
The simplest of the saturated cyclic hydrocarbons, or cycloalkanes, is cyclopropane, C3H6, the molecules of which are made up of three carbon atoms to each of which two hydrogen atoms are attached. Cyclopropane is somewhat more reactive than the corresponding open-chain alkane, propane, C3H8. Other cycloalkanes make up a part of ordinary gasoline.
Several unsaturated cyclic hydrocarbons, having the general formula C10H16, occur in certain fragrant natural oils that are distilled from plant materials. These hydrocarbons are called terpenes and include pinene (in turpentine) and limonene (in lemon and orange oils).
The most important group of unsaturated cyclic hydrocarbons is the aromatics, which occur in coal tar. Although the aromatics sometimes exhibit unsaturation, that is, the addition of other substances, their principal reactions bring about the replacement of hydrogen atoms by other kinds of atoms or groups of atoms. The aromatic hydrocarbons include benzene, toluene;, anthracene, and naphthalene. See Aromatic Compounds.
Contributed By:
K. P. Müller
H. Reinhard