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(or alkenes), a homologous series of unsaturated hydrocarbons of the general formula Cn H2n, with an open chain and one carbon-carbon double bond; they belong to the class of acyclic compounds. Ethylene, CH2 ═ CH2, is the precursor of the series; therefore, olefins are also known as ethylene hydrocarbons. Ethylene and its nearest homologues—propylene, CH2 ═ CH—CH3; butenes, C4H8; amylenes, C5H10; and so on —are often called alkylenes. According to the Geneva system, the names of olefins are derived from the corresponding names of alkanes by substituting the ending “-ene” for “-ane” and inserting a numeral in the formula to indicate the position of the double bond. For example, ethane → ethene (ethylene), propane → propene (propylene), and butane → 1-butene (I) and 2-butane (II).

Structural isomers exist for a number of olefins, beginning with butene; in addition, olefins can exist as geometric isomers because of the presence of a double bond in the molecule. Olefins differ little from alkanes in their physical properties. They have slightly lower boiling points and slightly higher densities than alkanes containing the same number of carbon atoms per molecule. Lower olefins (from C2 H4 to C4H8) are gases; those up to C18H36 are liquids; and higher olefins are solids. All olefins are colorless and virtually insoluble in water, have limited solubility in alcohols, and are readily soluble in hydrocarbons and ethers.

Olefins combine readily with halogens to form oily liquids; hence their name (from the French oléfiant, “oil-forming”). They also combine with hydrogen, hydrogen halides, and water along the double bond. Halohydrins (for example, ethylene chlo-rohydrin) are produced upon interaction of olefins with aqueous solutions of chlorine or bromine. Oxidation yields glycols (ethylene glycol, propylene glycol, and so on) and oxides (for example, ethylene oxide and propylene oxide). Olefins readily undergo isomerization, as well as polymerization and copolymerization, with the formation of valuable products. An important property of olefins is their high alkylating action.

The main industrial sources of olefins are the products of refining of petroleum and natural gas. Under laboratory conditions, olefins can be prepared by dehydration of alcohols:

CH3—CH2OH → CH2 ═ CH2 + H2O

as well as by pyrolysis of carboxylic acid esters or by the Wittig reaction.

Because of their high reactivity and availability and their low cost, olefins are widely used in petrochemical synthesis, the manufacture of plastics, and the preparation of certain types of synthetic rubber, chemical fibers, and other commercially valuable products.


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