Hydrocarbons

I

Introduction

Hydrocarbons, in organic chemistry, family of organic compounds, composed entirely of carbon and hydrogen. They are the organic compounds of simplest composition and may be considered theoretically as 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 behaviour into saturated and unsaturated compounds.

II

The Alkane Series

The saturated open-chain hydrocarbons form a homologous series called the alkane, or paraffin, series. The composition of each of the members of the series corresponds to the formula C_nH_{2n +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, alkalis, 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 semi-solids or solids. Petroleum contains a great variety of saturated hydrocarbons, and such petroleum products as petrol, 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.

III

The Alkene Series

The alkene, or olefin, series of chain hydrocarbons are those in which a double bond exists between two carbon atoms. The general formula for the series is C_nH_2n, where n is the number of carbon atoms. As in the alkane 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 found to some extent in natural products, and 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 ethene, 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 (methylbuta-1, 3-diene), C₅H₈.

IV

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 ethyne, C₂H₂.