الألكينات و الألكاينات (الفصل 3 ) Alkenes and Alkynes

Alkene One of the class of acyclic hydrocarbons containing one or more carbon-to­carbon double bonds. Alkenes (also called olefins) and alkynes (also called acetylenes) together constitute the family of organic compounds called unsaturated hydrocarbons, since they contain less than the number of hydrogens found in the corresponding saturated compound, alkane. When the double bond is present in a nonaromatic ring (alicyclic hydrocarbon), the compound is termed a cycloalkene. Hydrocarbons containing more than one double bond are termed dienes, trienes, and so forth, or collectively, polyenes.

In naming alkenes by the system of the International Union of Pure and Applied Chemistry (IUPAC), the longest chain containing the double bond is identified. The presence of the double bond is indicated by changing the “-ane” ending of the alkane having the same number of carbon atoms to “-ene,” and the position of the double bond is indicated by a prefixed number. Examples are given in the table, with common or nonsystematic names which are still frequently used given in parentheses.

The lower alkenes and dienes which have up to five carbon atoms are gases at room temperature and pressure. Higher alkenes are colorless liquids or solids. Like other hydrocarbons, alkenes are insoluble in water. Liquid alkenes have specific gravities well below 1.0. Alkenes may undergo polymerization, cyclization, and addition reac­tions. A major share of structural and elastic polymers are based on homopolymers or copolymers of alkenes and dienes. Alkenes and dienes cyclize readily under various conditions.

Addition reactions of alkenes are among the most important in the entire field of organic chemistry. Industrially, high-octane gasoline is made by the acid-catalyzed alkylation of the three-and four-carbon alkenes. A variety of alkylated aromatics are made by the alkylation of benzene with olefins.

The commercially important alkenes are produced on a large scale in the petroleum industry by thermal or catalytic cracking processes. In the laboratory, most methods for the preparation of alkenes involve some type of elimination reaction, in which atoms or groups on adjacent carbon atoms are removed with concomitant formation of the carbon-carbon double bond.

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